637. Unit-jar. For quantitative experiments on the effects of discharge, Lane's unit-jar has frequently been employed. One of its forms is represented in Fig. 396. It consists of a very small Leyden phial, having two knobs a, b, one connected with each coating, the distance between them being adjustable by means of a sliding rod. To measure the charge given to a jar or battery, the latter is placed upon an insulating sup

tween a and b, each spark being produced by the escape of a definite quantity of electricity from the outer coating of the battery, and indicating the addition of a definite amount to the charge of the inner coating. The charge is measured by counting the sparks.

Snow Harris modified the arrangement by insulating the unit-jar instead of the battery. One coating of his unit-jar is connected with the battery, and the other with the conductor of the machine. The battery thus receives its charge through the unit-jar1 by a succession of discharges between the knobs a, b, each representing a definite quantity of electricity.

Both arrangements, as far as their measuring power is concerned, depend upon the assumption that discharge between two given conductors, in a given relative position, involves the transfer of a definite quantity of electricity. This assumption implies a constant condition of the atmosphere. It may be nearly fulfilled during a short interval of time in one day, but is not true from one day to another. Moreover, it is to be remembered that, as dissipation is continually going on, the actual charge in the battery at any time is less than the measured charge which it has received.

1 Lane's arrangement might have been described by saying that the outer coating of the battery receives its negative charge from the earth through the unit-jar.

PUNCTURE OF CARD AND GLASS.

629

638. Mechanical Effects.-The effects of discharge through bad conductors are illustrated by several well-known experiments.

1. Puncture of card. A card is placed (Fig. 397) between two points connected with two conductors with are insulated from one another by means of a glass stem. The lower conductor having been connected with the outer coating of a Leyden jar which is held in the hand, the knob of the jar is brought near the upper conductor. A spark passes, and another spark at the same instant passes between the two points, and punctures the card. In performing this

experiment it is observed that, if the points are not opposite each other, the perforation is close to the negative point. This want of symmetry appears to be due to the properties of the air. When arrangements are made for exhausting the air, it is found that, as the density of the air is diminished, the perforation takes place nearer to the centre.

The piercing of a card can very easily be effected by Holtz's machine. Its two conductors are connected with the two coatings of a small Leyden jar. The discharges between the poles will then consist of powerful detonating sparks in rapid succession; and if a sheet of paper or card be interposed, every spark will puncture a minute hole in it.

2. Perforation of Glass. To effect the perforation of glass, a pane

of glass is supported on one end of a glass cylinder in whose axis there is a metallic rod terminating in a point which just touches the pane. Another pointed rod exactly over this, and insulated from it, is lowered until it touches the upper face of the pane. A powerful spark from a Leyden jar or battery is passed between the two points, and, if the experiment succeeds, a hole is produced by pulverization of the glass.

The experiment sometimes fails, by discharge taking place round the edge of the glass instead of through its substance. To prevent this, a drop of oil is placed on the upper face of the plane at the point where the hole is to be made; but this precaution does not always insure success, and, when the experiment has once failed, it is useless to try it again with the same piece of glass, for the electricity is sure to follow in the course which the first discharge has marked out for it.

639. Explosion of Mines.-If a strongly charged Leyden jar be discharged by means of a jointed discharger which has one of its knobs covered with gun-cotton, when the spark passes between the jar and this knob, the gun-cotton will be inflamed. Ordinary

FIRING MINES.

631 cotton mixed with powdered resin can be kindled in the same way..

A similar arrangement is often used for exploding mines. A fuse is employed containing two wires embedded in gutta percha, but with their ends unprotected and near together. One of these wires is

connected with the outer coating of a condenser, and the other is brought into communication with the inner coating. The discharge is thus made to pass between the ends within the fuse, and to ignite a very inflammable compound by which they are surrounded. Sometimes one of the wires, instead of being connected with the outer coating, is connected with the earth by means of a buried wire.

CHAPTER XLIX.

ELECTROMETERS.

640. Object of Electrometers.-Electrometers are instruments for the measurement of differences of electrical potential. The gold-leaf electroscope, the straw-electroscope, and other instruments of the same type, afford rough indications of the difference of potential between the diverging bodies and the air of the apartment, and more measurable indications are given by the electrometers of Peltier and Dellmann; but none of these instruments are at all comparable in precision to the various electrometers which have been invented from time to time by Sir Wm. Thomson.

641. Attracted-disc Electrometers, or Trap-door Electrometers.— We shall first describe what Sir Wm. Thomson calls "Attracted-disc Electrometers." These instruments, one of which is represented in Figs. 400, 401, contain two parallel discs of brass g, h, with an aperture in the centre of one of them, nearly filled up by a light trap-door of aluminium ƒ, which is supported in such a way as to admit of its electrical attraction towards the other disc being resisted by a mechanical force which can be varied at pleasure. The trapdoor and the perforated plate surrounding it must have their faces as nearly as possible in one plane when the observation is taken, and, as they are electrically connected, they may then be regarded as forming one disc of which a small central area is movable. There is always attraction between the two parallel discs, except when they are at the same potential.

Let their potentials be denoted by V and V', the electrical densities on their faces by p and p', and their mutual distance by D. We have seen (§ 617) that, in such circumstances, p and p' are constant (except near the edges of the discs), opposite in sign, and equal, and that the intensity of force in the space between them is everywhere