METHODS OF MAINTAINING AN ELECTRIC CURRENT

194.] The principal methods of maintaining a steady electric current are—

1. The Frictional Electric Machine
2. The Voltaic Battery
3. The Thermo-electric Battery
4. The Magneto-electric Machine

The Frictional Electric Machine

195. Here, electrification is produced between the surfaces of 2 different substances, such as:

• glass and amalgam
• ebonite and fur

By the motion of the machine one of these electrified surfaces is carried away from the other.

Both are made to discharge their electrification into the electrodes of the machine, from which the current is conveyed along any required circuit.

In the ordinary form of the machine a circular plate or a cylinder of glass is made to revolve about its axis. Let us suppose that the revolving part is a plate of glass. The rubber is fixed so that it presses against the surface of the plate as it rotates.

The surface of the rubber is of leather, on which is spread an amalgam of zinc and mercury.

By the friction between the glass and the amalgam, the surface of the glass becomes electrified positively, and that of the rubber negatively.

As the plate revolves the electrified surface of the glass is carried away from under the rubber, and another part of the surface of the glass, previously unelectrified, is brought under the rubber to be electrified.

As long as the oppositely electrified surfaces of the glass and the rubber remain in contact, the electrical effects in the neighbourhood are very small, but when the glass is removed from the rubber, strong electrical forces are developed.

The potential of the rubber becomes negative, and as, on account of the amalgam, it conducts freely its electrification is at once carried off to the negative electrode.

At the same time the potential of the electrified glass becomes highly positive, but as the glass is an insulating substance it does not so readily part with its electrification.

The positive electrode of the machine is therefore furnished with a comb, consisting of a number of sharp pointed wires terminating near the electrified surface of the glass.

As the potential at the surface of the glass is much higher than that of the comb there is a great accumulation of negative electrification at the point of the comb, and this breaks into a negative electric glow accompanied by an electric wind blowing from the comb to the glass.

The negatively electrified particles of air spread themselves over the positively electrified surface of the glass, and cause the electrification of the glass to be discharged. It is possible, however, that part of them may be carried round with the glass till they are wiped off by the rubber, though I have not been able to obtain experimental evidence of this.

Thus the rotation of the machine carries the positive electrification of the surface of the glass from the rubber to the comb.

The negative electric wind of the comb either:

• neutralizes the positively electrified surface, or
• is carried round with it to the rubber
  • In thi way, there is a continual current of positive electricity kept up from the rubber to the comb, or, what is the same thing, of negative electricity from the comb to the rubber, or, since the mode of expressing the fact is indifferent, we may, if we please, describe it as consisting of a positive current in the one direction combined with a negative current in the other the arithmetical sum of these two imaginary currents being the actual current observed.

The action of the machine thus depends on the electrification of the surface of the glass by the rubber, the convection of this electrification, by the motion of the machine, to the comb and the discharge of the electrification by the comb.

196.] The strength of the current produced depends on the surface-density of the electrification, the area of the electrified surface and the number of turns in a minute.

The electromotive force of the machine is the excess of the potential of the comb above that of the rubber. The most convenient test of the electromotive force of an electrical machine is the length of the sparks which it will give. During the passage of the electrified surface from the rubber to the comb it is passing from places of low to places of high potential, and is therefore acted on by a force in the direction opposite to that of its motion. The work done in turning the machine therefore exceeds that necessary to overcome the friction of the rubber, the axle, and other mechanical resistances by the electrical work done in carrying the electricity from the rubber to the comb.

At every point of its course the electricity on the surface of the glass plate is acted on by a force the value of which is measured by the rate at which the potential varies from one point to another of the surface. If this force exceeds a certain value it will cause the electrification to slide along the sur- face of the plate, and this will take place under the action of a much smaller force than that which is required to remove the electricity from the surface. This discharge along the surface of the plate may be seen when the electric machine is worked in a dark room, and it is evident that the electricity which thus flashes back is so much lost from the principal current of the machine. In order that the machine may work to the best advantage this slipping back of the electricity must be prevented. The slipping takes place whenever the rate of variation of the potential from point to point of the surface exceeds a certain value. If by any distribution of the electrification the rate of variation of the potential can be kept just below this value all the way from the rub- ber to the comb the electromotive force of the machine will have its highest possible value.

THE REVOLVING DOUBLER

In most electrical machines flaps of oiled silk are attached to the rubber so that as the plate revolves the electrified surface as it leaves the rubber is covered with the silk flap which extends from the rubber nearly up to the comb. These silk flaps become negatively electrified and therefore adhere of themselves to the surface of the glass. If in any part of the revolution of the plate, the rate of increase of the potential is so great that a slipping back of the electrification occurs, the positive electricity which so slips back neutralizes part of the negative electrification of the silk flap and so raises the electric potential just behind the place where the slipping occurred. In this way the slope of the electric potential is equalized and the electromotive force of the machine is raised to its highest possible value, so as to give the longest sparks which a machine of given dimensions can furnish.

When the silk flaps are removed the slope of the potential becomes much greater close to the rubber than at any other place, the electricity slips back on the glass just as it leaves the rubber and very little electricity, and that at a comparatively low potential, reaches the comb. In the best machines, in which the slope of the potential is uniform from the rubber to the comb, the length of the spark must depend principally on the distance between the rubber and the comb. Hence a machine which, like Winter’s, has the rubber and the comb at opposite extremities of a diameter of the plate will give a longer spark than one from a machine whose plate has the same diameter but which like Cuthbertson’s has two rubbers and two combs, the distance between each rubber and its comb being a quadrant.