The input circuitry of a SQUID voltmeter system has been modified by using a shorted signal coil which acts as a low-pass filter with a cut-off frequency of less than 1 Hz. This enables the SQUID to be used under adverse noise conditions in order to measure voltages generated in an unscreened sample. The incorporation of a superconducting switch is also described which enables sample voltages to be isolated without changes in the applied conditions. The frequency response of the low-pass filter is measured in the magnetometer mode.