CHAPTER XIV.

THE LEYDEN JAR.

In our further study of the transformation of energy self-induction plays a most important rôle. The coefficient of induction multiplied by one half of the square of the current represents the energy which is stored up in the medium surrounding the wire carrying the current. Any change in the value of the current produces a change in the amount of this energy. Another factor which is never absent on electrical circuits is what is called capacity. A Leyden jar has capacity. We ordinarily say that a certain amount of electricity can be stored up in the jar. The larger the jar, the greater the capacity. The Atlantic cable has a large capacity. It is a long, cylindrical Leyden jar; the conducting wire forms the inner coating and the water the outer coating. In the case of a thunderstorm, the upper layer of clouds forms one coating of a condenser and the surface of the earth the other, while the air between takes the place of the glass in an ordinary Leyden jar or of the gutta-percha of the Atlantic cable. The telegraph wires strung on poles also possess capacity. The wires form one surface of the condenser and the ground the other.

The insulator between two charged surfaces of metal is called the dielectric, and modern inquiry is

largely directed to the study of what goes on in the dielectrics under rapidly alternating charges on the metals. There is no doubt that a stress occurs in the dielectric under heavy charges, for the glass walls of Leyden jars are often broken, and we see how the air is cracked, so to speak, by discharges of lightning. The ordinary Edison lamp which is used to light our houses, consisting of a glass bulb inclosing a carbon filament, has considerable capacity. It is a small Leyden jar. If one holds the bulb in one's hand and presents the brass base of the lamp to the conductor of an electrical machine, after a moment of charging one can obtain a shock by touching the brass base of the lamp with one hand while the bulb is held in the other.

When we survey the path we have followed in studying the electric current and the various transformations of energy which are manifested by the rate of change of electro-magnetic induction, we perceive that our attention has been directed mainly to the electrical manifestations on closed metallic circuits. Indeed, to the ordinary mind a wire seems to be the essential part of an electric circuit. Thus, when we discharge a Leyden jar by connecting the outer coating to the inner coating by a wire, we are apt to fix our minds upon this wire as the seat of a momentary electric current, the energy of which is manifested by the spark which results when the jar is discharged. We know that there is a current in the wire when the jar is discharged, for it will melt a wire and decompose water.

Faraday, in 1832, made experiments on the quantity of electricity yielded by the discharge of a Leyden jar; and stated his results as follows: "The decomposition of a single grain of water requires 800,000 discharges of a large Leyden battery. This, if concentrated in a

single discharge, would be equal to a very great flash of lightning, while the chemical action of a single grain of water on four grains of zinc would yield electricity equal in quantity to a powerful thunderstorm." *

The prevailing impression is that more electricity can be obtained from a percussion cap filled with moist salt sand in which a piece of zinc wire is immersed, not touching the copper of the cap, than from a discharge of lightning. One can send a signal across the Atlantic cable with such a minute battery; but it is said one can not do this with a spark from a Leyden jar. This last assertion, however, is a mistake. It can be done by means of the spark from a Leyden jar. All that is necessary is to properly transform this spark in the following manner:

Coat any large thin glass vessel on the outside with tin foil and fill the vessel with water. Now connect the outside tin-foil surface with one conductor of an electrical machine and the water inside the vessel with the other conductor of the machine. After a few turns of the machine the Leyden jar becomes charged; and if the water on the inside is connected by means of a wire with the tin foil on the outside, a spark passes when the end of the wire is brought near the tin foil.

Instead, however, of allowing the spark to dissipate itself in light and noise, let us connect the tin foil with one end of a bobbin of well-insulated fine wire of a thousand feet or more in length, but wound compactly on a hollow bobbin. In the centre of this bobbin, entirely disconnected and insulated from the fine-wire bobbin, we will place another coil of coarse wire five or six feet in length wound once around a bundle of iron

* Dr. Bence Jones, Life of Faraday.

wire. Across the ends of this coarse wire we will place a small incandescent lamp of from five to six candle power. Now, if the other end of the fine-wire bobbin is brought near the inside of the Leyden jar, a spark jumps and is dissipated through the thousand feet of the fine wire. The little lamp connecting the ends of the coarse coil lights up for an instant. If, instead of the lamp, two platinum wires are placed in acidulated water, and are connected with the ends of the coarse coil, a quantity of bubbles of oxygen and hydrogen gas is given off from each of the platinum wires. The water is decomposed, just as it is by two or three strong chemical or voltaic cells.

We see, therefore, that it is merely a question of transformation. An electric spark can do all that a battery or a dynamo can do. It works, however, for a very short interval of time. It has the characteristic of brilliancy but not of persistence. A simple calculation will enable us to form an idea of the horse power in a spark from a Leyden jar of about a gallon capacity, the glass of which is about one sixteenth of an inch thick and which is charged so that it will give a spark of about two inches long.

Such a spark discharged through our bobbin containing about a thousand feet of wire will light up brilliantly a six-candle-power lamp connected with the coarse-wire bobbin which occupies the centre of the fine-wire bobbin. Now we know from accurate experiments that the spark lasts a few hundred thousandths of a second-it may be three hundred thousandths. We know also a horse power would light from thirty to forty of our little lamps. If there were no loss in transforming the spark, the spark would be equal to one thirtieth of a horse power acting for three hundred

thousandths of a second; but there is a loss in transformation of nearly fifty per cent, so the horse power in our spark is twice what we have supposed-two thirtieths or one fifteenth of a horse power.

In a subsequent paper I shall show that we have reasons for believing that the energy of an electric spark an inch in length amounts to thirty or forty horse power; for waves are sent out in the ether in all directions, and these waves are of great energy. At present, however, we are concerned only with a direct transformation of an electric spark into horse power which can be directly measured.

Now, if a spark two inches in length from a gallon Leyden jar is equivalent to one fifteenth of a horse power, what must be the horse power in discharges of lightning which are many hundred feet in length? We have all heard the bells of telephone apparatus ring violently at each discharge of lightning, and, on timing the interval between the flash and the thunder, we find, knowing that sound travels about a thousand feet per second, that the discharge must have occurred a mile away. We should find, on making the necessary calculation, that it would take some hundreds of horse power to produce this electrical effect from the distance of a mile.

Incandescent lamps also often blink at each discharge of lightning from a storm centre at least a mile away. When the discharges occur within a thousand feet the lamps may be nearly extinguished for an instant ; therefore the lightning, even at a distance of a thousand feet, holds in check it may be a thirty-horse-power steam-engine which is turning the dynamo machine and supplying the lights. I have no doubt that a discharge of lightning five hundred feet long, if properly directed