USA > Maine > York County > Parsonsfield > A history of the first century of the town of Parsonsfield, Maine > Part 10
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Franklin's hypothesis led him to devise many experiments, among them the charging of several jars simultaneously by the so-called cascade method. He was, however, greatly surprised when he found that two negatively electrified bodies repel each other, and he could account for the fact only by assuming that matter which has no electricity is self-repellent, and so as the electricity is by degrees withdrawn from the bodies, the repellent action of the matter becomes more and more apparent. To Franklin we owe the very ingenious demonstration that in case of a charged body the electricity is confined entirely to the surface. He brought a cork, suspended by a silken thread, in contact with the interior of a charged silver cup. On removal he found no trace of electricity on the cork.
Aepinus, in 1759, attempted to subject Franklin's hypothesis to a mathematical treatment. He had no means of knowing the law of the force between electrified bodies as regards distance, and so he made a general assumption that the force diminished when the distance increased. But though this assumption was want- ing in the precision which alone could lead to the highest results, his labor was not unfruitful. He replaced the glass, which Franklin had employed in the con- denser with air, and thus showed that the action was not dependent on glass as such, as Franklin had assumed. He also showed that there is no sharp line of demarkation between conductors and non-conductors; that all bodies conduct in some degree and that all offer some resistance to the passage of electricity through them.
Meantime, Symmer, in England, had revived the almost forgotten hypothesis of Du Fay. He was led to do so by observing that when he withdrew his silken stockings, of which he wore two pairs, of different colors, at the same time, that those of the same color repelled one another while those of unlike colors attracted one another. He much relied on an experiment of Franklin's devising, which is now well known and often shown. It consists in sending a powerful spark from a Leyden jar through a piece of card-board or through several sheets of paper held together, when it is found that instead of a smooth perforation the
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outside sheets are blurred outwards. Symmer and those who held to his view of the nature of the discharge, looked upon this as indicating the passage of two electricities in opposite directions at the same time. Symmer thought that the puzzling observations of Guericke concerning what we now call inductive action became clearer in the light of his explanation. Though at first, Symmer's theory attracted little attention, it was shown by Wilke that it offered a clear explana- tion of the discharge of the Leyden jar and other condensers. There were, how- ever, acknowledged difficulties in the way of accepting either Franklin's or Symmer's views, and these difficulties have not even yet wholly disappeared.
Guericke and Gray had each observed that if an insulated conductor be placed near a charged body, though not in contact with it, the former becomes electri- fied. Winkler had endeavored to account for this in accordance with the pre- vailing doctrine of the time by the assumption that there is an electrical exhala- tion, or atmosphere which goes out from the rubbed body, to which the effect may be referred. John Canton was very active in experimentation and contrib- uted much to the overthrow of this doctrine. He showed that the kind of elec- trification produced in a given case depends on the character of the surfaces in contact and not at all on the nature of the underlying bodies, and, moreover, that when an insulated conductor is made to approach a charged body, the portion nearest the charged body exhibits electricity of the opposite kind. Wilke showed, in addition, that if the insulated conductor be now removed from the presence of the charged body, all signs of electrification disappear; and Aepinus showed that the same is true for non-conductors. He ascribed the effect to the action of the electricity of the charged body within a certain space about the latter, which he called the "sphere of electrical activity." Whether this action takes place through the intervention of the air, of course he did not know. Beccaria, in 1767, at Turin, investigated the same problem, and the result was that he ascribed the peculiar properties of charged insulators to a power of restoration (electrici- tas vindex) of the condition which had been disturbed. This explanation, if such it can be called, is one which belongs to the methods of philosophy rather than to science. He showed that two equally and oppositely charged bodies, separated only by a thin plate of insulating substance, can exert no outside influence, but that if they be separated from each other, they both manifest a charge. Volta noticed that the two charged bodies do not lose their electricity, but that during their near approach it ceases to act outward. As he employed, for the experi- ment, a plate of resin to separate the two movable conductors he remarked that the electrical action continued to act for a long time, and he hence called the arrangement a "perpetual electrophorous." The apparatus still remains a most important and useful one in our laboratories. Volta found that if the electrified plate of his electrophorous be a partial conductor, the electrophorous may, by its use, have its initial charge greatly strengthened.
In 1778, Lichtenberg invented a double electrophorous, so contrived as to pro-
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duce both states of electrification at the same time, and to make the electricity so produced available, and both he and Volta observed that by an obvious use of the apparatus, the charges would continue to increase so long as the work of applying and removing the movable conductors is kept up.
In 1787, Bennett, in England, made use of varnished metal plates in the con- struction of an " electric duplicator," which was simply a means of conveniently applying the principle of Volta's electrophorous. Cavallo, Bennett's country- man, employed metal plates unvarnished, but never brought quite in contact, making use of the thin plate of air intervening, when they approached for induc- tive action. Nicholson, in 1788, embodied the same idea in a revolving apparatus which carried a metal plate constantly, and in succession past two fixed plates. There were metallic wire connections so added that the electricity displaced by inductive action could be made available in another part of the apparatus. This device is able to build up the slightest difference of electrical states between the fixed plates, which may chance to exist, so as to produce very considerable effects. Essentially the same principles were embodied in an apparatus in which a fixed and revolving glass wheel were employed, the opposed surfaces of which were partly covered with tin foil. The foil was made, at suitable points, to touch fine metallic brushes so connected as to serve the same general purpose as the con- nections in his earlier apparatus.
No further notable advance was made in apparatus of this sort for a consider- able period, the reason doubtless being that the attention of electricians was irresistibly drawn to another class of phenomena in whose investigation Volta again holds a conspicuous place. However, in 1831, Belli, an Italian, produced an apparatus having much in common with that of Nicholson. It is, in fact, a self-charging and continuous by acting electrophorous, and it may properly be regarded as the forerunner of the "influence machines " which, in one form or another, have almost entirely supplanted the friction machines of earlier times. Our limits do not permit a description of this apparatus. There is no new ad- vance to record till 1865, when, as so frequently happens, remarkable progress was simultaneously made by two persons, Toepler and Holtz, independently of each other. The principle of induction is employed by both, and there is great similarity in the apparatus which they invented, though there are very important differences in details and operation. It will be sufficient for our purpose to say that while in the apparatus of Toepler the electric potential (disposition to dis- charge) of the conductors is brought about by the action of a fixed inductor on the metallic surfaces carried past and near to it, the Holtz machine consists essentially of two combined electrophori with a common rotating plate which represents the movable cover of the Volta device. A detailed description of either of these machines would be out of place. The invention of these means of producing electricity gave rise to a great number of researches which have resulted in advancing the theory of electrical action. Very large machines have
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been constructed on the plan of Holtz, some of them by the use of glass plates of extraordinary size, and some by the combination of many plates so as to con- tribute to one common effect. Hitherto, they have proved rather useful for scien- tific than for practical purposes.
In the early days of electrical experimentation it was not possible to detect the presence of very small amounts of electricity, for no very delicate apparatus existed for that purpose. The invention of the condenser, however, rendered the construction of such an apparatus possible, and Volta had the acuteness to take advantage of the fact. Nollet appears to have been the first to attempt to estimate the strength of the charges with which he dealt. For this purpose he made use of the fact which Du Fay had observed, that two threads hanging near each other and connected with the charged body are repelled so as to stand apart from each other, and that they diverge more and more as the charge is increased. Waitz attached small metallic balls to the threads which gave a means of com- paring the repulsive force with that of gravity, and so measuring the charge. The next important device was to place the charged body, or a metallic plate which was in contact with it, under the pan of a balance so that the attractive force might be counter-balanced by weights placed in the other pan. This was done by Gralath and others. Le Roy and d'Arcy contrived an apparatus which could stand partly immersed in water, after the manner of an hydrometer, and when put in communication with the charged body, it could exhibit the repulsive action by displacement upwards. The amount of weight which must be added to restore the former equilibrium measured the charge. Canton employed the number of sparks which one may obtain in the discharge of a Leyden jar, and also the repulsion which two suspended pith or cork balls can show. The first plan had but little success till it was carried out by Lane, in 1767, in an apparatus which still bears his name. To the outer coating of a Leyden jar is fastened a conductor carrying a screw, on whose extremity is a knob. This latter can be brought to any required distance from the knob connected with the inner coating, and thus the length of the spark can be regulated.
Henley, in 1772, replaced one of the suspended balls of the former repulsion aparatus with a metal strip from which the other ball was suspended by a light rod. Cavallo, in 1780, employed thin wires instead of threads to suspend the balls and placed them in a glass case with strips of tin foil opposite the balls, to increase the divergence by induction. Volta, in the course of the next year, used light straws instead of threads, and, finally, Bennett, in 1787, substituted gold leaf.
It is clear that none of the apparatus here briefly mentioned, can serve for measurements such as exact science demands, and so it was clearly impossible that theoretical results of much value could be obtained by its use. It is, how- ever, true that some of the conclusions reached by Aepinus and by Cavendish are in close agreement with the facts as they have since come to be established.
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Aepinus, as remarked before, did not know the law of the diminution in electri- cal action which depends on distance. Cavendish, however found evidence that it is as some negative power of the distance lying between 1 and 3. He assum- ed it to be the second. Mayer, in 1769, showed that this law holds for magnetic . poles. Priestly, in 1766, concluded that the law of inverse squares is valid, from Franklin's observation that the charge is wholly confined to the surface of the charged body. But Coulomb first showed beyond all doubt, both experi- mentally and theoretically, that this law is rigorously true. The course of his investigations led him to the construction of his so-called " torsion balance," an apparatus of the greatest value in the estimation of very small forces. The plan is to employ the force of torsion of a very thin wire or fibre, say of unspun silk, to antagonize the repulsive action of the charged bodies so as to bring them into the same relative position which they would have before being charged. By a long series of most exact experiments Coulomb established the two facts (a) that the force exerted between two charged bodies is directly as the quantities of their electricities, and (b) inversely as the square of their distance from each other. So far as theory is concerned he showed that the hypothesis of Franklin, or that of Symmer, will equally well account for the facts. Coulomb to some extent also discussed the distribution of electricity on the surfaces of bodies, and made many exact measurements on this subject. He also explained the peculiar action of points in effecting the discharge of charged bodies, as a necessary result of the distribution. In short he laid the foundation for the theory of static electricity which no subsequent researches have disturbed.
All the accumulated observations and discoveries up to this time were neces- sary to prepare the way for the rigorous treatment of the mathematicians. It was found that much of the mathematical work of Laplace, which had reference primarily to another field of inquiry, was applicable. Biot, in 1801, made a be- ginning when he solved the problem of distribution on the surface of a special geometrical surface, but Poisson, in 1811, commenced the real work of establish- ing a mathematical theory of electricity. He assumed as a basis that there are two incompressible fluids whose particles are freely movable, and that they obey the laws which Coulomb had established by experiment. He was able to calcu- late what would be the distribution of a charge given to two spheres in contact and the calculations agreed with the measurements executed by Coulomb.
In 1828, Green, in England, devised a new treatment of certain difficult elec- trical problems which introduced very great simplicity into the calculations. His work, however, attracted no attention till long afterwards, and it happened that Gauss, in 1837, rediscovered his general plan which has been since developed by many mathematicians. Through their labors we have come to attach a physical signification to what was at first a mere mathematical form of expression. A single illustration will suffice. The potential of one electrical mass upon another at any given distance is equal to the work which must be done to bring the masses
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from mutually infinite distances to their given positions, in opposition to their repulsions.
As has been said above, the various forms of electroscopes are unsuited to the exact measurements required in science. The only real measuring instrument thus far mentioned is the torsion balance of Coulomb, 1785. The successful use of this instrument, however, requires the highest skill and care on the part of the experimenter, and so in the course of time attempts were made to replace it with some more convenient device. Among the many proposed may be mentioned that devised and used by Sir Snow Harris in 1834, in which he directly weighs the pull exerted on a charged plate held near a similar plate, parallel with it. Here was made a commencement, which, at last, in 1855, by the genius of Thom- son, was developed into an " absolute electrometer " with scarcely anything to be desired in addition. Thomson has also invented other forms of electrometers by means of which inconceivable small amounts of electricity can be compared and measured. Indeed it is hardly too much to say that more is due to him than to any other man, both for his experimental devices and for his contributions to the theory of electricity. We must pass without special mention a great number and variety of instruments which serve the same general purpose as those of Thomson.
Some of the earlier experimenters, as we have seen, suggested the identity of the electric spark and the lightning flash. Franklin, whose attention was arrested by the peculiar action of metallic points in discharging the Leyden jar, or other charged body, conceived the idea, in 1749, of protecting buildings from the de- structive effects of lightning, by means of metallic conductors terminating in points. His plan at once attracted attention and rods were erected near Paris by Dalibard and Delor who succeeded in showing the identity of the spark with lightning. Meantime Franklin had been awaiting the erection of a church spire in Philadelphia, on which he hoped to raise his conductor. Impatient at the delay in completing the erection, he, together with his son, sent up a kite, armed with points and held by a hempen cord, and had the satisfaction of drawing a spark from a key which was tied to the cord. Everywhere the greatest interest was aroused and Franklin's bold experiment was repeated, notwithstanding its danger and the consequent warnings respecting it. At last Professor Reichmann was killed by a powerful flash from his apparatus, at St. Petersburg. Lightning rods were now speedily erected in various parts of the world, but most numer- ously in Philadelphia. As Franklin recommended they were generally pointed, but about 1766, Wilson in England contended that they should not be pointed, but should end in a rounded knob, and be led through the interior of the build- ing to some point below the highest part. Thus if the building were struck the rod would be able to convey away the charge without setting the building on fire. Beccaria supported the view of Franklin, contending that the points could attract no more electricity than they could safely carry away. Finally, the sensible view of Franklin, that the office of the rod is to discharge the cloud before the poten-
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tial rises high enough to discharge disruptively, prevailed. The erection of ex- perimental rods gave the means of studying the electrical conditions of the atmosphere. The most obvious accessories were some forms of electroscopes. Lichtenberg, however, devised a means of making the atmospheric changes self registering. It consisted in arranging a plate, such as was in use for the elec- trophorous, to be constantly moved by clock-work, while a small metallic ball, in conducting communication with the rod rested on it. When the plate was strown with light powder it would adhere at the points where the contact of the ball had occurred.
In 1785, Saussure established regular observations by means of which he showed that there is a regular period in the variation of the electrical condition of the atmosphere in fair weather. Schubler showed that there is a regular yearly period as well. With the exceedingly delicate instruments devised by Thomson and with the help of photography continuous registrations of the electrical con- dition of the atmosphere are now kept in many places, and the results are sub- jected to comparative study and discussion.
It must not be supposed that we have thus far mentioned all the means which may be employed to produce static electricity, many of which were in some measure known at the close of the last century. The so-called cramp fish had been known for many centuries, and the similarity of its effects to those of the Leyden jar suggested that the source of its power was electrical. Walsh, in 1772, showed that the suspicion was correct, and he succeeded in producing a spark from the electrical eel. Gray, in 1731, observed that many fused bodies become charged on solidifying. Aepinus showed that many bodies are charged by gently laying them on oiled silk and removing them. In 1781, Lavoisier and Laplace found that vapor of water evaporated from iron is charged. This led Saussure and Volta to suppose that atmospheric electricity is produced by evaporation from the earth. In 1840, a workman near Newcastle, noticed that when he made con- tact with a steam boiler and with the steam escaping from its safety valve, a spark was produced. This led Armstrong to construct a very powerful machine on this plan. Faraday, in 1843, showed that the effect is due to the friction of the minute particles of water carried along with the steam.
Aepinus and Wilke, in 1756, investigated the electrical phenomena exhibited by a crystal of tourmalin when it is heated. There are other crystals which show similar effects but no consistent explanation of all the facts has been reached.
As a matter of convenience, we have, thus far, treated only of the progress of discovery in electrical science and of the apparatus necessarily incident thereto. But it seldom happens that progress in science continues for any considerable time uninfluenced by the application of its results in the arts. And, conversely, such applications almost invariably react to stimulate scientific inquiry, for in such applications new conditions are constantly coming into view.
In very early times, even before any notion whatever of the laws governing
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electrical phenomena had been gained, the negro women of Africa had been ac- customed to bathe their sick children in water containing the gymnotus. Scribonius Largus, in the time of Tiberius, was accustomed to humbug his patients with the same prescription. But Kratzenstein, in 1744, appears to have been the first to apply electricity in surgery, artificially excited, for the cure of a disabled finger. Wonderful accounts were given of the new treatment and it soon became a sover- eign remedy for all sorts of diseases and afflictions, and the most reckless and intemperate use was made of it. The electric bath, the silent discharge from points, shocks from the Leyden jar, etc., were employed with success varying with the severity of the case and with the faith of the patient in the remedy and in its prescriber.
As already remarked, Franklin showed that the mechanical effects of lightning, such as the disruption of non-conductors, could be produced on a small scale by electricity artificially produced. He also investigated the effects when the dis- charge is effected through good conductors, as the metals. When these were in the form of fine wires he was able to heat them to incandescence and even to fuse and dissipate them. Priestly, in 1766, endeavored to employ this method of procedure to determine the relative conducting power of the metals, for plainly, those which were easiest heated, other things being equal, must possess the lowest conducting power. Harris, in 1830, and Riess, in 1837, endeavored in a similar way to determine the relation which exists between the heating and the specific resistance of the metals. The heating effect produced in a wire by the discharge of the Leyden jar, was applied, first in this country in 1831, to explode charges of powder in blasting. In 1855, frictional electricity was employed in military operations by Ebner.
Although the electric light as a means of general illumination, is of compara- tively recent date, and is produced by means quite different from any yet men- tioned, electric lighting was proposed as early as 1750, by one Grummert, a Pole. The brush-like discharge which is frequently seen on one conductor of the elec- trical machine when it is in good operation, and the small star-like glow which is at the same time seen on the other, were early noticed and studied. Watson, in 1753, explained the so-called fire of St. Elmo as being a similar discharge where the accumulation occurs by the action of natural causes. In 1766, Priestly examined the electric spark through a prism, and concluded that its liglit was of the same general character as that of the sun. Wollaston, however, found, in 1802, that the spectrum of the electric spark is traversed by a series of bright and dark lines, which we now know are due to the incandescent particles of the conductors and to the heated air through which the spark passes. By the aid of a rapidly revolving mirror, Wheatstone, in 1834, ascertained that the duration of the spark does not exceed the one one million one hundred and fifty-two thousandth part of a second. And by a similar use of the mirror he found that the velocity of electricity in a copper wire is about 288,000 miles per second. It must not be
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