Galvanometer Resistance and Linearity

The values of galvanometer resistance which are obtained by the two half-deflection methods (constant-voltage versus constant-current sources) are often found to differ because of nonlinearity in the galvanometer response. It is shown that the correct value is the geometric mean of the results of the two methods, if the initial deflections are the same in the two cases; also, the galvanometer resistance equals that value of the variable resistance in the two methods which gives the same second deflection. A new method which resembles the Kelvin method of obtaining the resistance of a galvanometer is reported. The source of the nonlinearity is apparently in the torsional response of the galvanometer coil suspension and not in a lack of uniformity in the magnetic field through which the coil turns.