The paper deals mainly with an inverstigation of the dependence on the exciting peak voltageVand the currentIof the output of a hot-cathode X-ray tube as measured by a “free-air” ionization chamber or by a photographic method. The relation may be expressed in the formOutput ∝IVnIn the case of two types of commercial X-ray tubes (Goolidge and Metalix) when excited by constant voltages ranging from 65 to 150 kV,nis found to increase from unity for unfiltered radiation from a tube with a thin window, to between 4 and 5 or more for heavily filtered radiation. The various types of valve rectification circuits suitable for X-ray work, and the influence of voltage and current waveform, are discussed. It is shown that for like values of peak voltage and milliammeter readings, the X-ray outputs for full-wave and half-wave rectification are the same over the voltage range 40 to 80 kV, while the output for constant voltage is about 1.8 times larger for unfiltered radiation.The various types of value rectification circuits suitable for X-ray work, and the influence of voltage and current waveform, are discussed. It is shown that for like values of peak voltage and milliammeter readings, the X-ray outputs for full-wave and half-wave rectification are the same over the voltage range 40 to 80 kV, while the output for constant voltage is about 1.8 times larger for unfiltered radiation.From an X-ray point of view it is concluded that the essential factors of voltage excitation are the peak value and the wave-form. The several methods of measuring high voltages are dealt with. As regards tube currents, the moving-coil milliammeter customarily employed only indicates the mean current and not the maximum current through an X-ray tube excited by pulsating voltage. It is concluded that while the moving-coil milliammeter is adequate for the measurement of sustained tube currents, it is better to turn to some form of ballistic meter for currents lasting less than about 1 second.