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Popular Science Monthly/Volume 83/November 1913/Discovery of Contact Electrification



THE discovery that the mere contact of two dissimilar metals causes them to become oppositely electrified seems to be everywhere attributed to Volta, though Nicholson in the first volume of his "Journal" published in 1802, calls attention to the fact that both Bennett and Cavallo, in England, had made experiments upon contact electrification previous to its supposed discovery by Volta. The fundamental experiment from which Volta made this discovery is said by Auerbach in Winkelmann's "Handbuch der Physik" to have been announced by Volta in 1795, in Gren's Neues Journal der Physik, Vol. II., p. 144. The experiments which Volta, himself, seems to have regarded as fundamental in his theory of contact electrification were published in a postscript to a letter to Gren in Volume IV. of the Neues Journal. These experiments were not only the same in character, but were performed in the same manner and by means of the same apparatus as experiments which had been performed about ten years earlier by Bennett, and which had been published in a book to which Volta was a subscriber.

The actual discoverer of contact electrification seems to have been the Rev. Abraham Bennett, curate of Wirksworth, Derbyshire, who is known in the history of electricity as the inventor of the gold leaf electroscope, which still bears his name, and of a multiplier for increasing by induction the intensity of a given charge so as to render it measurable by an electroscope.

In 1789 Bennett published a small book entitled "New Experiments on Electricity," in which he gives an account of many of his discoveries and describes the construction of his electroscope and doubler, as well as the mechanical improvements made in the latter by Dr. Erasmus Darwin and William Nicholson. This book was published by subscription and contains a list of 394 subscribers, including many of the best known scientific men of the day, and among the rest, "Mr. Volta, Professor of Nat. and Exp. Philosophy." Volta had then been for ten years a professor in the University of Pavia, and had corresponded for some years with English physicists, notably Priestley and Cavendish, and only two years later was made a foreign member of the Royal Society.

Section VII. of Bennett's book is devoted to "Experiments on the Adhesive Electricity of Metals and Other Conducting Substances." In performing these experiments Bennett made use of Nicholson's improvement upon his own "Doubler," an invention which he had described in Vol. 77 of the Philosophical Transactions. Previous to this Volta had increased the sensitiveness of an electroscope by mounting condensing plates upon it, and Cavallo had still further increased its sensitiveness by using a double condenser. Bennett's first doubler consisted of three brass plates, one of which was mounted upon the standard which supported the gold leaves of his electroscope. The others were provided with insulating handles and were varnished on one side. When the electroscope had been given a small charge, one plate was laid with its varnished side upon the electroscope plate and touched with the finger. Tt thus received by induction a charge opposite to the charge of the electroscope. It was then raised from the electroscope by its insulating handle, and the other plate was laid with its varnished side upon it and touched with the finger. It accordingly received by induction a charge like the charge of the electroscope. It was then touched by its edge to the electroscope plate with which it divided its charge. Both unmounted plates were then discharged, and the process was repeated. By sufficient re]3etition the charge of the electroscope could be built up to any desired intensity.

Bennett's doubler was improved by Dr. Erasmus Darwin by mounting the plates upon horizontal arms which could be swung into and out of position readily, and later by mounting the plates vertically and moving them back and forth by a rack work in a direction always parallel to each other. In this form it was used by Darwin in the study of atmospheric electricity.

Bennett had noticed that his plates nearly always had a residual charge of electricity which made it possible to build up a charge on his electroscope without giving it a preliminary charge. To get rid of this he improved Darwin's form of the doubler by leaving the plates unvarnished and depending upon the air for insulation. He found that this made it possible for him to thoroughly discharge the apparatus, so that no charge could be built up until a preliminary charge had been given to the electroscope.

Soon after this Nicholson built a doubler in which two of the plates were fixed and the movable plate was mounted on an arm and turned about an axis by a crank, by which the proper contacts wore also automatically made. This doubler Nicholson presented to Bennett, and it was used by him in his investigation of the "adhesive" effects of electricity. Bennett's original figure of this doubler is here reproduced. In this figure is the fixed plate which remains permanently insulated and upon which it was desired to build up the charge. is the movable plate which is carried on an arm from an axis which may be turned by a crank, and is a fixed plate which may be either insulated, joined to or joined to earth. is a ball which serves to counterpoise the crank and the plate so that may be stopped in any position. When the crank is turned passes alternately in front of and parallel to and . The contacts are so arranged that when is parallel to it is joined to earth and so charged oppositely to . It is then revolved in front of , at which instant is joined to earth and is charged by induction oppositely

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to and like . As again comes in front of and are joined and is earthed. now receives by induction a greater charge than before, since has now received the greater part of 's charge. By successive revolutions of the charge may thus be built up to any desired intensity upon .

Bennett performed many experiments with this doubler, and after learning how to discharge it completely he tested the electrification induced upon metal plates by being placed in contact with various substances, both solid and liquid. He concludes that different substances "have a greater or less affinity with the electrical fluid," and he then undertakes his experiments on this adhesive affinity in the case of different metals.

At the beginning of Section VII., page 91, he says:

Having fully proved by a frequent repetition of experiments, that the positive or negative spontaneous charge of the doubler depended upon the absorption or repulsion of the electrical fluid by the approximation of its parallel plates, and that by applying larger plates covered with minium or flour its electricity might be changed at pleasure, it easily occurred, that if the spontaneous electricity in the beginning of the process was sufficiently weak, the mere contact of metals or other substances having a different adhesive affinity with the electrical fluid might also change it, and a new and interesting employment for the doubler be discovered.

This supposed effect of contact was confirmed by the following experiments, in which the doubler and electrometer were deprived of electricity, and used with the precautions and improvements mentioned in the last section.

Experiment I

The spontaneous charge of the doubler having been negative, and being deprived of this charge by the usual method, the plate was placed parallel to the plate , but so that was not connected with the earth. The plate was then touched with the blade of a knife, and the plate B at the same time touched with the point of a soften'd iron wire. With sixteen revolutions the gold leaf diverged about one third of an inch positively.

Experiment II

The doubler being deprived of electricity as before, and the plate placed as in the last experiment, the knife was applied to instead of , and the soft iron wire to instead of , which opened the gold leaf negatively at 15 revolutions.

These experiments were repeated very often, and the electricity changed each time, being always positive in the plate touched by the knife.

To distinguish so minute a difference of adhesive electricity, as that which might be supposed between two metals so nearly alike as hardened steel and soft iron, wou'd appear incredible had not the frequent repetition of experiments confirmed it.

Being now well convinced of this fact I tried many other substances with various success, sometimes the charge wou'd change regularly for a long time together, by applying the opposed substances to A and B alternately, as in the above experiments; and sometimes with other substances the charge wou'd be quite uncertain.

Bennett gives his experiments with six pairs of substances, each pair being tried about ten times. The charges given by contact to the plate A of the doubler were as follows: steel, iron wire ; lead ore, lead ; lead, iron wire ; lead ore , ironwire ; tinfoil , iron wire ; zinc , iron wire .

He then tried charging the plate of his doubler by a single substance while was earthed. He found to take a positive charge from lead ore, gold, silver, copper, brass, regulus of antimony, bismuth, tutenag, mercury, various kinds of wood and stone. Zinc and tin gave negative charges to his plate.

Here is apparently the beginning of that arrangement of substances which has since come to be known as Volta's Contact Series. It is well to bear in mind that these experiments were published in 1789, two years before Galvani made his celebrated observation on the twitching of frogs' legs which finally led up to the controversy through which Volta discovered the electric current.

The next experimenter to investigate the subject of contact electrification was apparently Tiberius Cavallo. Cavallo was an Italian by birth, but was a resident of London and a prominent member of the Royal Society. Cavallo published "A Complete Treatise on Electricity," which went through a number of editions. In the fourth edition, published in 1795, he adds a new volume containing the important discoveries in the subject since the publication of the third edition. Among these he gives first place to the investigations of Galvani and Volta on animal electricity, and mentions the fact that Volta suspected the phenomena might be caused by the contact of two dissimilar metals. He refers to Bennett's experiments, but says that others who have repeated them have obtained inconstant results. Finally he hit upon a different method of experimentation which enabled him to detect with certainty the electrification due to metallic contact. In his section devoted to experiments on metallic substances he says:

After many fruitless attempts, and after having sent to the press the preceding part of this volume, I at last hit upon a method of producing electricity by the action of metallic substances upon one another, and apparently without the interference of electric bodies. I say apparently so, because the air seems to be in a great measure concerned in those experiments, and perhaps the whole effect may be produced by that surrounding medium. But though the irregular, contradictory, and unaccountable effects observed in these experiments do not as yet furnish any satisfactory theory, and though much is to be attributed to the circumambient air, yet the metallic substances themselves seem to be endowed with properties peculiar to each of them, and it is principally in consequence of those properties that the produced electricity is sometimes positive, at other times negative, and various in its intensity.

The discovery of those properties of metallic bodies opens a new field of useful investigation, and renders more manifest the general or extensive influence of a fluid wonderful in its nature and action. But how far they will enable us to explain the phenomena of animal electricity, and of other operations of nature, are considerations which will be noticed after the recital of the experiments.—In this account I shall endeavor to select and methodise the experiments, in the best manner that the irregularity of their results seems to admit of.

Exp. I. A piece of zinc, which weighed little more than half an ounce, was dropped ten times successively upon an insulated tin plate. This plate was then brought in contact with the plate of the multiplier: the lever was worked, and after ten additions of electricity, the plate [1] communicated to the electrometer a sufficiently sensible quantity of positive electricity, which shows that the tin plate had been electrified negatively by the contact of the zinc. This experiment was repeated four times within the space of half an hour, and was constantly attended with the like effect; but on the following day the effect was found to be less conspicuous, for three times twenty additions just enabled the plate to communicate a sensible degree of positive electricity to the electrometer. In short, the different states of the atmosphere seem to be much concerned in the result of this experiment, and yet the whole effect can not be attributed to it; but of this further on. Before, however, I proceed to the narration of other experiments, it will be necessary to dwell a little longer on the above-mentioned operation, not only to render it more intelligible, but likewise to avoid repetitions. The tin plate used in the preceding as well as in many of the subsequent experiments, measures eight inches in diameter; and is fastened to a small piece of wood about three inches in length. Two glass sticks covered with sealing-wax are cemented into this piece of wood, and their other extremities are cemented into a larger piece of wood, which forms the stand or basis of the instrument. The operation is as follows: I hold this apparatus by the last mentioned piece of wood in my left hand, and keeping the plate in an horizontal situation, let the piece of zinc or other metallic body, fall upon it from my right hand, which I hold a few inches above the plate; then by inclining or shaking the plate, the piece of metal is caused to fall upon the table or upon a chair; from whence I take it up, and again let it fall upon the tin plate, and so on.

Cavallo repeated this experiment with a considerable number of metals, and with great precautions to guard against any other source of electrification. He found that repeated touching with an insulated body gave no greater effect than a single contact. He tried lifting his pieces of metal with iron tongs, or in a metal spoon, and found that in some cases this changed the sign of the electrification. He then performed a long series of experiments on the effect of heating the metals, and found a change in their electric properties due to temperature. In the case of bismuth, he was able to change the sign of the electrification produced on the tin plate by heating the bismuth very hot. His experiments upon the temperature change in contact electrification were almost the only ones made for a hundred years, and were probably the most important ones that have yet been published. At the end of his experiments he stated the following conclusions:

1. The contact of one metallic substance with another generally produces electricity.

2. The quantity and quality of the electricity so produced, is various according to many circumstances which seem to concur in the production of it, or in great measure to influence it.

3. Those circumstances are, the various nature of the metallic substances, their various degrees of heat, the state of the atmosphere, and the other body concerned in the experiment, viz. the hand of the operator, etc. Each of those causes has a share in the result of the experiment; for the variations of any one of them, when everything else remains unaltered, produce different effects. Thus in different states of the atmosphere, the very same metallic substances treated in exactly the same manner, produce a greater or less quantity of electricity. Thus also, by only heating or cooling the metals, the electricity may be varied in quantity and even in quality.

I am inclined to suspect, that different bodies have different capacities for holding the electric fluid, as they have for holding the elementary heat; if however the experiments relative to this subject be carefully tried, under all the variety of circumstances which the combination of the above-mentioned causes is capable of producing, I do not doubt but that all the phenomena observed in the preceding pages may hereafter be reconciled to one, or to a few, simple laws, which will at the same time assist the farther investigation of the science of electricity.

These experiments of Cavallo's could not have been made later than the year of publication, viz., 1795. While Cavallo says in his discussion of animal electricity that Volta suspects that the phenomena of muscle contraction which Galvani and he were studying might be caused by the contact of the two dissimilar metals which were used in making the connection between the muscle and the nerve in many of their experiments, yet Volta seems not to have actually experimented with contact electrification until after the publication of Cavallo's treatise, and then to have begun with the repetition of Bennett's experiments made ten years before.

There seem to be many diverse statements as to how Volta arrived at his theory of contact electricity, but his own story of it is given in a so-called letter to Dr. Gren, which was published in volumes III. and IV. of Gren's Neues Journal der Physik, in the years 1796-98. These journals are not accessible to the present writer, so their exact date can not be given, though Vol. IV. was concluded in 1798. Volta's letters to Gren are translated in the Philosophical Magazine of 1799, from which the extracts here given are quoted.

In the first part of Volta's letter, which was published in Vol. III. of Gren's Journal, Volta says:

The contact of different conductors, particularly the metallic, including pyrites and other minerals as well as charcoal, which I call dry conductors, or of the first class, with moist conductors, or conductors of the second class, agitates or disturbs the electric fluid, or gives it a certain impulse. Do not ask in what manner: it is enough that it is a principle, and a great principle. This impulse, whether produced by attraction or any other force, is different or unlike, both in regard to the different metals and to the different moist conductors, so that the direction, or at least the power, with which the electric fluid is impelled or excited, is different when the conductor is applied to the conductor , and to another . In a perfect circle of conductors, where either one of the second class is placed between two different from each other of the first class, or, contrariwise, one of the first class is placed between two of the second class different from each other, an electric stream is occasioned by the predominating force either to the right or to the left—a circulation of this fluid, which ceases only when the circle is broken, and which is renewed when the circle is again rendered complete.

Farther along he says:

We might consider this mutual contact of two different metals as the immediate cause which puts the electric fluid in motion, instead of ascribing that power to the contact of the two metals with the moist conductors. . . . In both suppositions the result, as may be easily seen, is the same. But though I have reasons for adopting the first as true rather than the second, yet the latter represents the proposition with more simplicity, and it may be convenient to adhere to it in the explanation, as it affords a readier view of it.

In a postscript to this letter published the next year (1797 or 98) Volta says:

Some new facts, lately discovered, seem to shew that the immediate cause which excites the electric fluid, and puts it in motion, whether it be an attraction or a repulsive power, is to be ascribed much rather to the mutual contact of two different metals, than to their contact with moist conductors. But though it can not be denied that in the latter case there exists an action, it is proved that it exerts itself in a far more considerable degree when two metals mutually touch each other. There arises by the mutual contact, for example, of silver and tin, an action or power by which the former communicates the electric fluid, and the latter receives it; or the silver suffers it to escape and the tin attracts it. This produces, when the circle is rendered complete by moist conductors, a stream, or continual circulation of the fluid. When the circle is complete, there is an accumulation in the tin at the expense of the silver; which indeed is very small, and far under the point necessary to enable it to announce itself by the most delicate electrometer. I have however been able, by the assistance of my condenser, constructed on a new plan, and still better by Nicholson's doubler, to render it very perceptible: I shall here communicate the result obtained by my experiments, which I made some time ago with great satisfaction.

Exper. I. The three plates of the doubler are of brass. I took two strong wires, one of silver and the other of tin, and brought the former into contact with the movable plate, and the other with one of the fixed plates; while they both rested on the table, or, what is better, on moist pasteboard, or any other moist conductor, so as to be in communication by the intervention of one or more conductors of the second class. I suffered the apparatus to remain some hours in this state, then removed the two wires and put the machine in motion. After 20, 30 or 40 revolutions (or more when the atmosphere was not dry, or the insulation imperfect) I brought one of my straw electrometers into contact with the movable plate, and observed indications of positive electricity (+ E) which arose to 4, 6, 10 degrees, and more. If I suffered it to touch the fixed plates, I had the corresponding indications of the opposite kind of electricity (—E).

The silver, therefore, poured the elastic fluid into the brass plate when it had been some time in contact with it; and the tin attracted it from the other plate, which was also of brass, while in contact with it. This was confirmed by the following experiment, which is a real experimentum crucis.

II. I reversed the experiment, so that the silver was in contact with one of the fixed plates, and the tin with the moveable one. The electricity which I obtained from the latter, after the apparatus had remained a sufficient time in that position, was negative ; while that of the fixed plate was positive .

III. This is the reverse of the former. The piece of tin was applied to one of the fixed plates, and the moveable one was insulated from all metallic contact. The result was now reversed; that is, the fixed plates were electrified negatively, and the moveable one had positive electricity.

Volta then varied the experiments, just as Bennett had done, by applying the tin wire only to the movable plate and testing its charge, and then to the fixed plate, and repeating the process. He then reph;ced the movable brass plate of the doubler by a tin plate, and using brass and tin wires for touching the plates, he found that he got a charge by touching his brass plate with a tin wire and his tin plate with a brass wire, but got no effect when he touched the plates with wires of their own metal. He then says:

We must therefore conclude that the contact of two metals of a different kind with moist conductors, without the mutual contact of these metals themselves (which is wanting in the sixth experiment, where brass is in contact with brass, and tin with tin), produces nothing or almost nothing; and that, on the contrary, the mutual contact of the two metals of a different kind, which takes place in the fifth experiment, produces the whole, or almost the whole, effect.

The above considerations seem to make it certain that though Volta was apparently the first to recognize the existence of a current in a circuit composed of two metals and an electrolytic conductor, he has no claim to be regarded as the first discoverer of contact electrification. This honor should undoubtedly be accorded to the Rev. Abraham Bennett, while the discovery of the variation of the phenomenon with temperature is due to Tiberius Cavallo.

  1. Cavallo used his own form of multiplier in which the plates were not named as in Bennett's figure.