This is a 20 Tera Ohm Insulation Tester I made around 20 Years Back. It had even Polarization Index for Transformer Insulation. It generated 1000V DC Regulated Voltage and the pA leakage Current was Measured. See more here Insulation tester or Tera ohm meter.
A regulated, low ripple, high voltage of very low current capability is applied by instrument on a DUT (device under test). The leakage current in nA or pA (picoamps, nanoamps) is measured and readout, The Innovation i did was to measure the current in Vref and not Vin of ICL7107, this made the 1/x computation.
Also using the ICL7650 Chopper Stabilized amplifier must be done as shown in data sheets with grounded guard rings. pA will leak on the PCB itself if layout is not done keeping this in mind.