A highly sensitive method of measuring alternating currents of the order of microamperes and of any frequency is described. The current to be measured is passed through the filament of a diode valve. The main heating current of this valve is supplied by an independent oscillator working at the frequency of the current under measurement. The change in the plate current of the valve is measured, thus indicating the amplitude of the alternating current being measured. Theoretical and practical details of the methods of working are given and particulars of the method by which the calibration was tested. It is suggested that the method is particularly applicable to the determination of electro-magnetic field strengths by measuring the currents induced in an aerial.