The law of the moving-iron instrument

The law of the moving-iron instrument has been quoted in the form: Torqu=¿I2dL/d¿, where I is the current in the coil and d/d¿ the rate of change of inductance with angular deflection. This law is deduced on the assumption that the current remains constant for a small change in deflection, but since a change in inductance is produced by a change in current, it does not seem justifiable to assume the current to be constant. The law then takes a more complicated form. Experiments are described showing that the simple law does not hold for a simple electromagnetic system.