An electrometer valve, combined with an ordinary pentode or triode valve, both acting as cathode followers, can provide a valve voltmeter of very high input impedance, low input capacity and of wide range. By using two further ordinary pentodes as "tail" valves the range can be extended to ±500 V without difficulty and with an input to output magnification of 0.99. The input grid current is not increased by the wide range, while the presence of a very closely following output facilitates guard ringing and the reduction of input leakages and capacities. The arrangement will have several uses and has been especially designed for use for measurements of the atmospheric potential gradient using a radioactive collector, where it is more convenient and more rapid in response than most other methods.