The paper describes a thermionic voltmeter the action of which depends upon the emission of secondary electrons from the anode of a new type of valve. The capacity of the instrument and its power factor are exceedingly small, so that it is very suitable for the measurement of high potentials at high frequencies.The voltmeter has a straight line calibration within the limits investigated by the writer, the gradient of the line being independent of the frequency of the alternating potential. The intercepts of the line on the axes are, however, linear functions of the frequency. The readings of the instrument depend upon the wave-form of the potential. It may thus be used to compare the "peak factors" in an oscillatory circuit at different times.