The Electric Glow.

142.] It can be proved by the mathematical theory of electricity that if a conductor having on its surface a sharp conical point is placed in a perfectly insulating medium and electrified, the surface-density of the electricity will increase without limit for points nearer and nearer to the conical point, so that at the conical point itself the surface-density, and therefore the electro- motive force, would be infinite. But this result depends on the hypothesis that the air or other surrounding dielectric has an invincible insulating power, which is not the case, and therefore as soon as the electromotive force at the conical point reaches a certain limiting value the insulating power of the air gives way, and there is a disruptive discharge of electricity into the air. A small portion of air close to the conical point thus becomes electrified. The electrified system now consists of the metal conductor with its conical point, together with a rounded mass of electrified air, which covers the point and acts as a sort of sheath to it, so that the boundary of the electrified system is no longer pointed.

This electrified system, if it were solid, would retain its charge, for the electromotive force is not great enough at any place to produce disruptive dis- charge, but since the air is fluid, and since the electromotive force is greatest in the line of prolongation of the conical point, the electrified particles of air move off in that direction. When they are gone other unelectrified particles take their place round the point, and the point being no longer protected by electrified air, there is another discharge, as at first. Thus there is continually kept up an influx of uncharged air to the point, a luminous discharge of electricity from the point, called the Electric Glow, and a stream of charged air in the direction of the prolongation of the axis of the cone called the Electric Wind. By checking the influx of air behind the point we may weaken the glow and by increasing the current of air byTHE ELECTRIC WIND.

blowing we may make the glow stronger.

143.] The electric wind which blows from the conical point may be made to drive a little windmill, or if the conductor is made of two wires crossed and having their sharpened ends bent backwards, as in Fig. 32, and supported so as to be capable of rotating, the reaction of the electric wind will make the cross rotate in the direction of the arrows. It is only close to the electrified point that the motion of the electrified air is in any de- gree influenced by its electrification. At a short distance from the point the electrified air becomes mixed with other air, and is car- ried about by the ordinary currents of the at- mosphere as an invisible electric cloud.

Fig. 32.

If we calculate the force due to the electrification of a large body of air at a consid- erable distance from other electrified bodies, we shall find that it is not capable of producing effects on the motion of so large a mass which are at all comparable to the effects of the slight variations of density and other causes which produce the movements of the atmosphere. Hence the motion of thunder clouds is due almost entirely to atmospheric cur- rents and is not sensibly affected by their electrification.

144.] When an electrified portion of air comes near the surface of a con- ductor, it induces on that surface an electrification opposite to its own and is attracted towards the surface, but since the electromotive force is small the electrified particles may remain for a long time in the neighbourhood of the conductor without being drawn into contact with it and discharged.

To detect the presence of this electrified atmosphere clinging to a conductor we have only to insulate the conductor and connect it with an electrometer. If we now blow away the electrified air from its surface, the electrometer will indicate the electrification of the conductor itself, which is of course of the opposite kind to that of the electrified air. Thus, if we hold in the hand a hollow metal cylinder over an electrified point, we may electrify the air within it. If we then place it on an insulated stand in connexion with the electrometer, the electrometer will remain at zero till the electrified air is removed, which may be done by blowing air through the cylinder. The electrometer will then indicate the electrification of the cylinder, which is of the opposite kind from that of the electrified air.

ACTION OF POINTS

145.] The glow is more easily formed in rare air than in dense air, and more easily when the point is positive than when it is negative. This and many other differences between positive and negative electrification seem to depend upon a condition analogous to electrolytic polarization in the stratum of air in contact with the electrode. It appears that the electromotive force re- quired to cause an electric discharge to take place is somewhat smaller where the electrode at which the discharge begins is negative, but that the quantity of electricity in each discharge is greatest when the electrode at which the discharge begins is positive.

146.] A fine point may be used instead of a proof plane to determine the nature of the electrification of any part of the surface of a conductor when electricity is induced upon it in presence of another electrified body. For this purpose the point is fixed to the conductor so as to project a few millimetres from its surface. If the part of the surface to which it is fixed is electrified positively the point gives off positive electricity to the air, and the conductor loses positive electricity or gains negative electricity. This may be ascertained either by removing or discharging the inductor and ascertaining the character of the charge of the induced body, or by connecting the induced body with the electrometer and observing the change of potential as the point throws off its electricity.

It has been found that some vegetable thorns, prickles, or spines act more perfectly in throwing off electricity than the finest pointed needles which can be procured. The action of the point may be assisted by blowing air from a blowpipe over the point, and in this way we may prevent the electrified air from dis- charging itself on the surface of the inductor. The Electric Brush. 147.] The electric brush is a phenomenon which may be produced by elec- trifying a blunt point or a small ball in air so as to produce an electric field in which the tension diminishes as the distance from the ball increases, but not so rapidly as in the case of a sharp point. The brush consists of a suc- cession of discharges, ramifying as they diverge from the ball into the air, and terminating either by charging portions of air or by reaching some other conductor. The brush produces a sound, the pitch of which depends on the interval between the successive discharges, and there is no current of air as in the case of the glow.

DISRUPTIVE DISCHARGE