CHAPTER XV.

ELECTRICITY.

370. =Origin of the Term.=--The ancients observed that when certain substances were rubbed together singular phenomena were produced. One of these substances was amber, and as the Greek name for this is ηλεκτρον, the power which is thus excited into action has been called electricity.

371. =Attraction and Repulsion in Electricity.=--One of the most common effects of electricity is attraction. If we rub a tube or rod of glass with woolen or silk it will attract light articles, such as cotton, feathers, lint, etc., so that they will adhere to it. But repulsion is also an effect of electricity under certain circumstances. In order that the explanation of these two opposite effects may be clear to you, I will detail some of the experiments which exhibit both. Suppose that we have a pith ball, A, Fig. 257, suspended by a silk thread, B, from a stand, C. I must premise that silk does not readily let electricity pass over it, or is a non-conductor, and therefore any electricity communicated to the pith ball will remain there unless something be brought in contact with it or very near it. If now you rub a glass tube, thus exciting electricity upon it, and then bring it near the ball, it will attract the ball to it, and then in a moment repel it, so that it will stand off from the tube and retreat from it if you follow the ball with the tube. Why is this? It is supposed that there is a subtile fluid on the electrified glass, some of which passes to the ball as it touches the glass, so that the ball and the glass are in a similar condition. But the particles of the fluid repel each other; and this is the reason that the ball is repelled from the glass as soon as it becomes charged with a part of the electricity of the glass. For the same reason if two pith balls hanging from a standard become electrified from a glass tube or rod they will repel each other, for they are in the same electrical condition.

372. =Vitreous and Resinous Electricity.=--Suppose now that you rub a rod of sealing-wax with woolen or silk and hold it near a pith ball which has been electrified from glass. It will attract the ball. The reason is that an electricity is excited on the sealing-wax of a different kind from that which is excited on the glass. The former is called _resinous_, and the latter _vitreous_ electricity. They are supposed to be two fluids, which have a strong attraction for each other, while, on the other hand, the particles of either fluid are repellent to each other. It is this attraction between the two fluids which causes in the case just stated the sealing-wax to attract the ball to itself. We can illustrate this attraction in another way. Take two pith balls and electrify them, the one from glass and the other from sealing-wax. Brought near together they will attract each other, because they have two unlike electricities. This, you see, is just the reverse of the effect produced in an experiment cited at the conclusion of § 371, in which the electricities were alike in the two pith balls. Again, if you bring the rubbed sealing-wax near to the ball electrified from glass, the ball will be attracted, and the same effect will follow if you bring the rubbed glass near to the ball electrified from sealing-wax.

373. =Franklin's Theory.=--In § 372 is developed the theory now commonly received in regard to electricity. The theory of Franklin was different. He supposed that there is but one electric fluid, and that all bodies are in their usual state charged with a certain portion of it, some having more than others, according to their capacity for electricity. While a body is in its usual state there is no manifestation of electricity. The fluid is in a quiescent condition, because its particles are prevented from repelling each other by the attraction which exists between them and the particles of the substance. But this quiescence can be disturbed by friction and other causes. Thus if a glass rod be rubbed with a piece of silk, the natural equilibrium is disturbed, the glass having an excess and the cloth a deficiency of electricity. The glass is therefore said to be _positively_ and the cloth _negatively_ electrified. The equilibrium can be restored in the case of a positively electrified body by having its excess drawn off, and in the case of a negatively electrified body by having its deficiency made up by receiving electricity from other bodies. Though this theory is discarded, the terms positive and negative derived from it are retained, being applied to the two fluids[6] or electricities, and they are often designated by the two signs + and -.

374. =Upon What the Kind of Electricity Excited Depends.=--It depends on what a substance is rubbed with whether vitreous or resinous electricity is excited in it. Thus smooth glass rubbed with woolen cloth or silk will be positively electrified; while if it be rubbed upon the back of a cat it will exhibit negative or resinous electricity. So, also, if a resin, as gumlac or sealing-wax, be rubbed with silk or woolen cloth, it will be charged with resinous electricity, but it will be charged with vitreous or positive if it be rubbed with sulphur. The terms vitreous and resinous are therefore incorrect, for they are based upon the idea that one kind of electricity is always excited on glass, whatever the friction may be made with, and that the other kind is always excited on resins. The most decided illustration of the incorrectness of these terms we have in the fact, that while smooth glass rubbed with silk or woolen cloth becomes charged with positive (vitreous) electricity, roughened glass rubbed with the same gives us negative (resinous) electricity. Below I give a table of substances, any one of which has positive electricity developed on it when it is rubbed with any substance below it on the list, and negative when rubbed with any substance above it:

1. Cat-skin. 2. Polished glass. 3. Woolen cloth. 4. Feathers. 5. Wood. 6. Paper. 7. Silk. 8. Sealing-wax. 9. Amber. 10. Roughened glass. 11. Sulphur.

375. =Conductors and Non-Conductors.=--Electricity passes over the surface of some substances very readily; while over others it moves with very great difficulty, and therefore very slowly and sparingly. The former are termed conductors, and the latter non-conductors. As in the case of heat, so with electricity there are no substances which are wholly non-conducting. The best of all the conductors are the metals, those least liable to oxydation being the most perfect. Next come charcoal, water, living substances, flame, smoke, steam. The best non-conductors are gumlac and gutta-percha. Then come amber, resins, sulphur, glass, silk, wool, hair, feathers, cotton, paper. Non-conductors are sometimes called _insulators_, from the Latin word _insula_, as they serve to confine electricity within certain bounds, and prevent its escaping. Thus in the experiments with pith balls, already cited, the silk threads by which they are suspended prevent the electricity from escaping from them. So the glass knobs on which the wires of the telegraph rest are insulators, preventing the electric fluid from escaping down the poles into the ground.

376. =Electricity Always on the Surface.=--There is a marked difference between heat and electricity in the manner in which they are disposed of. Heat pervades all the particles of substances, and in its conduction spreads through them, while electricity in its ordinary movements operates altogether on the surface. A hollow ball, therefore, can contain as much electricity as a solid, and a hollow conductor of electricity is just as effectual as a solid one. The following experiment exhibits in a very striking manner this disposition of electricity to occupy the surface alone: Let _a_, Fig. 258, be a metallic ball supported by a glass stand, _b_; and let _c c_ be metallic caps which will just cover the ball, having non-conducting handles, either glass or gumlac. Now, after having charged the ball with electricity, let the caps held by the insulating handles be carefully placed over the ball. On withdrawing them it will be found that the electricity of the ball has all passed to the outer surface of these caps.

377. =Electrics and Non-Electrics.=--It will be observed, on looking over the list of conductors and non-conductors, that among the non-conductors are those substances in which electricity is easily excited by friction, such as glass, amber, silk, etc. These were therefore called electrics. The conductors, on the other hand, were called non-electrics, it being supposed that electricity could not be excited with them. But this has been found not to be true. For example, if a metal be insulated by being placed on a pillar of glass or of gumlac, so that the electricity, when excited, can not pass off readily, its generation can be made manifest. It is probably true that every substance is more or less an electric, it being difficult to make this manifest in the case of conductors, because the electricity passes off as fast as it is generated.

378. =Electricity Every Where Active.=--I have said that there is electricity in all substances, each having its own capacity for it, but that in the usual condition of substances the electricity is in a state of equilibrium, and therefore of quiet. We see this quiet disturbed whenever there is a thunder-storm, when we rub glass or silk, or a cat's back, or when we work an electrical machine. But the active state of electricity is not limited to such palpable demonstrations as these. Electricity is undoubtedly in action every where and always, although we can seldom appreciate and measure its action. Wherever there is motion there is a disturbance of the equilibrium of electricity, and a consequent return to this equilibrium. And this change from the one state to the other must be the constant cause of important changes and operations in the world around us, and in our own bodies. Let us look at some of the indications of this universality electrical action. The friction of any electric upon another awakens it. The friction of the belts upon the drums in cotton factories does it quite freely. Every stroke of India rubber upon paper as you erase a pencil mark excites electricity. The blowing of air upon glass does the same. So, also, does the blowing off of steam from an engine. Electricity has been excited even upon ice by rubbing it when cooled down to 13° below zero. Experiments upon the air have shown that there is usually some free electricity in it, the atmosphere being generally in a positive state, especially when the air is dry and clear. It is constantly generated from one source and another. It is generated every where by evaporation. Every gust of wind, causing friction of the particles of the air upon various substances, generates it. Motion of every kind probably generates it. Chemical action, as you will see in another part of this chapter, generates it every where. It is generated also in the operations of life, and in some animals there are special organs--electrical batteries--for the generation of this agent.

379. =Induction.=--A remarkable influence is exerted by an electrified body upon another body in its usual state when brought near it, and this influence is called induction. I will illustrate this by Fig. 259. Let A be a metallic ball standing on a glass pillar, and charged with positive electricity. Let B be a metallic cylinder supported upon two glass pillars. Now if A be placed near B, but not near enough for the electric spark to pass from it to B, it will destroy the equilibrium of the two electricties in B, the negative electricity being accumulated at the end near A, and the positive at the remote end. This is because the positive electricity in A repels its like in B and attracts the unlike fluid. You observe that there is a pair of pith balls suspended at each end of B, and also at the middle. The two balls at the positive end repel each other because they are charged with the same electricity, and so with the balls at the negative end. But the balls hanging from the middle are not affected, because they are on middle ground between the two electricities. Here is no communication of electricity from A to B, but only an influence upon the quiescent balanced electricities of B. Accordingly, if the surplus electricity of A be discharged by putting the hand or any good conductor upon it the influence will cease, the equilibrium in B will be restored, and the pith balls will all hang straight down. The same effect will be produced if A be withdrawn to a distance from B, and the influence will be renewed if A be brought near again.

If instead of one conductor we use two, B and C, Fig. 260, and have them in contact, we shall have the negative electricity on B and the positive on C. Now if we withdraw C from B we may have the two electricities separate, B being charged with the negative and C with the positive.

380. =Electrical Machine.=--You are now prepared to see how the common electrical machine operates. There are two kinds--the plate and the cylindrical. The plate machine, Fig. 261, has at _p_ a large plate of glass, and at _r_ a rubber which consists of two brass plates lined with leather which is stuffed, the pressure of which upon the glass is regulated by a screw. Above this rubber is a brass ball, _d_, and a brass chain connects the rubber and the ball with the floor, or, in other words, with the earth. At _c_ is what is called the prime conductor--a hollow brass cylinder with rounded ends, having attached to it a rod with points, as seen at _a_. There is a similar rod attached to it on the other side of the glass plate. The different parts of the instrument are supported on glass pillars, _g g g_, standing on a wooden platform. The lower part of the plate is covered with a case of silk, which, being a non-conductor, prevents the electricity on the glass from being lost in the air, and also serves to keep the plate free from dust. The rubber is covered with an amalgam of tin, zinc, and mercury, this being found very effectual in exciting electricity. The operation of the machine is this: As the plate revolves positive electricity is collected upon the glass, and negative electricity upon the rubber. The former, as it comes to the points at _a_, goes to them and passes on by the rods to the prime conductor, while the latter passes from the rubber by the chain to the earth. The points at _a_ are of great service in collecting the electricity, because the fluid is always much more ready to go to points than to conductors of a blunt shape.

The cylinder machine is represented in Fig. 262, _a a_ being a glass cylinder, which can be turned rapidly by the multiplying wheel, _b b_. At _c_ is a piece of silk, and on the rear part of the cylinder is the rubber. At _d_ is the prime conductor.

381. =Experiments.=--Many experiments may be tried with the electrical machine. I will cite a few of them:

If pith balls be attached to the prime conductor, as seen in Fig. 261, they will stand out from each other as soon as the machine is worked, because they are both charged with the same kind of electric fluid.

Let a small figure with its head covered with hair be placed upon the prime conductor. As soon as the conductor becomes charged with electricity the hair stands out, as represented in Fig. 263, for the same reason that the pith balls diverged in the previous experiment.

So, also, if you place on the conductor a figure having attached to it strips of tissue-paper, they will diverge in the manner shown in Fig, 264.

Let a metallic plate, _a_, Fig. 265, be suspended by a chain to the prime conductor, and another plate, _b_, be supported upon a conducting stand. If figures of paper or pith be placed between these plates as the machine is worked they will move about briskly between the plates, being alternately attracted and repelled by the communication of the electricity.

The experiment represented in Fig. 266 is a very beautiful one. Let _a b_ be a brass rod with an arch, _g_, by which it can be suspended from the end of the prime conductor. To this rod are suspended three bells, the two outer ones by chains, and the middle one by a silk thread; also two clappers, _d_ and _e_, by silk threads. The middle bell has a chain, _f_, connecting it with the table--that is, with the earth. The operation of the apparatus is this: As soon as the outer bells become electrified they attract the clappers. These, on touching the bells, receive a portion of their electricity, and are repelled. They therefore strike against the middle bell, to which they impart the electricity which they received from the outer bells. They swing back again then in the same state that they were in at first, and now are attracted again by the outer bells. This goes on so long as the electricity is communicated.

Let there be pasted upon a slip of glass a continuous line of tin-foil, going back and forth, as represented in Fig. 267, and let there be a ball, G, connected with one end of, the foil. The word light is made upon it by cutting out with a sharp knife little portions of the foil. If now with your finger on one end of the line of foil at _a_, you present the ball G to the prime conductor, the electric fluid will run along the whole length of the line from G to _a_. In doing this the letters are beautifully illuminated, a spark being produced at each interruption of the line. So rapid is the passage of the electricity that the whole appears to the eye simultaneously illuminated.

382. =The Insulating Stool.=--This consists of a wooden top, _a_, Fig. 268, supported by glass legs, _c c_. It can be made simply by boring holes in the four corners of a piece of board sufficiently large to admit the necks of bottles. Many amusing experiments can be tried with this. A person standing upon it can be highly charged with electricity by holding a chain connected with the prime conductor. The hair will rise up as represented in Fig. 263, and he can give electric shocks to other persons from any part of his body.

383. =Electricity Discharged from Points.=--I have already, in giving an account of the electrical machine, spoken of the readiness with which electricity is received by points. It is discharged from them with equal readiness; so that, if a metallic point be attached to the prime conductor, the electricity will be carried off into the air nearly as fast as it is received upon the conductor. And as it passes off it creates a current in the air as it strikes upon it. The reaction of the air upon the electrical currents can be very prettily exhibited with the apparatus represented in Fig. 269, which consists of a cap, A, resting upon the point of a rod, and having pointed wires branching out from it in a wheel-like arrangement. You observe that the points are all bent one way. If this apparatus be set upright upon the prime conductor, the wheel can be made to revolve rapidly by working the machine. As the reaction of the air against the gases issuing from the rocket makes it rise, so the same reaction against the electricity issuing from these points causes the circular motion. If electricity be discharged from a point in a darkened room it appears like a brush of light, as represented in Fig. 270.

384. =Leyden Jar.=--The Leyden jar, Fig. 271, is so called because it was contrived at Leyden. It was suggested by an accidental result of an experiment tried there with the electrical machine. It consists of a glass jar coated upon the inside and the outside to near the top with tin-foil, and having a metallic rod passing through the cork, with one end touching the inner coating, and the other surmounted by a brass ball or knob. The jar is charged by holding the knob near to the prime conductor while the machine is worked. The electricity passes by the metallic rod to the inside coating of the jar, and accumulates there. This is positive electricity. In the mean time there is an accumulation of negative electricity on the outside coating. But how is this? It is by the repulsion of positive electricity for itself, and its attraction for its opposite, negative electricity. As you hold the jar in your hand positive electricity is repulsed from its outside through your arm earthward, while negative electricity is attracted to it by the positive which is within. The two fluids get as near to each other as possible. They are prevented from coming actually together by the non-conducting quality of the glass. If a slip of tin-foil were made to connect the inside foil with the outer, there would be no accumulation of electricity on the inside, for as fast as it passed from the prime conductor to the inside it would pass out over the bridge of foil to the outside, and down your arm and body to the earth.

If there were no communication of the outside with the earth the jar would not be charged. No electricity would pass to it, because the positive electricity which is on the outside can not be driven off, and no negative electricity can be received. To make this plain, suppose that the jar, _a_, Fig. 272, having a bent rod, is suspended to the prime conductor, _b_. Here you have the inside tin-foil connected with the source of positive electricity. But the outside is insulated. No electricity can pass from it or to it. It has both positive and negative electricity, but they are in equilibrium. If there were a preponderance of negative electricity there, it would attract positive electricity to it as near as possible, and so the latter would enter the jar from the conductor. But there is no such preponderance, and so, though a little may enter--a spark or two--there will not be enough to charge the jar sensibly, because there is no attraction in that direction. But bring now another jar, _c_, near to the outside coating of _a_, and there is a movement at once in the electricities. The positive electricity has a chance now to pass off from the outside of _a_ to the inside of _c_, leaving therefore a preponderance of negative electricity on the outside of _a_, which exerts an attractive influence on the positive electricity of the conductor drawing it to the inside of the jar.

385. =Discharge of the Leyden Jar.=--The jar may be discharged by making a communication between the inside and outside by means of any conductor. It may be done with the discharging-rod (Fig. 273). This has two slender metallic rods, with brass knobs at their ends, and jointed at _a_, so that the knobs can be separated to different distances. The handle is glass, so that as the electricity passes through the rods none of it may be communicated to the hand. In discharging the jar one knob is placed upon the outside foil, and the other is brought near to the knob of the jar. The two fluids now rush together from their attraction, and in doing so a bright flash is produced, going from the knob of the jar to that of the discharging-rod, and with this a report. You can yourself be the conductor to discharge the jar. If, having one hand upon the outside of the jar, you bring the other near its knob, the fluids meet in you as they do in the discharging-rod, and a shock will be experienced in proportion to the amount of charge in the jar. Any number of persons can together receive the same shock. To do this they must join hands, and the person at one end of the row must touch the knob of the jar while the person at the other end has his hand upon the outside.

You may touch either the knob of the jar or the outside coating _separately_, and the power that is in it remains quiet; but the moment that you touch both it bursts forth, because a bridge is made upon which the two fluids can meet.

In a dry air the charge in the jar can be retained for some time, the communication between the two electric fluids being very slow through the medium of air. It is otherwise when there is much moisture in the air, for water is a good conductor. For this reason, if you let the moisture from your breath come upon the jar between the outside coating and the rod, the jar will be discharged soon, though imperceptibly, the moisture making a medium of communication between the inner and outer electricities.

386. =The Electrical Sportsman.=--In this contrivance, Fig. 274, the discharge of the Leyden jar is very prettily exhibited. The jar, _c_, has a rod with two branches. On the end of one of these, B, are suspended pith balls cut in the shape of birds. On the other is a knob by which the jar can receive its charge from the prime conductor. After it is charged it is placed on the stand with its knob, _b_, near the gun, _a_, of a metallic figure. The suspended birds, you observe, stand out from each other, because they are charged with the same fluid, positive electricity, and therefore are repellent. Now when the chain, _e_, which is connected with the outside of the jar, is made to touch the foot of the metallic image, the connection between the inside and outside of the jar is established. Of course there is an instantaneous flash between _a_ and _b_, and the birds, losing their electricity, fall, and hang as they did before the jar was charged.

387. =Electrical Battery.=--By combining together a number of jars, having the insides all connected together, as seen in Fig. 275, with metallic rods, and the outsides connected together in a similar manner, we have what is termed an electrical battery. By such an arrangement we can accumulate a large amount of electricity, which can be discharged in the same way essentially as in the case of the single jar.

388. =Light of Electricity.=--The light produced by electricity is not occasioned by any thing like combustion. It depends obviously upon the resistance which is offered to its passage. Thus when the electric fluid passes through air from the prime conductor to the knob of the Leyden jar it causes a flash of light, but when it arrives at the knob the flash ceases. What is the reason of the difference? In both cases it has the resistance of the air, for when it comes to the knob it passes over the _surface_ of the knob and rod; but in the latter case it is so diffused in its conduction over the metallic surface that it meets with much less resistance from the air. By experiments with the air-pump it is found that the denser the air is the more vivid is the spark; and if electricity be passed through a glass vessel from which the air has been mostly exhausted we have the streams of light seen in the aurora borealis, which are so strikingly in contrast with the vivid flashes of the lightning. In the experiment, § 381, in which the word light is made by the passing electricity, we have a striking illustration of the production of the spark by the resistance of the air. If the foil were one continuous surface the electricity would be diffused over it without giving any light. It is only where the electric fluid has to leap through the air from one portion of foil to another that the light is seen.

389. =Sound of Electricity.=--The report of electricity is a sort of crack or snap from the sudden condensation of the air by the rapid passage of the fluid. The rolling of thunder is occasioned by the reverberation of the first sound among the clouds. The nearer the flash is to us the more like a crack is its first sound as it comes to our ears.

390. =Mechanical Injuries from Electricity.=--When any great amount of electricity meets in its passage with any imperfect conductor it does much violence to it. Thus it rends wood, scatters water, breaks glass, etc. Various experiments have been tried illustrating the manner in which mechanical injuries result from electricity. Thus if it be made to pass through a card or several leaves closely pressed together, there is a burr on each side of such a character as to show that two forces moving in opposite directions have made their passage.

391. =Heat Produced by Electricity.=--Electricity always produces in its passage some amount of heat, probably by its mechanical effect. When it is diffused over a large conducting surface the heat is not sufficient to be observable; but if it be confined to the surface of a small wire the heat may be sufficient to melt or even burn it. Various effects can be produced by the heat thus caused by the passage of electricity. Gunpowder may be exploded by it. Alcohol and ether may be readily ignited by it, especially the latter. Gas can sometimes be lighted by pointing the finger to an opened burner after walking across the room two or three times briskly, rubbing the feet upon a thick carpet.

392. =Franklin's Discovery.=--It had very early been conjectured that the electricity produced by the electrical machine is identical with lightning; but it was reserved for our countryman Franklin to prove the fact. A tall spire which was being erected in Philadelphia in 1752 he conceived might be used in his investigations, but before it was completed the sight of a boy's kite in the air suggested to him another plan. He made a kite by stretching a silk handkerchief over a frame, and sent it up as he saw a thunder-shower rising, his only companion being his son. Having raised the kite, he attached to the end of the hempen string a key, and also a silk ribbon, by which he insulated his apparatus, as seen in Fig. 276. He now watched with much anxiety the result. A cloud arose, which he supposed, from its appearance, was well charged with electricity, and yet no effect was seen. Franklin began to despair; but he soon saw some loose fibres of the hempen string bristling up, and, applying his knuckle to the key, received just such a spark as he had often received from the conductor of an electrical machine. The discovery was made, and Franklin was at once overcome with emotion at the thought of the immortality which it would give his name. He felt very much as Archimedes did when, after making one of his grand discoveries as he lay in a bath, he went home saying all the way, Εὕρηκα! Εὕρηκα! The fame of the discovery, made in a manner so simple and yet so original, spread every where, and prompted to many experiments by other philosophers. One, Professor Richman of St. Petersburg, fell a victim to his investigations. While he was attending a meeting of the Academy of Sciences he heard the sound of distant thunder, and hastened home to make some observations with an apparatus which he had erected. While doing this a charge of electricity flashed from the conducting rod, and piercing his head killed him instantly. His assistant, who stood near, was struck down, and remained senseless for some time, and the door of the room was torn from its hinges.

393. =Lightning-Rods.=--It was the discovery of Franklin which led to the custom of attaching lightning-rods to buildings. The object of a lightning-rod is to conduct any electricity in a cloud that may come over the building down into the ground. For this purpose the rod should terminate in the air in points, as these, as you saw in § 380, so readily receive the electric fluid. The rod should be separated from the house by wooden supports, and it should pass so far into the ground as to have its end in the midst of continual moisture. The points should be gilt, in order to preserve from corrosion, or they may be made of silver or platina. Lightning is very apt to go down in chimneys, as smoke is a very good conductor; and therefore it is well to have the rods go up by chimneys, especially if they are to have fire in them during the summer. Lightning-rods often undoubtedly are of service when there is no obvious passage of the lightning down them, by quietly and continuously receiving electricity upon their points, and passing it down into the earth.

394. =Galvanic or Voltaic Electricity.=--This form or mode of electricity I will barely notice here, reserving its full consideration for Part Second, where it appropriately belongs. The history of its discovery is interesting. The first dawning of Galvanism is to be found in an experiment noticed by Sulzer, a citizen of Berlin, in 1767. He states that if a piece of zinc be placed under the tongue, and a piece of silver upon it, on being brought in contact a metallic taste is perceived, and a shock is felt by the tongue. Sulzer attributed the effect to some vibratory motion occasioned by the contact of the metals, and, satisfied with this fanciful explanation, pursued the inquiry no farther. The statement excited but little notice until other facts of a similar character were brought out in 1790 by Galvani, professor of Anatomy at Bologna. He observed that the legs of some frogs, which had been obtained for his invalid wife, were convulsed, when near an excited electrical machine, on touching the nerves with a knife. In contrast with the example of Sulzer, he was led to examine the matter further. He found that the effect was produced when no electricity was communicated from the machine, by establishing a connection between the nerves and the muscles by some conductors. For example, when a strip of zinc was placed in contact with the nerve which goes to the lower extremities, and a strip of copper in contact with the legs, on bringing the two together at the other end the legs would be convulsed, being drawn up, as represented in Fig. 277 (p. 306). But Galvani did not get at the true explanation. He supposed this to be an exhibition of animal electricity, regarding the muscles as being a sort of Leyden jar, the nerve being the medium of communication with the inside.

395. =Volta's Pile.=--The observations of Galvani awakened much interest in all scientific minds, and of course there was much of inquiry, observation, and experiment. Professor Volta, of Pavia, went a step farther than Galvani toward the true explanation, in referring the effects to the contact of dissimilar metals, and he was led by this view of the subject to construct his _pile_ or battery--called after him the voltaic pile--the object of which was to produce a much greater amount of electricity than could be obtained by the contact of only two pieces of metal. The pile is made of circular pieces of copper, zinc, and cloth, the cloth being moistened with salt-water. They are arranged as represented in Fig. 278. First a disk of copper is laid down, then upon this one of zinc, then one of cloth, and so on in the same order, the top of the pile ending in a plate of zinc. If you touch one end of the pile with a moistened finger and the other end with a finger of the other hand, you will feel a shock like that from a Leyden jar. The communication between the two ends of the pile may be made by wires, as seen in the figure. Volta afterward changed this to the form of a cup battery, the plates of metal being immersed in a series of cups in a mixture of sulphuric acid and water. There have been various improvements from time to time, but the arrangement is in all the different batteries essentially the same. Although Volta accomplished so much he did not arrive at the truth in full. His "contact theory," as it is called, so long received as the true theory, gradually gave way to the true explanation, viz., that the electricity produced is owing to chemical action.

396. =Difference Between Frictional and Voltaic Electricity.=--The electricity produced by the friction of the electrical machine is more intense than that of the voltaic battery. Voltaic electricity, on the other hand, is much more abundant, and is more continuous and lasting. As it is therefore more steady and more easily controlled than frictional electricity, it is used in the working of the Telegraph.