A high-tension supply suitable for all types of Geiger counter must supply up to 5000 volts, free from A.C. ripple and other electrical disturbances, and it must be possible to rely on the voltage remaining constant to within one or two parts in a thousand over considerable intervals of time. With the apparatus here described these requirements have been fulfilled sufficiently well for satisfactory operation of Geiger-Müller tube-counters from ordinary A.C. mains, and since the method of keeping the voltage constant does not appear to have previously been used for this purpose, it seemed desirable to record it briefly.