Fig. 275.
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.
Fig. 276.
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.