Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/452

If $S$, the capacity of the condenser, is given in electrostatic measure as a certain number of metres, then $R$ is also given in electrostatic measure as the reciprocal of a velocity.

If $S$ is given in electromagnetic measure its dimensions are ${\frac {T^{2}}{L}}$ and $R$ is a velocity.

Since the condenser itself is not a perfect insulator it is necessary to make two experiments. In the first we determine the resistance of the condenser itself, $R_{0}$, and in the second, that of the condenser when the conductor is made to connect its surfaces. Let this be $R'$. Then the resistance, $R$, of the conductor is given by the equation

${\frac {1}{R}}={\frac {1}{R'}}-{\frac {1}{R_{0}}}$.

This method has been employed by MM. Siemens.

Thomson's^[1] Method for the Determination of the Resistance of the Galvanometer.

356.] An arrangement similar to Wheatstone's Bridge has been employed with advantage by Sir W. Thomson in determining the

Fig. 36.

resistance of the galvanometer when in actual use. It was suggested to Sir W. Thomson by Mance's Method. See Art. 357.

Let the battery be placed, as before, between $B$ and $C$ in the figure of Article 347, but let the galvanometer be placed in $CA$ instead of in $OA$. If $b\beta -c\gamma$ is zero, then the conductor $OA$ is conjugate to $BC$, and, as there is no current produced in $OA$ by the battery in $BC$, the strength of the current in any other conductor

1. Proc. R. S,, Jan. 19, 1871.