For measuring P.Ds. of 2,000 volts, or higher, such as are now maintained between the underground mains with certain electric light systems, the leaves may be conveniently made out of lead foil instead of gold-leaf.

experimentally

56. No Force Inside a Closed Conductor Produced by Exterior Electrostatic Action.-This fact may be illustrated with the cage composed of wire gauze cc (Fig. 94) mounted on an insulating stand, which will be found described in detail at the end of § 65, page 216. Inside the cage are suspended one pair of pith balls by means of silk fibres which are fairly insulating, and a second pair by pieces of cotton, which is relatively a conductor. Outside the cage are suspended one or more pairs of pith balls by silk fibres or by pieces of cotton. If now the cage be electrified, the

Fig. 94.-Apparatus for Proving that the pith balls outside the

Electrostatic Force inside a Closed
Conductor is Nought.

cage will diverge as in the figure, but those inside give no evidence whatever of any electric force even when the cage is so highly electrified that sparks can be drawn from it. If the openings in the cage be very large, it may be possible to produce some slight effect inside by powerful outside electrostatic action, but if the meshes of the wire-gauze be very small, or, better still, if the sides of the metal box be continuous throughout, then no matter

how thin they may be, or how sensitive may be the apparatus for detecting electric force that is placed inside the box, no effect whatever can be produced on the apparatus by external electrification. A continuous metal box then, even if its sides be made of the thinnest gold leaf, completely screens bodies inside it from external electrostatic action.

If, however, a current be flowing along the surface of a long hollow conductor, then, whether or not the ends of the hollow tube be closed metallically so as to make it into a box, electric forces are exerted inside the tube.

57. Potential due to Exterior Electrostatic Action is Uniform at all Points inside a Closed Conductor.— This result follows from the conclusion arrived at in the last section, for, if it be impossible by means of external electrostatic action to produce a force on any measuring instrument placed inside a closed conductor, no voltmeter could give evidence of any P.D. inside the conductor produced by external electrostatic action. the potential at all points inside a conductor, subjected to outside electrostatic action, must be uniform, and have the same value that it possesses at a point on the surface of the conductor.

Hence

as

If the conductor has a hole in its surface large, for example, as the opening at the top of a coffeepot-the preceding will still be true, except for points in the space just inside the pot close to the opening, where the potential will differ somewhat from the uniform potential inside the pot. When, however, the surface of the conductor is as continuous as the wires of a meat-cover made of wire gauze, or even as continuous as the wires of a bird-cage, the potential is practically uniform at all points inside.

If then there be a number of electrical apparatus inside a metallic box, or inside a metallic room, some, or all, of the apparatus being connected with the box and some, or all, being insulated from it, the internal distribution of potential will be quite independent of the potential of the box relatively to the earth, and will remain quite

unaffected if the potential of the box be raised or lowered by outside action. Altering the potential of the box by outside action alters the potential of every object inside it by exactly the same amount, and, therefore, leaves the P.D. between any two objects inside the box unchanged.

The preceding only applies when the electric forces exerted on the box are produced by electricity at rest. For when a current is made to flow along a metal tube, the potential varies from point to point in the space inside the tube in the same way that it varies from point to point along the metal of the tube itself through which the current flows.

58. Voltmeters must be Enclosed in a Conducting Case. In order that a voltmeter may correctly measure the P.D. between the needle and the inductors, it is necessary that no electric force shall be exerted directly on the moving system by bodies external to the voltmeter. The needle and the inductors of an electrostatic voltmeter must, therefore, be surrounded by a screen constructed of conducting material. Such a screen can be constructed of strips of metal foil stuck fairly close together inside the glass shade which covers up the instrument (Fig. 93, page 192), and the screening can be improved by connecting the vertical strips together by horizontal strips of metallic foil, as in Fig. 95. When such a coated glass shade is used with a gold-leaf electroscope, it acts both as inductors and as screen.

A good test of the power of any such coated glass shade to act as an electrostatic screen consists in joining the terminals of the voltmeter together by a piece of wire, and bringing a highly excited ebonite rod close to the voltmeter, when, if the screening is practically perfect, no deflection of the moving system will be possible.

Nor if the screen acts well will a deflection of the moving system be able to be produced by placing a voltmeter, whose terminals are connected together in the way just described, on an insulating stand, and electrifying the voltmeter as a whole so highly with an "electrical

machine," that sparks can be drawn from any part of the voltmeter on approaching the finger. For the instrument measures the P.D. between its terminals, and this P.D. must be zero when the terminals are joined by a wire, whether the instrument be electrified or not.

It will be found, however, that when the metallic foil is stuck on the glass

the

shade, as indicated in Figs. 93 and 95, so that the moving system can be fairly well seen at a distance through openings between the strips, the screening action, although considerable, is by no means complete, and that when the area of the metallic coating becomes sufficiently large compared with the area of the glass as to render the screening practically perfect, there is considerable difficulty in seeing the moving

system sufficiently well to enable small changes in the deflection to be observed at a distance.

Mr. Mather and the author, therefore, experimented on methods of coating the whole of the interior of the glass shade with a transparent varnish that should be sufficiently conducting to act perfectly as an electrostatic screen, and yet hard enough that the inside of the glass could be cleaned when desired without risk of the var nish being rubbed off. And this, they find, can be satisfactorily accomplished in either of the following ways:

Method No. 1.--Dissolve ounce of transparent gelatine in 1 ounce of glacial acetic acid by heating them together in a water bath at 100°C. To this solution add half the volume of dilute sulphuric acid, which has been prepared by mixing 1 part of strong acid with 8 of distilled water by volume, and apply the mixture while still warm to the glass shade, which should be previously polished and warm. When this film has become very nearly hard apply over it a coating of Griffith's anti-sulphuric enamel, the chief ingredient of which is resin dissolved in fusel oil.

Method No. 2.-Thin the gelatine solution, prepared in the manner previously described, by the addition of acetic acid (say, 2 volumes of acid to 1 of solution), and after polishing the glass, float the thinned solution over the glass cold. Drive off the excess of acetic acid by warming, allow the glass to cool, and repeat the floating process, say, twice. Thin the anti-sulphuric enamel by the addition of ether, and float it over the gelatine layer applied as just described. Expel the ether by heating, and apply a second layer of this thinned antisulphuric enamel.

It is advisable to varnish the inside of the glass shades or glass fronts, not merely of electrostatic voltmeters, in one of the ways just described, but of current voltmeters, ammeters, or indeed of any instrument where the electrification of the glass produced by cleaning it on a dry day might cause a deflection of the pointer of the instrument a cause of error that has been noticed with electrical measuring instruments placed in hot dry engine-rooms of electric-light stations.

59. The Potential of a Conductor." The potential" of a conductor is an abbreviation for the P.D. between the conductor and the earth. To measure this we must connect one terminal of the voltmeter. by a wire to the conductor, and the other terminal to a system of gas-pipes, or better, to a system of water-pipes. If the potential of a body be high, it can be roughly