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Re: Nanoamp meters we already own

Original poster: "Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>

Tesla list wrote:
Original poster: William Beaty <billb@xxxxxxxxxx>

DIGITAL NANOAMP METER 1998 W. Beaty 

Interesting idea. The meter could have this scale, but maybe someone
imagined that it would be useless... Or simply the precision can be low.

I use a simple technique to measure resistances of materials as wood,
paper, glass, and even some plastics, using only low voltages.
It is simply a common operational amplifier with the + input grounded
trough a 10 M resistor and the - input connected to the output trough
another 10 M resistor. The resistors may have capacitors across them
to reduce noise and interferences, and the offset adjustment of the
amplifier may be used for more sensitive measurements. If I connect the
material to be measured between a -10 V supply and the - input of the
op. amp., a  tiny current flows. But this tiny current produces an
easily measurable voltage, at low impedance, at the output of the op.
amp.
With the values mentioned (-10V, 10M) The output voltage is:
Vout = 10*10M/Rx
Or: Rx=100/Vout MOhms
1 V means 100 MOhms
100 mV means 1000 MOhms
10 mV means 10000 MOhms

Antonio Carlos M. de Queiroz