Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/453

is independent of the resistance in $OA$. Hence, if the galvanometer is placed in $CA$ its deflexion will remain the same whether the resistance of $OA$ is small or great. We therefore observe whether the deflexion of the galvanometer remains the same when $O$ and $A$ are joined by a conductor of small resistance, as when this connexion is broken, and if, by properly adjusting the resistances of the conductors, we obtain this result, we know that the resistance of the galvanometer is

$b={\frac {c\gamma }{\beta }}$

where $c,\,\gamma$, and $\beta$ are resistance coils of known resistance.

It will be observed that though this is not a null method, in the sense of there being no current in the galvanometer, it is so in the sense of the fact observed being the negative one, that the deflexion of the galvanometer is not changed when a certain contact is made. An observation of this kind is of greater value than an observation of the equality of two different deflexions of the same galvanometer, for in the latter case there is time for alteration in the strength of the battery or the sensitiveness of the galvanometer, whereas when the deflexion remains constant, in spite of certain changes which we can repeat at pleasure, we are sure that the current is quite independent of these changes.

The determination of the resistance of the coil of a galvanometer can easily be effected in the ordinary way of using Wheatstone's Bridge by placing another galvanometer in $OA$. By the method now described the galvanometer itself is employed to measure its own resistance.

Mance's^[1] Method of determining the Resistance of the Battery.

357.] The measurement of the resistance of a battery when in action is of a much higher order of difficulty, since the resistance of the battery is found to change considerably for some time after the strength of the current through it is changed. In many of the methods commonly used to measure the resistance of a battery such alterations of the strength of the current through it occur in the course of the operations, and therefore the results are rendered doubtful.

In Mance's method, which is free from this objection, the battery is placed in $BC$ and the galvanometer in $CA$. The connexion between $O$ and $B$ is then alternately made and broken.

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