Theory of the force or torque of soft-iron electrical instruments

The inductance formulae F = ¿I2¿L/¿x and T = ¿I2¿L/¿¿ are shown to be correct for iron-cored instruments if the magnetization of the iron does not exceed the maximum value permissible for accurate indication. For higher magnetizations the force or torque is higher than the value given by the formulae, and becomes twice as great when the iron approaches saturation.