Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/390



ELECTROMAGNETIC UNIT OF RESISTANCE.

On the Determination of the Resistance of a Coil in Electromagnetic Measure.

758.] resistance of a conductor is defined as the ratio of the numerical value of the electromotive force to that of the current which it produces in the conductor. The determination of the value of the current in electromagnetic measure can be made by means of a standard galvanometer, when we know the value of the earth's magnetic force. The determination of the value of the electromotive force is more difficult, as the only case in which we can directly calculate its value is when it arises from the relative motion of the circuit with respect to a known magnetic system.

759.] The first determination of the resistance of a wire in electromagnetic measure was made by Kirchhoff. He employed two coils of known form, $$A_1$$ and $$A_2$$, and calculated their coefficient of mutual induction from the geometrical data of their form and position. These coils were placed in circuit with a galvanometer, $$G$$, and a battery, $$B$$, and two points of the circuit, $$P$$, between the coils, and $$Q$$, between the battery and galvanometer, were joined by the wire whose resistance, $$R$$, was to be measured.

When the current is steady it is divided between the wire and the galvanometer circuit, and produces a certain permanent deflexion of the galvanometer. If the coil $$A_1$$ is now removed quickly