240.]

DEVELOPMENT OF HEAT.

487

is inversely as the conducting power. Snow Harris (Phil. Trans. 1827, 21), by means of an air-thermometer with a large bulb, across which were passed in succession wires of different metals but of equal length and thickness, found that when equal quantities of electricity were discharged through these wires, the heating effects were as follows. The metals which stand first on the list are the best conductors, and they emit the least heat:—

It will be seen that by alloying the metals with each other, the conductivity is often greatly reduced. Great care should therefore be taken to ensure the purity of the metals in experiments of this nature.

If different quantities of electricity be transmitted through the same wire, it is found that the rise of temperature is proportional to the square of the quantity transmitted in equal times: for example, if the thermometer, with a given charge, rise 10°, a charge of twice the power will raise it four times as much, or 40°.

By sufficiently reducing the thickness of the conductor at one part of the circuit, the heat may be raised so far as to fuse the wire, or even to convert it into vapour.

The amount of electricity required to produce this effect, when measured by a unit jar, is found to be equally powerful, whether it be diffused over a large or small surface; the intensity (i.e. quantity which passes through a given space in a given time) is the same in the wire in both cases, though the density of the charge on equal surfaces of the jar is, very different. Where large quantities of electricity are needed, a corresponding extent of coated surface is requisite; this may be obtained either by employing a single jar of large dimensions, or several smaller ones, the inner surfaces of which are connected by wires, and the outer surfaces likewise united by placing them upon a sheet of tinfoil, or on a metallic tray. By discharging such a battery through thin metallic wires-of silver, steel, platinum, or copper, for instance they will be fused and dispersed.

488

LEYDEN BATTERY.

[240.

The arrangement represented in fig. 197 shows one method of employing such a battery for the deflagration of metallic wires: nine jars are in this case

FIG. 197.

B

represented; they are enclosed in a wooden case, B, and rest on tinfoil, which communicates with the earth through the chain c. The battery is charged from the prime conductor P. The internal coatings of all the jars are connected by cross wires. In order to direct the discharge of the battery, a wire passes from its inner coating to the insulated upper arm, f, of the discharger A; a second wire passes from the ball b, to one of the insulated wires on the stand of the

universal discharger D. The wire for deflagration, w, is fastened to a card which is also supported on a little stand insulated by glass; and the communication with the external coating of the battery is continued by a wire connected with the other insulated support of the universal discharger D; thus the conducting communication is complete with the exception of the interval between a and b. When the battery is adequately charged, the lever is withdrawn, the ball a and its attached wire are thus released, and fall through a hole in the metallic arm ƒ, which is connected with the inner coating, and the circuit is completed when the balls a and b come into contact.

It must be observed that in all cases of conduction the charge passes through the whole thickness of the rod or wire, and is not confined to its surface: it therefore makes no difference whether the metal is in the form of wire, or is extended over a large surface as leaf. The induction at any part of the wire during the discharge is mainly from one transverse section of the wire to the contiguous section that immediately precedes and that follows it.

The dispersion of the conductor by the passage of high charges of electricity leads us to consider next what Faraday has termed the disruptive discharge.

(241) b. Disruptive Discharge.-This mode of discharge is attended by sudden and forcible separation of the particles of the medium through which it occurs; and it is attended with evolution of light and heat. It is best seen between two conductors separated by a dielectric, such as two metallic balls in air. In these cases, when a sudden bright spark passes, the discharge is as complete as if it had been effected by direct metallic communication. The particles of the intervening dielectric are brought up to a highly polarized state, until at length the tension on one particle rising higher than the rest, and exceeding that

241.]

DISRUPTIVE ELECTRIC DISCHARGE.

489

:

which it can sustain, it breaks down the balance of induction is thus destroyed, and the discharge is completed in the line of least

resistance.

In all these cases, portions of the solid conductors are detached, and by their ignition increase the brilliancy of the spark. This transfer of material particles by the spark is easily proved, for if sparks be caused to pass between a gold and a silver ball, the surface of the gold becomes studded with particles of silver, and vice versa. If an iron chain be laid on a sheet of white paper, and a powerful discharge be sent through it, each link will leave upon the paper a stain, arising from the portions of the metal which have been detached: and if the discharge be effected over a plate of glass, particles of the metal are frequently forced into it. The experiment may be varied by suspending the chain in a dark room, and passing the discharge through it; brilliant deflagration of the iron will be seen at each link.

If the sparks be taken between wires composed of different metals, and the light of each spark be viewed through a prism, the spectrum will in each case exhibit the bright lines due to the light of the corresponding metal in the state of vapour (106).

Sparks attended with disruption may also take place in the midst of liquid dielectrics. More rarely disruption from the discharge occurs in solids: occasionally this is exemplified in the Leyden jar itself, the tension upon the glass now and then rising so high that the glass becomes perforated. Across this fracture discharge always afterwards occurs; so that no effective charge in a battery can be maintained till the cracked jar is removed. This disruption of glass may be produced at pleasure by bending a wire so that its point may press against the side of a tube or other vessel filled with some liquid dielectric, such as olive-oil. On charging the wire from the prime conductor, and applying a ball to the outside of the tube opposite the end of the wire, a spark passes, and a minute perforation is produced.

Great expansion of the air occurs from the heat developed at the moment of the discharge, as is shown in the following experiments. Paste a strip of tinfoil on glass, cutting it through in two or three places with a knife; place a few wafers or other light bodies over the interrupted points, then discharge a jar through the tinfoil, and the wafers will be immediately scattered in all directions. If a card or half quire of paper be placed in the direction of its thickness in the track of the discharge, the card or the paper will be burst outwards on both sides.

Many pleasing experiments may be made by causing a succes

490

MEASUREMENT OF THE VELOCITY OF ELECTRICITY.

[241.

sion of discharges to occur through such interrupted conductors; a beautiful display of the electric light may thus be exhibited in a darkened room.

(242) Velocity of Discharge.-Of the velocity of the spark discharge some notion may be formed from the brief duration of its light, which cannot illuminate any moving object in two successive positions, however rapid its motion. If a wheel be thrown into rapid rotation on its axis, none of its spokes will be visible in daylight, but if the revolving wheel be illuminated in a darkened room by the discharge of a Leyden jar, every part of it will be rendered as distinctly visible as though it were at rest. In a similar manner, the trees even when agitated by the wind in a violent storm, if illuminated at night by a flash of lightning, appear to be absolutely motionless.

By a very ingenious application of this principle, Wheatstone has shown that the duration of the spark is less than the onemillionth part of a second. The apparatus is the same in principle as the revolving wheel.

By a modification of the apparatus, Wheatstone was also enabled to measure the velocity with which the discharge of a Leyden jar was transmitted through an insulated copper wire. He estimated the rate of its passage at 288,000 miles in a second (Phil. Trans. 1834, 589). For this purpose he employed an insulated copper wire about half a mile long, through which a Leyden jar was discharged. This insulated circuit was interrupted at three points; one of these interruptions was within a few metres of the inner coating of the Leyden jar; the second was in the middle of the wire; and the third within a few metres of the outer coating of the jar. The parts of the wire at which these three breaks in the circuit occurred were all arranged side by side on an insulated disk, so that the three sparks could be seen simultaneously. In fig. 198 a wire is represented

as proceeding from the knob of the jar to an insulated rod; when the charge attains a certain tension, a spark passes between this rod and a small knob attached to the axis of a small revolving mirror, m: to one extremity of this axis, the wire which passes to the outer coating is fastened; but the discharge is made to traverse the whole length of the two intervening long contorted portions of wire before it reaches the outside of the jar. The three sparks, if viewed by the naked eye, appear to be simultaneous. If viewed through the glass plate, e,

243.]

VELOCITY OF ELECTRICITY.

491

in a small steel mirror, m, to which is given a regulated but extremely rapid revolving motion on an axis parallel to its surface, the sparks appear no longer as dots of light in the same horizontal line, but present the appearance of three bright lines of equal length. The two outer ones commence and terminate in the same horizontal line, but the middle one occurs later than the other two, and the angular position of the mirror has had time to advance slightly before the middle spark appears, which consequently exhibits an image slightly displaced. As the velocity of rotation of the mirror is recorded by the register, b, and the amount of this angular deviation of the image of the central spark is easily ascertained, the retardation of the discharge by the copper wire, or, in other words, the velocity with which it travels along it, can be estimated.

This experiment has another important signification, to which due weight appears hardly to have been given; for it affords a convincing proof of simultaneous action and reaction in the operations of electricity, and of its existence as a stress at the same moment that a positive influence leaves the inner coating, an equal amount of negative influence leaves the outer coating, and these two neutralize each other at the central point of the conductor, after the lapse of an extremely minute but still appreciable interval of time. It appears from this experiment that Franklin's theory (227), though in many cases a simple and convenient mode of explaining facts, is not the true representation of the phenomena. The theory of two fluids seems by this experiment to be demonstrated.

The velocity of the electric discharge is, however, found to vary with the intensity of the charge, and with the nature of the conducting medium (Faraday, Phil. Mag., 1854, [4] vii., 197). The duration of the discharge may be prolonged by causing it to take place through bodies of inferior conducting powers. A charge of a given amount, if transmitted slowly, may, by the prolonged period through which its heating powers can be applied to a combustible, be made to ignite bodies, which the same charge, if more quickly transmitted, would only have dispersed. For example, let two metallic wires be brought within an eighth of an inch (3mm.) of each other, and let a little loose gunpowder be placed over the interval-the powder will simply be dispersed if the charge of a Leyden jar be sent through the wires; but if a few inches of wet string be interposed in any part of the circuit, the discharge will be prolonged sufficiently to fire the powder.

(243) Striking Distance.—In air, whatever be its density, the same charge produces, cæteris paribus, induction to the same extent. But the distance through which the discharge of equal quantities of electricity takes place in the same gaseous medium, varies inversely as the pressure. This might be anticipated, since under a double pressure double the number of particles of air