On Methods of Varying the Sensitiveness of Ballistic Galvanometers

Methods of changing the sensitivity of ballistic galvanometers in a definite ratio by adding series resistance.—On closed circuit, the added series resistance does not change the throws in the inverse ratio of the circuit resistances because of the simultaneous change in electromagnetic damping. However, if the added resistance $r_2$ required to reduce a given throw to half value is determined by measurement so that $\frac{r_2}{G}=n_2$ (where $G$ is the original total resistance of the galvanometer circuit) is known, then $n_5$ and $n_{10}$ and the corresponding resistances $r_5$ and $r_{10}$ required to reduce the throw to one-fifth and one-tenth, can be obtained from a table which is given. A slight amount of air damping does not affect the results, but for galvanometers with a special damping coil this table does not hold. The complication due to the change of the damping may, however, be readily eliminated if a special switch is used to open the galvanometer circuit immediately after the induced charge has passed. In this case even with strongly damped galvanometers, experiment shows that the deflections are inversely proportional to the total resistance of the galvanometer circuit.