A cheap and compact 100-megohm resistor for the measurement of direct or alternating voltages of power frequency may be constructed from thin-film resistance units similar to those commonly used as “grid leaks”. The construction and performance of such a resistor are described. Errors due to corona or leakage along supports appear to be absent. There is a change of resistance with loading, due to the self-heating of the resistor. The magnitude of this change is approximately proportional to the square of the applied voltage and amounts to −2.6 per cent at 100 kV. By taking this change into account, voltages up to 100 kV may be measured correctly to 1 or 2 parts in 1000.