OK. It’s a little modern for Leeds but the 610C does the impossible: it measures resistance, voltage, charge (in coulombs) and current down to 100 pico-amperes full scale – that’s .0001 billionths of an ampere. A wonderful and versatile instrument. A dozen years ago I used a 610C to study the grid current of common vacuum tubes. I was motivated by the rather informal limits listed on common tube testers. It turns out that “modern” output tubes (6L6, 6550, etc) have grid leakages (gas current + grid emission) of around 1/3 of a micro-amp per watt of dissipation. Older ones (211, 845 etc) have up to 1 micro-amp per watt.
Eye-opening considering that the Hickok 118A/B and many other common tube testers reject at 3 micro-amps. Clean calibrated 610C’s go for $1000-$2000.