The other wonderful technology of yesteryear is the differential voltmeter. Fluke made several types of these, and were the precursor to the DVM. The big advantage is that they are null reading, and when nulled the input resistance is infinite, limited only by leakage paths - so in practice 10^10 to 10^11 ohms.
See for example
https://www.radiomuseum.org/r/fluke_...eter_893a.html