482

LANE'S ELECTROMETER.

[236.

with the outside: whilst the unit jar is becoming charged from the machine (say that its outer surface is rendered positive, as represented in the figure), an equal

FIG. 190.

bu

P

quantity of positive electricity is passing off from the interior along the wire, w, attached to the inside of the jar, B, which is to be loaded with a definite quantity, the interior of the small jar becoming negatively charged by induction: as soon as the negative charge in the unit measure rises sufficiently high, it discharges itself between the adjusted balls, a, b, without affecting the charge in the jar, B. A second charge is now given to the unit jar, which discharges itself when it rises to the same amount as before: during each successive charge of the unit jar, a corresponding quantity of positive electricity passes from its exterior into B, so that by counting the number of sparks that pass between a and b, the number of equal quantities or arbitrary units which have been given to the jar, B, is ascertained. Supposing the adjustment of the balls, a and b, to remain the same, the jar в may be made to receive, for any number of times successively, equal amounts of electrical charge, by causing an equal number of discharges of the unit jar to take place in each case.

Other means have been proposed for ensuring an equal accumulation of electricity in a jar. Lane's discharging electrometer is the simplest of these. One

FIG. 191.

form of this apparatus is shown in fig. 191: its principle of action will be at once apparent. L is an ordinary Leyden jar, in the ball a of which a hole is drilled to receive the brass pin of the electrometer; a bent glass arm, b, carries upon its lower extremity a brass socket, c, through which slides an insulated rod carrying a brass knob on either extremity; one of these balls, f, can be placed at any required distance from the knob of the Leyden jar. A chain or wire, w, effects a communication between the sliding rod and the outside of the jar. If the interval between A and f be maintained the same, the jar will always require the same amount of charge before the discharge takes place between these two balls, A, and f. The quantity of electricity in the charge is proportional to the distance between the balls: with an interval of 10 millimetres the quantity would be double that required when the distance was only 5 millimetres.

The attraction between two charged surfaces has been measured by an ingenious modification of the common balance devised by Snow Harris. A light disk of gilt wood is substituted, as shown in fig. 192, for one of the pans of the balance; beneath it is a second similar insulated disk: the suspended disk and the balance beam, through its support, are connected with the exterior of a Leyden jar, and the lower insulated disk with the interior of the jar. By charging the Leyden jar with definite quantities of

237.]

ELECTRICAL BALANCE.

483

electricity by means of the unit jar, the laws which regulate the attraction were experimentally determined. One or two of the more important results may be given as an illustration of the mode of proceeding.

FIG. 192.

If a Leyden jar charged with a certain quantity of electricity produce between the disks an attraction sufficient to support 02 grm., it will when charged with double the quantity support four times the amount, or o 8 grm.; with three times the quantity it will raise nine times the amount, or 18 grms.; consequently, if the extent of charged surface continue constant, the attraction increases as the square of the quantity.

When two equal and similar jars are used instead of one jar, and the same quantity, say ten units, is distributed over them, the attraction will be diminished to, and with three jars to of what it was when a single jar was employed. For instance, a quantity which on one jar would support 18 grms., would, if diffused over two similar jars, support only o'45 grm.; and if diffused over three, it would support only o2 grm. If, therefore, the quantity remain constant, the attraction is inversely as the squares of the charged surfaces of the jars. When the distance between the disks was altered, it was found, for charges of equal superficial density, that the attraction varied inversely as the square of the distance, the attraction being 4 times as great at a distance of I centimetre as it was at 2 centimetres.

(237) Specific Induction.-It has been shown that the induction between two conducting plates, one of which is insulated while the other communicates with the earth, is facilitated by diminishing the thickness of the dielectric which separates them, and that the insulated plate is enabled to receive a higher amount of charge by reducing the number of particles of the dielectric

484

SPECIFIC ELECTRIC INDUCTION.

[237

which undergo polarization. It is evident from this circumstance that the polarization is attended with a certain amount of resistance. Faraday discovered that this resistance varies in amount with the material of the dielectric employed; some substances becoming polarized more readily than others. The relative facility of induction through the different bodies as compared with a common standard constitutes their specific inductive capacity. A plate of shell-lac, for example, of a centimetre in thickness, allows induction to take place across it twice as readily as does an equal thickness of atmospheric air, and sulphur with a facility equal to that of shell-lac.

The following Table represents, according to Snow Harris (Phil. Trans. 1842, 170), the specific inductive capacity of various bodies

FIG. 193.

S

The fundamental fact may be shown by the following simple experiment (fig. 193). About 1 inch, or 4 centimetres above the cap of a gold-leaf electroscope suspend an insulated disk of metal, and communicate a small charge to the insulated disk; the gold-leaves immediately diverge by induction. Between the disk and the electroscope substitute for the dielectric air, a body the specific induction of which is greater than that of air, such, for example, as a plate of shell-lac, s, an inch (25 millimetres) in thickness, and mounted on an insulating handle; the leaves will immediately diverge more widely, because induction towards the instrument takes place more freely; on removing the shell-lac the leaves of the electroscope return to their original divergence. The effect is precisely similar to that which would be produced by bringing the charged plate nearer to the electroscope in air. Similar phenomena occur

if a mass of sulphur or of resin is substituted for the shell-lac.

In good conductors no such polarization can be traced, and in imperfect conductors, such as spermaceti, the results become indistinct.

With gaseous bodies no difference in specific inductive capacity is found to exist; it is remarkable that the chemical nature of the gas has no influence; all gases having the same inductive capacity as common air. No variation in temperature, in density, in dryness, or in moisture, produces any change in this respect.

239.1

ELECTRICAL DISCHARGE BY CONDUCTION.

485

FIG. 194.

The apparatus with which Faraday investigated these curious phenomena was a kind of Leyden phial (fig. 194), consisting of two concentric metallic spheres, A, A, insulated from each other by a stem of shell-lac, B. Any dielectric could in succession be placed between the spheres, whether the subject of experiment were solid, liquid, or aëriform, as by connecting it with the air-pump by means of the stop-cock, s, it could be exhausted, and the interval filled with any gaseous medium, with the same facility as with a liquid (Phil. Trans. 1838, 9). Two of these jars having been prepared, a charge was given to one of them, after it had been filled with the body the inductive capacity of which was to be determined, and the charge was then divided with the second similar apparatus, in which the interval between the spheres was filled with air only. The charge in each case was measured by means of a carrierball and Coulomb's electrometer.

(238) Various Modes of Discharge.—We pass on now to consider the different modes in which the electric equilibrium is restored after it has been disturbed: this restoration may be effected in one of three ways, for the excited body may be discharged either by (a) conduction, by (b) disruption, or by (c) con

vection.

(239) a. Conduction.-When a charged Leyden jar is discharged in the usual way through a discharging-rod, the electricity passes quietly through the wire of the discharger by conduction, but traverses the interposed air by disruption, in the form of a spark attended with noise.

All bodies, shell-lac and glass not excepted, possess a certain amount of conductivity, which gives rise to the phenomenon termed the residual charge of a jar or battery. If a jar be charged strongly, and allowed to remain undisturbed for a few minutes, and then be discharged and again allowed to stand for a few moments, a slight apparent renewal of the charge will take place, and a second smaller spark may be obtained from it. This Faraday considers to be due to the penetration by conduction of a portion of the charge into the substance of the dielectric. Each surface of the glass acquires a weak charge, one of positive, the other of negative electricity; but as soon as the constraint which caused this penetration of the electricity is removed, it returns towards the nearest surface and produces the slight recharge, or residual charge.

As no bodies are perfect insulators, so none are perfect conductors, for even the metals offer a certain measurable resistance

486

LATERAL SPARK.

[239.

to the transmission of electricity. The following experiment will serve to illustrate this point. Charge a large Leyden jar (fig. 195), and arrange a metallic wire, w, from 120 to 150 feet, or

tricity takes the shorter course from a to b, and overcomes the high resistance of the stratum of air interposed between the balls, owing to the resistance experienced by the discharge to its passage along the continuous conducting wire, w.

This resistance, even in good conductors, often occasions the spark to pass between two contiguous conductors, and produces what has been called the lateral spark, which can be elicited, even if the conductors subsequently unite below. For example, in the arrangement shown in fig. 196, at the moment a spark FIG. 196.

passes from P to the ball, a, a minute spark will be seen to pass between the wire and the loop, b, if they be sufficiently near each other. This lateral spark may acquire sufficient power to ignite gunpowder or other combustible matter. In fact, momentary as is the duration of the discharge, induction takes place towards all surrounding objects whilst electricity is in motion, as well as when it is at rest.

If in a darkened room a thin insulated wire be made to terminate at each extremity in a metallic ball, and on one ball large sparks be thrown, whilst from the other ball the sparks are allowed to pass off to some contiguous conductor, the air will be seen to become feebly luminous from induction along the whole course of the wire every time that a spark passes.

(240) Development of Heat.-The passage of electricity through conductors is attended with evolution of heat, the amount of which