Abstract: A method of testing a coin in a coin testing mechanism, comprising subjecting a coin inserted into the mechanism to an oscillating field generated by an inductor, measuring the reactance and the loss of the inductor when the coin is in the field, and determining whether the direction in the impedance plane of a displacement line, representing the displacement of a coin-present point which is defined by the measurements, relative to a coin-absent point representing the inductor reactance and loss in the absence of a coin, corresponds to a reference direction in the impedance plane. The reactance and loss measurements may be taken by a phase discrimination method.