540B Thermal Transfer Standard

This instrument is used to compare the RMS value of an AC signal with an equivalent DC signal. The comparison is conducted using a thermal element and there are various voltage ranges available, up to 1000 V.

These were standard equipment for many years but have now been largely superseded by multimeter-style devices that offer better accuracy and ease of use.

This one might work, if the NiCd cells haven't given up and if you could still obtain mercury cells.

There is an external input for use with high frequency converters and another for AC/DC current shunts .