A simple method of testing diode valves for passing no reverse current at voltages up to 20,000 is described. The anode of a standard valve, known to pass no reverse current, is connected to that of the valve under test, and the filaments are joined to the terminals of a high-tension alternating voltage supply. A low-reading electrostatic voltmeter, with parallel condenser, is arranged to indicate if the valve under examination passes reverse current.A description of a method of using a diode valve to measure the decrement of a highly damped oscillation is given. This consists in arranging the valve connections of the circuit so that the first positive peak is obtained. The valve connections are then reversed, and the first negative peak is found. This method is applied to the measurement of the highly damped oscillations which occur after the primary current is broken in a magneto.