In mounting a moving coil to give a variation of 1 per cent. or so on a mutual inductance, it is necessary to determine at least two quantities - the length of the arm carrying the moving coil and the separation of its axis of rotation from the axis of the fixed coil. A simple method is described for the simultaneous determination of these quantities, only one preliminary set of experimental results being necessary. A simple reciprocal property is pointed out which leads to an improvement in mechanical design.