Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/299

 electrification capable of affecting his electrometer. Instruments on the same principle have been invented independently by Mr. C. F. Varley, and Sir W. Thomson.

These instruments consist essentially of insulated conductors of various forms, some fixed and others moveable. The moveable conductors are called Carriers, and the fixed ones may be called Inductors, Receivers, and Regenerators. The inductors and receivers are so formed that when the carriers arrive at certain points in their revolution they are almost completely surrounded by a con ducting body. As the inductors and receivers cannot completely surround the carrier and at the same time allow it to move freely in and out without a complicated arrangement of moveable pieces, the instrument is not theoretically perfect without a pair of re generators, which store up the small amount of electricity which the carriers retain when they emerge from the receivers.

For the present, however, we may suppose the inductors and receivers to surround the carrier completely when it is within them, in which case the theory is much simplified.

We shall suppose the machine to consist of two inductors $$A$$ and $$C$$, and of two receivers $$B$$ and $$D$$, with two carriers $$F$$ and $$G$$.

Suppose the inductor $$A$$ to be positively electrified so that its potential is $$A$$, and that the carrier $$F$$ is within it and is at potential $$F$$. Then, if $$Q$$ is the coefficient of induction (taken positive) between $$A$$ and $$F$$, the quantity of electricity on the carrier will be $$Q(F-A) $$.

If the carrier, while within the inductor, is put in connexion with the earth, then $$F = 0$$, and the charge on the carrier will be $$-QA$$, a negative quantity. Let the carrier be carried round till it is within the receiver $$B$$, and let it then come in contact with a spring so as to be in electrical connexion with $$B$$. It will then, as was shewn in Art. 32, become completely discharged, and will communicate its whole negative charge to the receiver $$B$$.

The carrier will next enter the inductor $$C$$, which we shall suppose charged negatively. While within $$C$$ it is put in connexion with the earth and thus acquires a positive charge, which it carries off and communicates to the receiver $$D$$, and so on.

In this way, if the potentials of the inductors remain always constant, the receivers $$B$$ and $$D$$ receive successive charges, which are the same for every revolution of the carrier, and thus every revolution produces an equal increment of electricity in the receivers.