Comments on Hoadly and Wilson’s Electrical Pamphlet, 28 January 1759
Comments on Hoadly and Wilson’s Electrical Pamphlet
AD: American Philosophical Society; italicized text summarized from Dr. [Benjamin] Hoadly and Mr. [Benjamin] Wilson, Observations on a Series of Electrical Experiments (London, 1756). (Yale University Library)
In August 1756 Benjamin Hoadly and Benjamin Wilson, English electrical experimenters, published a 76-page pamphlet entitled Observations on a Series of Electrical Experiments. In it they developed a “doctrine of resistances” to explain the resistance any material substance or body offers to electrification, whether positive or negative, or to an increase or decrease in the body’s electrical potential. They described the “electrical fluid” as subtle, elastic, and all-pervasive, and in the final pages announced their conclusion that it was the identical all-pervasive fluid that Newton had propounded and to which he had given the name “aether.”
When Franklin learned of this pamphlet’s publication he told Collinson of his interest4 and he ultimately secured a copy, probably sometime after reaching England in the summer of 1757. His own major contributions to electrical science were all in the past, yet he always managed to find time in his busy life to keep track of new developments, and he obviously read the Hoadly and Wilson pamphlet with great care. Some things in it he found perplexing, others he thoroughly disapproved, and he undertook in his methodical way to set down his reservations and objections. In his own copy he carefully numbered the 301 paragraphs of the pamphlet and then, using the appropriate numbers, wrote an eight-page memorandum containing his questions and comments. In the margins of his copy opposite more than half of these debated paragraphs he also drew short parallel lines to call attention to particular phrases or sentences to which his comments were directed.5
Franklin’s memorandum of criticisms is unintelligible when read without immediate access to what Hoadly and Wilson had previously written. To reprint here the full seventy-six pages of their pamphlet would be an inappropriate assignment of space, hence the editors have undertaken a compromise. The Hoadly and Wilson text is reproduced in summarized or abstracted form, set in italic type. Each section in the summary is preceded by inclusive numbers representing those Franklin assigned in his copy to the corresponding paragraphs of the original. Whenever Franklin’s memorandum has a comment, this is set in roman type immediately following the summary of the questioned passage. The summaries are rather longer than the editors would have preferred, but this length has seemed necessary both for the sake of clarity and in order to do as much justice as possible to the authors’ ideas and findings.6
As Franklin’s note at the end of his memorandum shows, he first read it to Wilson and then, on Jan. 28, 1759, gave him a copy. For this reason the paper is included in this edition as of that date. Dr. Hoadly had died, Aug. 10, 1757, so was not involved in the oral discussions that must have taken place when Wilson received Franklin’s criticisms. Later in 1759 the surviving author published a revised edition of the pamphlet.7 In the prefatory note he explained that he had “thought it expedient to make some alterations, and to add some new experiments.” Comparison of the two editions shows that of Franklin’s twenty-eight specific criticisms Wilson ignored all but seven. In five instances he made relatively small changes or incorporated explanatory footnotes; twice only he made major changes. These are all described below in footnotes at the appropriate places, as are a few additional revisions involving Franklin’s writings, but no attempt is made here to discuss other changes or additions in the second edition.
[January 28, 1759]
1–2. The authors cite Newton’s theory of an all-pervading “fluid” called “aether,” which is “much rarer within the dense Bodies of the Sun, Stars, Planets, and Comets, than in the empty celestial space between them,” and becomes denser the farther away it gets from these bodies. By causing each body to try to go “from the denser parts of the aether towards the rarer,” this aether causes gravity.
2. How is it discovered that the Ether grows denser in proportion to its Distance from the Sun Planets, &c.8
3–13. The earth is surrounded by aether, causing gravitation towards it; aether pervades the “pores” of all bodies, its density in inverse ratio to the density of each body; hence we may conclude that some resistance will arise to efforts to alter the density in any body and some force will be necessary. It is agreed that electrical phenomena are caused by a similar elastic fluid, and the term “electrified” means that the amount of this “electrical fluid” in a body has been either increased over or decreased below the amount “that naturally belonged to it.” There is resistance to such change and “a limit, beyond which we cannot encrease or diminish the natural quantity of this electrical fluid, in each particular body.”
13. Is this an Effect of Resistance in the Body, or is it a Consequence of the Atmosphere’s being extended beyond the Distance at which it can be retain’d by the Attraction of the Body? An Experiment will easily determine this, viz. When the Prime Conductor is fully charg’d, try if any Electricity is to be obtain’d from the Back of the Globe, which has pass’d by the Points.
14–36. There is also resistance to the return of the electricity in a body to its natural state after electrification, and some external force is required to effect the return. Bodies differ in this resistance to electrical change: glass, wax, etc., and above all, air, resist the most; metals, water, animals, etc., the least. In the latter group, polished and extended surfaces resist most; rough surfaces, edges, and sharp points resist least. An experiment is devised to prove this theory of resistance: a highly polished iron bar with rounded ends and suspended in air by silk threads is electrically charged. This is to be discharged by advancing towards it a pointed body made of a low-resistance substance, bringing it constantly closer even until contact is established. But as this is being done the bar is discharged gradually without the violent discharge the experimenters want, because as the pointed body approaches the bar there is so little air between the two that there is little resistance to the discharge. So they substitute a blunt-ended, highly polished body and approach it to the bar “with some degree of quickness” so that the discharge will take place “at once, and not gradually; violently, and not with ease.”
35. Is there not as much Air in Proportion between a Point presented to the prime Conductor, and its opposite Point of the Surface of the prime Conductor, as between a blunt Body presented, and an equal Part of the Surface of the P.C. opposite to it?
37–63. The authors wish to determine the circumstances [shape, nature of surface, etc.] which affect the ease and speed with which a body is charged, discharged, or recovers to its “natural state.” They have devised a series of experiments involving a long iron bar tapered at both ends and balanced on a tall drinking glass. If they bring a charged tube close (about six inches) to its mid-point, it is electrified minus; if they bring the tube close to an end of the bar, it is electrified plus. If a second similar bar balanced on a glass is placed in line with the first with the points touching and the tube is brought close to the mid-point of the first, the first bar will be electrified minus and the second bar plus. If a third bar is placed at the other end of the first and the tube advanced to the first as before, this middle bar will be electrified minus and both end bars electrified plus. All this shows that the first bar has discharged through its tapered ends some of its natural quantity of electricity. Now cover its pointed ends with close-fitting glass caps. When the charged tube is again brought close to the mid-point the bar “will be electrified plus, instead of minus: which shews how much more strongly the extended surface of the bar, opposite to the excited tube, resists the fluid’s escape, than the tapered ends did.”
63. Will the Bar remain electrified plus when the Tube is withdrawn?9
64–68. Set two or all three bars in line as before and advance the charged tube to one end, not the mid-point, of the first or middle bar. All three will be electrified plus, the middle bar the most, the nearer end bar next in amount, and the further bar least of all. It would seem that if a fourth, fifth, and sixth bar, etc., are added, ultimately the most distant would not be affected at all, and “consequently that the virtue of the tube is limited, and can affect the fluid in these bars to a certain distance only.”
67. Is the Effect mention’d in this and the following Section [pars. 67 and 68],1 any other than a necessary Consequence of a certain limited Quantity of Electricity, being divided among a greater Number of Bodies, or extended over a greater Surface?
69–74. Consider the air surrounding the bars in these experiments. Though air strongly resists being electrified, the excited tube can and does overcome that resistance when it succeeds in electrifying the bar minus through the intervening air [a distance of about six inches] and, in the experiment with a single bar, forces electrical fluid into the surrounding air. The fluid seemingly stands in the air near the points of the bar and when the excited tube is withdrawn, most, but not all, of the electrical fluid returns into the bar. What remains “forms itself into a kind of atmosphere every way surrounding the bar with a nearly equal degree of density.”
74. How is it found that any of it returns in again? If any of it remains out, how does it appear that it forms an Atmosphere round the Body?2
75–77. When the single bar was electrified plus, there was enough fluid to electrify two bars applied at its other end; it follows that when a single bar is electrified plus by itself and continues so after the tube is withdrawn, the expelled fluid similarly forms an atmosphere around the bar. Therefore, whenever a body is electrified either plus or minus and remains so after the experiment is over, there are “similar atmospheres of the electrical fluid”3 surrounding them, ready to expand into any approaching body less resistant than air. This is why “bodies give very nearly the same signs” when electrified either plus or minus.
77. How does it appear that the Atmospheres surrounding Bodies electrified plus and minus, are similar? If they are both plus Atmospheres, how do they destroy each other on being brought together?
78–80. I say “very nearly” the same signs because on accurate observation there are enough differences to enable us to say which bodies are electrified plus and which minus. Consider the different circumstances the fluid is in around a body electrified plus or minus. When a bar is electrified plus the electrical atmosphere formed around it lies between the air and bar and both resist the fluid’s entry, the bar more forcibly. So the fluid exerts its effort at expansion outwardly and it gradually dissipates into the air. “Whilst it is doing this, and no longer, the bar will remain electrified plus.”
80. If the Electrical Atmosphere round a Body electrified plus, lies between the Body and the Air, must not the Air be push’d off, a Vacuum of Air created, and the Space occupied by the electrified Atmosphere be unfit to breathe in? But none of these Things happen. Put an Iron Rod into an empty Bottle; electrify the Rod, and none of the Air is driven out of the Bottle by the electrical Atmosphere of the Rod. This I have try’d by an accurate Experiment. I have also often breath’d in an electric Atmosphere, without perceiving any Unfitness in the Air for Respiration.4
81. “When a bar is electrified minus, the atmosphere formed round it, which during the action of the tube that electrified it, lies in the same manner between the air, and the bar,” will when the tube is withdrawn, try to expand in a contrary direction, that is, from the air into the bar. It will gradually flow into the bar and restore its “natural degree of density.” Only while this is going on will the bar remain electrified minus.
81. When a Bar is electrified minus, by having Part of its Quantity forc’d out on its Surface, and you draw off that Quantity by a Point or otherwise, how does it appear that any electrical Atmosphere remains between the Bar and the Air?5
82. When two balls both electrified plus are suspended and brought near each other, they repel each other and stand for some time at a distance, because the two atmospheres, each trying to expand, want more room to do it in. When the weight of the balls is insufficient to prevent it, they are driven apart until the atmospheres are dissipated and the weight of the balls brings them back to their natural position.
82. Are not the two Atmospheres confined to the Balls, not only by the surrounding Air, but by a mutual Attraction between each Atmosphere and its Ball? If not, why does the Atmosphere accompany the Ball in its swiftest Motion? Must it not then be one Effect of such an Attraction to keep each Atmosphere spherical; and then if the two Atmospheres are brought near together by the Weight of the Balls, would they not, by endeavouring to preserve their Sphericity keep the Balls at a Distance?
83. When two balls both electrified minus are suspended and brought near each other they similarly repel each other because “the condensed electrical fluid in the air, in order to force itself in at the surfaces of the balls between their two centers, crouds in, and forces them asunder,” until the atmospheres get all inside the balls and weight restores them to their natural position.
83. Two Balls suspended by silk Strings may be electrify’d minus, by touching them with the Coating of a charg’d Bottle, which you hold by the Hook. In that Case the Electrical Fluid which quits the Balls does not go into the Airs, but into the Coating of the Bottle, an equal Quantity at the same time passing from the Hook thro’ your Body to the Earth. Whence then arises the condensed Electrical Fluid supposed to be in the Air surrounding the Balls?
84. When two balls, one electrified plus and the other minus, are similarly suspended, “they will gradually come together and unelectrify each other.” The atmosphere of the ball electrified plus is trying to dissipate itself outwards and that of the other “to dilate itself from the air inwards to the center of the ball.” So the two atmospheres exert their forces in the same direction and the flow of the electrical fluid in each facilitates the flow in the other, and the two balls and the air between “very readily return to their natural states.”
84. The Word gradually seems not the most proper as the Balls rush together with double Quickness, compar’d to that with which either would fly to a Body in its natural State.
Query. One of the two Atmospheres being suppos’d to enter the Ball electrified minus, what is suppos’d to become of the other?6
85–89. Following this train of reasoning we can obtain a method of determining whether a body is electrified plus or minus, “even without unelectrifying it.” Fasten two cork balls to the ends of an 8-inch thread and double it over a bar before electrifying it so that the balls are hanging below it as near together as possible. Bar, thread, and balls then become in effect one body ready to be electrified. Suppose, first, we electrify it minus; the balls will repel each other. Now bring an excited tube a certain distance under the balls; “they will at first repel each other more,” because the force of the excited tube will condense their atmospheres more (till the resistance at their surfaces is overcome), and these atmospheres being increased, the mutual repulsion will increase.
89. How does condensing the Atmospheres more, make the Balls repel each other more, since it should seem that Atmospheres being condens’d would take up less Room, and thereby suffer the Balls to approach nearer? How is it known that the Atmospheres encrease while they are condensing? May not the Approach of the Balls, on bringing the Tube nearer to them, be rather ascrib’d to their receiving some Electricity which they wanted, and thereby recovering their natural State. See 90, 92. See also 97.7
90–93. As soon as this initial resistance is overcome, the tube drives the atmospheres into the balls “and consequently begins to unelectrify them,” that is, they become “less forcibly electrified minus.” On withdrawing the tube the balls will hang nearer together than before [par. 90]. Now suppose we electrify the bar-and-ball combination plus; the balls again repel each other. When an excited tube is brought under them as before, they will come nearer together, because air resistance cannot prevent the electrical fluid escaping from the balls electrified plus and creating an atmosphere around them. Then, the excited tube, acting “in concert with the air,” forces this atmosphere back into the balls [par. 92]. This appearance will continue as long as the bar-and-ball combination can be increasingly electrified plus by the tube; when the tube is withdrawn the balls will repel each other more forcibly than before because of their increased electrification.
93. Is not the Balls being electrify’d more forcibly than at first, owing to their having receiv’d an Addition from the Tube?
97–102. In the case of a bar electrified minus, if the tube is held under the balls long enough for them to cease repelling each other when it is withdrawn, and the tube is then again presented to them, it will electrify both balls and bar plus [par. 97]. It is reasonable to conclude from this that in the first experiment [see above, pars. 37–63], if the tube had been brought nearer and nearer to the mid-point of the tapered bar and finally made to touch it, the bar would have been electrified plus (instead of minus). Testing proves this conclusion correct. There must therefore be some middle distance at which the tube, instead of electrifying the bar either minus or plus reduces it to its natural unelectrified state. “This would appear a most amazing paradox … to any one, who did not know that bodies were capable of being electrified plus and minus,” that is, that the same excited tube brought towards the bar first electrifies it, then ceases to electrify it, and finally electrifies it again.
103–109. The cork-ball experiment is also useful to prove that the power of the excited tube (or other electrifying machine) to electrify bodies at a certain distance is limited. The comparative force with which the balls repel each other provides an index to the degree of electrification of the bar to which they are attached. Hang the balls at the end of the bar and bring the excited tube to the middle of it at a distance to electrify it minus. However long it is kept at that distance it can electrify the bar only to a fixed degree. This is shown by the fact that the balls will repel each other only to a certain distance and remain in that position as long as the tube is unmoved. This proves that the power of the tube is limited and is confirmed by a further set of experiments.
110–114. Hermetically seal a glass tube at one end; cement to the other end a brass contrivance to which an air pump for creating a vacuum in the tube may be attached and then removed without destroying the vacuum. Before the air pump has been attached, the outer and inner surfaces of the tube are equally exposed to the air and are in equilibrium. Attach the pump and exhaust the air from the tube; the outer surface is now exposed to the air and the inner surface exposed to the electrical fluid, “which I will suppose to be naturally dispersed in empty spaces void of all gross bodies, (as the vacuum is thought very nearly to be) as well as in the pores of gross bodies.”
114. Upon what is this Supposition founded?
Can we suppose the Airs to be exhausted, and the electrical Fluid left behind in just the same Quantity as before? Or, does some electrical Fluid enter during that Operation, to supply the Vacuity made by exhausting the Air?
115–119. The equilibrium is still preserved between the powers at these surfaces, there being no observable evidence to the contrary. We may conclude that the air in its natural condition does not affect the tube at its outer surface more than the electrical fluid does at the inner. Now take the air-exhausted glass tube off the air pump and have a person grasp it by the brass end while standing on the ground. Let another person bring an excited tube near the outer surface, say near to the hermetically sealed end. Immediately “lucid rays of light” will appear proceeding from the inner surface of that part of the exhausted tube which is nearest the excited tube and darting through the vacuum to the brass grasped by the hand.
119. If the electrical Fluid was dispers’d in the Vacuum as in 114, would it not be put in Motion by this Experiment, and appear at both Ends at the same Time? See 124.
120–125. If the tubes are kept steady at the same distance for some time, the light will disappear. After a further interval of time, if the excited tube is removed the rays of light will reappear, this time darting from the brass through the vacuum to the point on the inner surface from which they had first appeared. This light, too, will in time disappear. When the excited tube is brought nearer and nearer the exhausted tube, a fresh darting of rays appears, similar to the first, again continuing for a time only. When the excited tube is removed the returning flow of rays, stronger and in greater quantity, reappears and then ceases. Apparently, when any electrical force is offered to the outer surface of the exhausted tube, the electrical fluid on the inside flows to the brass and hand, “where it finds the resistance to its escape is the weakest.” Yet there is some resistance even there, hence some force must be given the electrical fluid, condensing it enough to overcome that resistance [124]. Furthermore, the inside surface of the tube must be losing some of its natural quantity of the fluid into the vacuum and thence into the brass, etc., unless more is supplied it from the excited tube.
126–144. Here follows an extended discussion to show that no electrical fluid passes from the excited tube through the glass of the exhausted tube onto its inner surface to replace that driven off. The conclusion is that this inner surface has now been electrified minus and remains so until the return flow of rays signals the restoration of the natural state to that inner surface. A further experiment demonstrates the correctness of this conclusion. An excited tube gradually loses its power, but we know that an iron bar suspended in air by silk threads can be kept uniformly electrified for an indefinite period by means of an electrifying machine kept uniformly in motion.8 Therefore, approach the exhausted tube to an “excited bar” (steadily charged by such a machine) close enough “to make the lucid appearance begin” and keep it steadily there. The rays continue only for a time, then die away as before, but upon removing the exhausted tube from proximity to the excited bar the return flow of rays again takes place. The situation here is the same as in the series of experiments with tapered bars: the second bar brought in contact with the first is here represented by the brass at the tube’s end and the person holding it and standing on the ground. The excited tube (or excited bar) has power “to dilate the electrical fluid” in the first bar in the one case, and on the inner surface of the exhausted tube in the other case, and to drive it into the second bar and the brass end of the tube respectively. In both experiments this power is limited; while it is effective, this electrical fluid will flow, but “when the electrical fluid in the brass, hand, &c. is sufficiently condensed to ballance the force of the dilating fluid in the vacuum,” no more will flow and the lucid light will disappear.
144. As the Brass and Hand communicate with the Earth, how does it appear that the Electrical Fluid is condens’d in them? See 229, 230.
145–146. So long as the exciting tube is held at the same distance the electrical fluid in the vacuum and the brass, etc., will be kept in equilibrium, for the same power that drove it to its present position can hold it there while the force remains unchanged. But when the tube is withdrawn “the force of the condensed part of the fluid in the brass drives that in the vacuum back again to the inner surface of the glass, where the tube had been applied,” and is in turn replaced by fluid from the brass, all this being manifest by light shooting from the brass to the glass until equilibrium is restored.”
146. Is not the Electrical Fluid rather attracted back by the minus side of the Glass, and drawn thro’ the Brass?
147. This effect is similar to what happened to the two tapered bars when the tube was withdrawn: equilibrium was “restored by the condensed part of the fluid in one bar’s gradually flowing into the dilated part in the other.”
147. Is it not rather drawn than press’d back?
148–165. It has been found that in order to electrify glass with considerable force it is necessary to cover its two surfaces with metal or some similar easily electrified material; but why these coverings are necessary has not hitherto been satisfactorily explained. Cover the middle of both sides of an oblong pane of glass with leaf gold but leaving two or three inches of uncovered glass at all edges. When any spot on the upper covering is electrified, the entire covering is equally electrified and the glass surface immediately below it is equally exposed to the electrical fluid with no air intervening. An electrifying machine brought into direct communication with the upper gold covering by means of a wire will exert its full force against the whole extent of the glass immediately under the covering. Another wire from the under covering to the ground will remove much of the resistance to the escape of electrical fluid through the under side of the glass. The uncovered areas of glass at the rims prevent electrical leakages around the edges from one covering to the other. Place a drinking glass under each corner of the pane; let one person standing on the ground hold a finger within an inch of the under covering; bring a wire ending in a knob from the electrifying machine to within an inch of the upper covering; now set the machine in motion. A spark will jump from the wire to the upper covering and at the same time one from the lower covering to the finger. Repeated sparks will appear for a time and then cease, even though the machine is kept in motion. It would seem that whatever had been “thrown in” at the upper covering had passed out in equal quantities at the lower, and that the electrical fluid had a free passage through the glass, which remained unelectrified. Indeed, after the wires are removed one may touch either covering without producing more than very slight signs of electrification. But if one touches both covers at once with fingers of different hands “he will receive so severe a stroke, as will convince him that the glass was very strongly electrified.” It is certainly true that as much fluid is thrown out at the under surface into the finger as was thrown into it at the upper surface from the wire; “but the reasoning upon this fact is evidently false, as it contradicts experiment.”9
166–173. Can our doctrine of resistances help us resolve this difficulty? Suppose we give the resistance of each surface of the glass the numerical value of six, and the power of the electrifying machine the value of nine. Electrical fluid will be forced into the glass with the net force of three and condensed there to that force and no more. It cannot escape around the uncovered edges of the glass. As much fluid as force three can throw into the upper surface will be thrown out at the lower one at the undercovering (where the resistance is least because of the ground wire) until the resistance at this surface is gradually reduced to three, and then all forces will be in equilibrium. As we saw was the case in the experiment of the exhausted tube, it was possible for the excited tube, though at a distance, to drive electrical fluid out of the inner surface of the glass, though none passed through. We may therefore here imagine it possible that the fluid penetrating to a certain distance into the upper surface may similarly act at a distance and force out some of that near the under surface until the resistance on the under surface is reduced to three and an equilibrium is reached. Sparks may therefore appear until then in the experiment as described, but not thereafter.
174–183. How can we conceive a quantity of elastic fluid lying in one part of the glass while “its neighbouring fluid in another part is attenuated”? Following the precedent of the experiment of the exhausted tube, withdraw the wire which communicated from the electrifying machine to near the top covering, but leave the finger in place near the bottom covering; no sparks jump from the finger to the lower covering. “This therefore seems to be an experimental Proof that there is some power remaining in the glass itself” to prevent a return flow of the fluid previously thrown into the finger.1 The machine had apparently condensed the fluid in the upper surface of the glass, causing the dilation of that in the under surface and its consequent passing out into the finger; if now the condensed fluid could dilate itself and pass out again at the upper surface, the fluid would likewise return from the finger to the lower surface, but the resistance at the upper surface prevents this from happening. It is the condensed fluid in the upper surface which prevents the return of the sparks. In the case of the exhausted tube, the excited tube did not have enough power to condense fluid in the glass, and when the excited tube was withdrawn the excited fluid in the atmosphere surrounding it was likewise withdrawn, permitting the return of the lucid appearance; hence the difference in the two experiments.
184–189. We know that the pane of glass has remained electrified and “we have all the reason in the world” to believe that as much fluid had passed out at one cover as had entered in the other. Hence the electrification of the glass does not arise from an increase or decrease in the whole quantity of electrical fluid as in the case of metals, etc. The only possible explanation is the continued condensation of fluid at the upper surface after the wire is withdrawn, and the continued attenuation at the under surface. That is, the upper surface is electrified plus and the under surface minus. There is some similarity here to the experiment of the tapered bars when the excited tube was applied to the middle of the center of three bars. Yet in that case, when the excited tube was withdrawn the three bars returned to their natural state, while in the present case some resistance remains in each surface of the glass preventing a similar return. Here, because the respective plus and minus electrification of the two glass surfaces continues after wire and finger are both withdrawn, there must be two atmospheres left standing on these surfaces, which are now surrounded with air,2 as in the case of the cork balls, and these atmospheres are endeavoring to dilate into the air. Whenever they can communicate, the plus atmosphere will drive the minus atmosphere into the glass surface electrified minus, as in the case of the cork balls.
189. There is, I imagine, seldom more than one of the Atmospheres subsisting at the same time, and that not 100th part, perhaps, of the Quantity contain’d in the positive Surface of the Glass. So that the Communication is to be formed between the positive and negative Surfaces, rather than between positive and negative Atmospheres.
When the Pane of Glass is charg’d, touch the positive side with your Finger, and you draw a small Spark and with it all the Atmosphere of that Side; but you create a Negative Atmosphere on the other side. Then touch the Negative Side, and by giving a Spark you destroy the negative Atmosphere, but at the same time a quantity flows out of the positive side of the Glass, and spreads on that Surface, creating a positive Atmosphere. This you may draw off in a Spark as before, and repeat by alternate Touches on the opposite sides 100 times running.3
190–191. The broad column of air in immediate contact with all uncovered surfaces of the glass separates these atmospheres, and we know that a pane of glass, properly covered, will remain electrified much longer than metals. There is only one way of unelectrifying the glass with violence: open a direct communication between the two surfaces, as was done when a person touched them simultaneously with fingers of his two hands. However quickly he does it a strong spark will appear.
192. To form a communication between the two surfaces gradually instead of suddenly, bend a wire in circular form so that the two ends may be brought to within two or three inches of the covers of the glass; taper the ends to fine points and afix a piece of sealing wax to the midpoint of the wire to hold it by. With this device “the glass will quietly and gradually return to its natural state,” and at the end the wire will not be electrified. If this experiment is done in the dark “a very visible stream of light” will appear at each end of the wire during the action. May we not conclude: 1. That nothing more has been done but gradually restore the equilibrium in the glass? 2. That the excess of electrical fluid on one side of the glass had exactly balanced the deficiency on the other, and that, therefore, during the electrification as much had been thrown out of the glass as thrown in? 3. That in the simultaneous touching of both covers by the fingers the violence was due only to the increased velocity of the movement of the fluid? 4. That this velocity had been great enough to kindle a strong spark, while by the new method there will be only a “continued stream of faint bluish light”?
193–201. We may make further observations on this subject: 1. If the wire from the electrifying machine is brought into contact with the upper covering but the ground wire left off from the lower, the glass cannot be electrified, because of the combined resistance of glass, cover, and surrounding air, and this is true however great the power of the machine. 2. If the ends of the bent wire are brought into direct contact, or nearly so, with the two coverings, the glass cannot be electrified, because the fluid has a free passage from one covering to the other. 3. Without such a communicating wire the two atmospheres will come together if the coverings come too close to the edges of the glass, and it cannot be electrified. 4. There is a great resemblance between the two atmospheres lying at the two surfaces of the glass, electrified respectively plus and minus, and the two atmospheres surrounding the cork balls in a previous experiment.
201. It seems this Resemblance does not hold as to Atmospheres in the two Cases. The different States of the opposite sides of the Glass, being within its Substance, and not depending on Atmospheres subsisting without. Thus a Number of Glass Plates ground truly flat, and coated, may be laid one on another, and electrified, so that every other two Faces in contact shall be positive, and every other two negative, and the whole Forces united be very great and give a terrible Blow when discharg’d proportion’d to the Quantity of Surface coated;4 and yet no Atmosphere could subsist, unless on the two outside Plates; for the Plates being in close Contact, exclude any Atmosphere.
202–208. Electrify two panes of glass coated as before and each supported by drinking glasses at the corners, and let them stand. 1. Connect the two upper surfaces by a wire; no alteration will occur because the two atmospheres act upon their respective upper coverings with no more force than before they were connected. Thus, when two balls both electrified plus are brought together they do not unelectrify each other but merely repel each other, as the two panes of glass would do if the force were strong enough. The same is true if the two under surfaces are connected by wire. 2. But if by “a cross communication” one wire connects the upper cover of one pane with the under covering of the other and another wire connects the two remaining coverings to each other, “the condensed atmospheres will neither of them be confined, but a free passage will be opened to them to dilate into the panes of glass.” The electrical fluid will circulate around and be reduced to the natural state in both glasses and wires.
208. The Change is here hinted to be in the Glass.
209–214. Numerous experiments might be made confirming the “uncommon appearances” occuring in the electrification of glass, but I stop here with a mere caution not to carry too far the comparison of the atmospheres on the electrified pane with those on the cork balls. We have seen that the variety in resistance to electrification of different bodies leads us to know why bodies act so differently in different situations according to the nature of the bodies they are contiguous to, and enables us to explain “the most amazing appearance of all,” that of the Leyden bottle. Our success so far encourages us to inquire where this resistance is exerted and from what power within the body it arises. We know by experiment that every body resists electrification, though some do it with greater force than others. We may reasonably conclude that resistance to the entry of electrical fluid is exerted at the surface where the attack is made, but resistance to the fluid’s exit is exerted at all internal surfaces at once, and if the force at any point is greater than the resistance there, then it will always enter or leave at that point.
215–220. There is an exact analogy to this resistance in the resistance of glass to the entrance of light at one surface and to its going out at the opposite surface; at both surfaces their resistance drives off great numbers of rays that might otherwise have passed through. The rays reflected back into the glass in their endeavor to get out are again resisted when they return to the first surface. Newton has shown that this force of resistance begins to operate on rays of light before they arrive in contact with either surface. We have reason to believe that the resistance to electrification, either plus or minus, of all bodies is of the same nature as that which not only prevents the entrance and exit of rays of light but throws them off with the same velocity with which they tried to enter or leave, and that the two kinds of resistance probably arise from the same cause. The experiments in Newton’s Optics serve greatly to confirm this opinion, and we have tried to establish it by electrical experiments.
221–234. Whence does this power arise? “Let a person standing on wax electrify a tube, and let another person standing on the ground take as many snaps, as he can from the tube.” Soon he can get no more. But if, when this happens, the person on the wax sets one foot on the ground and keeps it there, the other person “may take snaps from morning to night, if he pleases.” The following conclusions may be drawn: 1. The person standing on the wax has “naturally” a quantity of the electrical fluid in him, which in the experiment is thrown into the other person on the ground. 2. But the person on the wax has only a limited quantity of the fluid. 3. Therefore, the cause of the fluid’s passing out and causing the snap is that the fluid is condensed in the hand which touches the tube in rubbing it, and so long as this condensation can be made with a degree of force superior to the resistance against escape, it will continue to produce snaps. 4. The person standing on the ground must be considered as one body with the earth and will not be sensibly electrified however many snaps he takes [229]. 5. When the person on the wax touches his foot to the ground, he too becomes one body with the earth and the degree of density in his body cannot be sensibly altered however many snaps are taken [230]. 6. In these circumstances, therefore, neither of these persons can be electrified. 7. It follows that all animals, vegetables, water, minerals, and metals on or in the earth partake “of this common stock in the general course of nature”: without being sensibly electrified. ‘Whence therefore arises their resistance to being sensibly electrified?” Any body sensibly electrified, whether plus or minus, is surrounded by an atmosphere strong enough to balance every power endeavoring to electrify it beyond a certain degree; otherwise it might be electrified without limit. It is this atmosphere surrounding bodies when artificial force electrifies them that resists their being electrified more, and when it absolutely prevents such electrification, it must be equally as strong as the electrical fluid flowing from the excited tube or machine.
234. All Fluids naturally seek an Equilibrium. If the Body electrified receives no more after a certain Quantity, is it not owing to an Incapacity, in the Globe, of having more than a certain Quantity at one Time on its surface, rather than to any Resistance in the Atmosphere already communicated? Increase the Surface of the Body electrified, and you may give it an Additional Quantity from the Globe ad infinitum.
235–239. “In the ordinary and quiet manner, in which the imperceptible works of the Author of nature are carried on” this subtile, active, all-pervasive electrical fluid cannot be idle, but must be in constant, if imperceptible, action; i.e., it must be electrifying all bodies plus or minus, though not forcibly enough to give sensible signs of it. We may conclude therefore that all bodies have surrounding electrical atmospheres sufficient to balance the smaller force attacking them. In these atmospheres is placed the power which occasions the resistance to the bodies being electrified to a higher degree. That power is the elasticity of the electrical fluid—everywhere dispersed when “gross bodies” are not in the way, but likewise confined within bodies according to their different situations and neighborhood to other bodies. These atmospheres may be increased or diminished to a certain degree by art, and when this is done with violence “the natural contexture of the bodies is altered in proportion to the violence.” Thus we see, even with the small force of our electrical machines, not only bodies parting with their natural share of the fluid, but of many of their component particles, “which may be perceived by the smell they yield on being electrified, and the rays of light they throw out, which, mixing with the air, occasions real Fire.”
239. Since the Smell occasion’d by Electricity is the same, whatever be the Body electrify’d, is it not probably occasion’d by some other thing than the component Parts of those Bodies?5 May not real Fire be produc’d by Electricity without mixing with Air?
240. These are proofs that the atmospheres have been increased, since they can only happen by the “sudden dissipation of them by art” after an increase. Before this they kept the enclosed body “compact and entire”; if the force that increased them had been gradually withdrawn, the atmospheres would have gradually returned to their natural density and the bodies to their natural state “both with regard to their component particles, and their natural share of this fluid and of the rays of light within them, which were all disturbed and in action before.”
240. What is meant by the natural Share of Rays of Light in Bodies? See 278 and 283, and 286, and 288.6
241–248. The above remarks on the increase of density in atmospheres may be criticized as arising from theory, but this theory “has been very carefully raised from experiments”; it can be destroyed only by showing a fallacy in the reasoning or an experiment to contradict it. Further experiments are now offered in support of the doctrine. Glass is the most difficult of the common substances to electrify; it has the most resisting atmosphere on its surfaces, that is, the densest atmospheres. Metals resist electrification much less; their atmospheres are not the densest. Heat rarefies bodies and increases conductivity: when glass is heated it resists electrification no more than metals do, and when melted, no more than water. A subtle and elastic fluid must be rarefied sooner than such a dense body as glass, and it seems most probable that the resisting atmosphere around glass is rendered as weak as that of metals by heat, and therefore resists the passage of electrical fluid no more than metals do. This seems confirmed by the fact that, as glass gradually cools, its resistance to electrification increases until, when quite cold, it resists as forcibly as ever. Wax resists electrification perhaps as much as glass, certainly more than metals when its surface is smooth and polished; it can be melted with a small degree of heat, so its atmospheres are readily brought down to the same degree of rarity as those of metals and when heated only by friction it seems to become an electrical conductor.
249–250. The same is true of brimstone (sulfur); hence the results of the following experiment. Place a glass globe so as to communicate with one end of a metal bar suspended by silk strings and a sulfur globe to communicate with the other end of the bar. If both globes are equally rubbed, the bar will not be electrified at all: we cannot get a single spark from it.
250. This experiment seems mistaken. The rubbed Brimstone electrifies negatively; or, at least, differently from Glass. It is doubted, whether Brimstone warmed to any Degree by Friction, short of a melting Heat, will conduct.7
251–256. Brimstone is classed with glass, wax, resin, etc., in its resistance to electrification, but like wax does not resist heat of friction as glass does; therefore its surface atmosphere can be more easily reduced by friction to the same weakness as that of metals. When rubbing the sulfur globe has heated, and so attenuated, the atmosphere on its surface, the globe and the machine that moves it become conductors and carry off the electrical fluid thrown into the bar by the glass globe. It is the heat caused by friction on the sulfur globe that causes the effects described [in par. 250]; hence if the sulfur globe is kept unrubbed but the glass globe is rubbed, the bar will be electrified and a spark obtained from it. All this is strong confirmation of what we have been saying, that the resistance of all bodies to being electrified is exerted at their surfaces and is caused by atmospheres of electrical fluid which lie at these surfaces and differ in density according to the different nature and quantity of the bodies immediately surrounding them. It shows us that heat arising from rubbing attenuates the electrical atmosphere of the sulfur globe and takes off the sulfur’s natural resistance to electrification.
258–259. Set on glass an electrified Leyden bottle that has a hook in its coating; attach a clean chain to the hook; let a person grasp the coating with one hand and with the other hand bring the other end of the chain, and his finger and thumb that holds it, into contact with the wire of the bottle. There will be two possible courses for the electrical fluid to pass from the wire to the coating: the person’s body or the chain. If the links of the chain hang loosely the person feels the shock; if the chain is stretched taut the fluid will pass through it and he will feel no shock, however long the chain may be. It follows that the electrical fluid does not always pass from one body to another by the shortest way, but by the way of the least resistance, even if that is more roundabout.8
260–70. If the chain alone forms the communication and is spread on the table so loosely that the links barely touch each other, not only will there be a spark at the end where the chain touches the wire, but if the room is dark, a number of sparks will appear at places where the links do not absolutely touch each other. When the chain is stretched tight enough to make absolute contact between all links, only the spark at the end will appear. The appearance or non-appearance of these sparks shows us whether the fluid is passing through the chain. We may conclude that the resistance in the chain arises from the sum of the resistances at the different surfaces of the several links where the fluid had to break its way through. When the links were forced into contact with each other the chain acted as a single piece of metal and no such series of resistances arose, the person holding it was not affected on the discharge of the bottle, and no sparks appeared between the links. Now, fasten a wire of whatever length to the hook of the bottle as well as the chain; then let a person, grasping the bottle as before and leaving the chain loose on the table, bring the other ends of both the wire and the chain (and also his fingers) into contact with the wire of the bottle. Now we have three ways of discharging the bottle: the person, the chain, and the wire. We find that the fluid does not go through the person: he feels no shock. It does not go through the chain: there are no sparks between its links. It must be passing through the wire; there it had only one surface to break through: the one at the end where it met the wire of the bottle. No one who has not tried this experiment can imagine how taut the chain must be stretched to avoid having sparks appear between the links; its own weight is not enough. This confirms Newton’s assertion about the pressure needed to bring a piece of convex glass into absolute contact with a plane glass on which it is laid. From all these observations and experiments we conclude that the power which produces resistance to electrification, either plus or minus, “is the elasticity of these small atmospheres of the electrical fluid, which are formed at all their surfaces by the action between the particles of this fluid (both within and without bodies) and the component particles of the bodies: and therefore must be different at the surfaces of different bodies.”
270. Do Bodies in Vacuo resist being electrified? May not the Resistance arise rather from Airs, adhering closely to the Surfaces of Bodies.
271–274. Now we shall try to explain how light bodies [i.e., light in weight], at considerable distance from an electrified body, are drawn to and from that body. The electrical fluid surrounding an excited tube in the air is dilated to a certain distance; beyond that it must be condensed; that is, the fluid is rarer the nearer it is to the excited tube, and grows denser until at the limit of the tube’s influence it returns to its natural state. Any light body within this distance will be forced from the denser to the rarer part of the fluid surrounding the tube, but when in its approach it becomes sufficiently electrified to have an atmosphere around it similar to that of the tube, it will be driven back again. Whenever afterwards it comes near any body more easily electrified than itself and communicating with the ground, its electrical atmosphere will be dissipated and it will immediately return to the tube as before.
275–278. From all the above experiments it appears that the electrical fluid is as universal and powerful an agent at or near the earth’s surface as that fluid Newton calls aether. It is as subtle and elastic, it similarly pervades the pores of all bodies, is dispersed through whatever vacuum we can create, and from the appearance of thunder and lightning seems to be extended to great distances in the air. “We shall make no scruple therefore now to affirm, that these two fluids are one and the same fluid.’ It is much more philosophical to do so than to suppose two such fluids equally present everywhere and equally capable of producing the same effects. The word electrical is much too confining a meaning for so universal an activity as this is found to be. “Electricity means no more than the power we give bodies by rubbing them, to attract and repell light bodies” near them, as amber does when rubbed. This fluid not only does so but it heats them by putting their component particles and the particles of light and air within them into vibration, “and makes them throw out the rays of light that before lay hid,” and part with their sulfurous and volatile particles which, with the rays of light, on mixing with the air, “burst out into sparks of real culinary fire,” as the chemists call it. Furthermore, in passing through animals it occasions convulsions, tremors, pain, and sometimes death, and it even fuses glass and gold into an enamel.9
278. What is meant by Rays of Light that before lay hid? See, 240, 283, 286, 288, 289.1
279–301. It is as improper to call this fluid “fire” as it is to call air “sound.” When sound is produced the particles of air are put into vibrations which convey the idea of sound through the ear. So when this fluid throws the particles of a body into such agitations in the air that it grows hot, shines, glows, and is consumed away, we say the body is on fire, but this fluid is not fire, nor can fire indeed be called a Principle or Element in the chemist’s sense any more than sound can. The authors conclude with various arguments and illustrations of their theory of the identity of the “electrical fluid” with aether; they quote Newton twice at length in support, and end with a postscript asking the reader’s indulgence because of the difficulty of finding satisfactory terminology for the discussion of their subject, and referring the reader to Wilson’s treatise on electricity for some of the experiments mentioned here.2
Gave these Hints to Mr. Wilson, a Copy of which I had formerly read to him.
4. See above, VII, 25. BF probably saw the listing of the publication in Gent. Mag., XXVI (Aug. 1756), 406.
5. Franklin’s copies of this pamphlet and of the second edition of 1759, mentioned below, bound with other pamphlets on electricity, including marked copies of his own Exper. and Obser., 1751 and 1754 editions (see above, III, 118), are in Yale Univ. Lib. In marking the Hoadly and Wilson pamphlet old habits asserted themselves and he scrupulously corrected the printer’s errors he detected.
6. The editors confess to some diffidence in offering this condensed version of the original; Hoadly and Wilson’s pamphlet lacks some of that simplicity and clarity of style that was a hallmark of BF’s scientific prose. They take comfort, however, from the fact that in 1767, when Joseph Priestley summarized a part of this pamphlet in his History of Electricity (pp. 450–1), he concluded by observing: “On this theory I shall make no particular remarks, because I cannot say that I clearly comprehend it.”
7. Observations on a Series of Electrical Experiments. By Dr. Hoadly, and Mr. Wilson … . The Second Edition. With Alterations and the Addition of some Experiments, Letters, and Explanatory Notes. By B. Wilson. (London, 1759).
8. In the 1756 edition “aether” was stated flatly as existing “according to Sir Isaac Newton.” In the 1759 edition Newton “has supposed” there is such a fluid, and a footnote calls attention to the modification. While BF had not directly challenged the phraseology in this particular, it is possible that in conversation with Wilson he had advised some modification.
9. In response to BF’s question Wilson inserted in the 1759 edition after “minus” in the quoted passage the words “even though the tube be withdrawn.” In BF’s copy of this edition he underlined the added words and wrote “Quy” in the margin.
1. BF inserted his parallel-line check marks opposite both pars. 67 and 68, but numbered his question “67” only.
2. As will be seen below, this concept of an “atmosphere” surrounding an electrified body is central to the authors’ whole theory. Yet Wilson ignored this question in the second edition.
3. In his copy BF underlined the words here directly quoted.
4. In the 1759 edition Wilson changed the text to read “When a bar is electrified plus, the atmosphere found round it is extended in the air to a considerable distance,” instead of as formerly “lies between the air … and the bar.” He added as a footnote: “The electric atmosphere is not to be understood as lying between the air and the bar, as it is exprest in the former edition, but extended in the air all the way from the bar.”
5. In the 1759 edition Wilson changed the opening lines of the text to read: “When a bar is electrified minus, the atmosphere formed round it, which during the action of the tube that electrified it, is extended in the same manner in the air to a considerable distance from the bar.” He also added as a footnote: “This alteration was in consequence of the former.”
6. There is no parallel-line check mark in BF’s copy here, but the word “gradually” is underlined in pencil (BF’s markings are regularly in ink). In response to BF’s “Query,” Wilson added a long note in the 1759 edition, saying in part: “But this seems to follow from the principles we have already deduced, that a quantity, equal to what was in the ball electrified plus, is at the end of the experiment diffused in the balls electrified minus; and the remaining quantity which form’d the atmospheres is diffused in the air, or the bodies from whence it at first flow’d, in order to crowd into the body electrified minus.”
7. For the pertinence of the paragraphs BF cites at the end of this comment see below where these paragraphs are summarized.
8. Machines for creating an electric charge less laboriously than by rubbing a glass tube had come into common use both in Europe and America. For BF’s description of an early electrical machine devised in Philadelphia, see above, III, 134.
9. A footnote here in the 1759 edition reads: “See Dr. Franklin’s letters upon this subject.” Wilson doubtless referred especially to Exper. and Obser., pp. 2, 23–4, 72, reprinted above, III, 157, 355–6; IV, 27.
1. A footnote here in the 1759 edition explains that Wilson conceived the condensation of the electrical fluid as being “made at or exceedingly near the surface” of the glass, but whether inside or outside he could not say without further experimentation. This view, he pointed out, differs considerably from BF’s, and he cited Exper. and Obser., 1751 edit., pp. 72–5 (above, IV, 27–30), to show that BF conceived “a kind of partition” of denser glass running through the middle of its thickness, through which the particles of electrical fluid in one half could not pass to reach the other half.
2. At this point in par. 189 BF placed his parallel-line mark, and he added the numbers “82, 83, 84,” to refer back to those paragraphs.
3. This comment led to the first of Wilson’s major revisions in the 1759 edition. He inserted the following footnote: “The reader will please to observe, that in the other edition, the reasoning upon the electric atmospheres, is not altogether agreeable to experiment. I say, not altogether, because though there are really such electric atmospheres extended over each surface of the glass, yet they are neither so powerful nor so extensive, as they are there supposed to be. I have therefore chose to vary and illustrate that part, which relates to the experiment, by a farther analogy between the exhausted tube and the pane of glass, as it appears to be the clearest and most satisfactory method. And I do think if my worthy friend [Hoadly] had now been living, he would readily have concurred in this alteration.” He then entirely omitted pars. 189–191 of the first edition and substituted two full pages of text to show parallels between what happens when the relative positions of the excited and exhausted tubes are variously changed and what happens when some or all of the positive charge on the upper surface of the pane of glass is drawn off.
4. It was for such a device as this that BF and his Philadelphia friends had invented the name “electrical battery” in or before 1749 (see above, III, 357), almost certainly by analogy to the artillery battery (a number of cannon at one emplacement under a single command) which he had been instrumental in erecting for the defense of Philadelphia at the time of the Association of 1747.
5. For BF’s earlier discussion of this matter, see above, IV, 32–4.
6. In the 1759 edition Wilson made a small concession to BF by omitting the words “rays of” in the expression “and of the rays of light within them,” near the end of the paragraph. The references are to other paragraphs in which Hoadly and Wilson had used the same or a similar expression, but Wilson made no corresponding changes in them.
7. In the 1759 edition Wilson beat a full retreat on this experiment and made his second major change in the text. He omitted entirely pars. 249–256 dealing with sulfur and added the following note at the end of par. 248: “In the other edition, there is an experiment or two with a sulphur globe, which was taken from the works of Dr. Franklyn: But as the experiments were never repeated either by Doctor Hoadly or myself, and the reasoning upon them was liable to objections, and not necessary in the present case, I have chose to leave that part out in this 2d edition.” The reference to BF’s works in this note is to a letter from Ebenezer Kinnersley to BF, Feb. 3, 1752, proposing certain experiments with a sulfur globe, and to BF’s report back, March 16, 1752, confirming Kinnersley’s anticipated result with one of the experiments; Supplemental Experiments and Observations on Electricity, Part II, pp. 99–106; above, IV, 263–5, 275–6.
8. BF made no comment here, but in the 1759 edition Wilson added the following note: “Doctor Watson and Doctor Franklyn are therefore mistaken in regard to the Leyden experiment, where they advance that the electric fluid always makes the shortest circuit.” Actually, as early as September 1753 BF had suggested that lightning goes “considerably out of a direct course” on its way to the ground, “taking in all the best conductors it can find.” This letter was published in 1754 in New Experiments and Observations on Electricity, Part III, p. 124 (above, V, 76–7), and so was available to Wilson. Writing to John Lining, March 18, 1755, BF had given Kinnersley credit for proving experimentally that the charge from a Leyden jar followed the better conductor, not the shortest course (above, V, 523), but this letter was not published until the 1769 edition of Exper. and Obser., p. 322.
9. In the 1759 edition Watson changed the last part of par. 278. He dropped the statement that the gold and glass became incorporated and formed “an enamel,” and substituted the statement that they became “so closely united, that Aqua Regia [a mixture of nitric and hydrochloric acids used to dissolve gold] itself, has no effect upon the metal.” He added in a footnote that he had made the original statement on the strength of BF’s reported experiment (Exper. and Obser., 1751 edit., pp. 64–6; above, IV, 21–3), but upon later repeating it himself he concluded that the gold was not actually vitrified but brought into such close contact with the surface of the glass and by such a great force as to support his point about the strength of the electrical fluid at the surface of all bodies.
1. These references are to other paragraphs mentioning the rays of light within bodies.
2. In 1752 Collinson had sent BF a copy of Benjamin Wilson, A Treatise on Electricity (2d. edit., London, 1752). BF later sent it to Cadwallader Colden and Colden had returned it with critical comments on Newton’s and Wilson’s concepts of “Aether.” See above, IV, 333, 355, 374–5.