Not exactly part of the thread, but the principle is sound.....many years ago I helped develop the Ground Resistivity meters, used by Archaeologists to measure earth resistivity to help locate buried walls and back filled trenches, where the background resistance and the resistance change could be similar. My late friend Keith Pickard devised the "back off" method , where the moving coil meter had a voltage fed to one side, proportional to the resistivity between two probes, and the other side of the meter was fed with a switched DC "backoff" voltage, using an accurate DC source. The nominal range of the meter was 0 - 1Kohm, in 10 X segments of 100 R FSD .. we also had a X 10 range to measure 0 - 100R in 10R steps.
This could be easily scaled to very low resistances. Our instrument used a 4 wire method with a Constant current fed via 2 wires and the sensor was the other two. Since my involvement, over 40 years ago, electronic components have developed to make it very simple to scale in your application.
We used a constant current of 10m/A AC..... to overcome polarisation of the probes in the ground, and to give us a fighting chance of measuring the return AC voltage, which was synchronously rectified. Just an Idea
