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1998 W. Beaty

Common digital voltmeters (DVMs) can measure nano-amperes. How?!! Just misuse them: use their voltage setting to measure current.

My DVM (digital voltmeter) has a 200 microamps setting, but some sorts of electrostatic effects deal with currents far below 1uA. My old 20uA panel meter is better for this, but sometimes I want to see things which barely budge its needle. I discovered a setting on my DVM meter which is 10,000 times more sensitive! The 200mV range on my DVM is also a 20 nanoamp current meter.

I tested this idea by putting a 20uA panel meter in series with my DVM, then setting the DVM to 200mV (volts, not amps!). small current which sends the voltmeter to 200mV reading will move the 20uA meter slightly. I estimate that the 20uA panel meter is indicating about 1/50 of a microamp (20 nanoamps), while the digital voltmeter reads "200". Aha, the DVM has a 10meg input impedance, so if its voltage range is instead used as a current meter, the 200mV range is the same as a 20 nanoamp range. So if the DVM indicates 1mV, it is actually measuring 100 picoamperes!

Does anyone have good ideas for applications for a digital meter with a 20nA full scale range? Maybe use it along with a 100v DC supply to make an ohmmeter with a full scale range of 5 giga-ohms, then use it to measure the resistance of wood, cloth, plastic, etc. Maybe we could detect the current which goes through thin glass. Or the nano-ammeter could be used alone to sense the air ions from a VandeGraaff machine that's on the far side of the classroom. Or take the meter outside, connect it to a big sheet of foil supported by insulators, and try to detect up the current/m^2 sky current? Oooo, if those Radio Shack DVMs with the RS-232 outputs can sense picoamps, then we could GRAPH the sky current over 24 hours, or watch pulses of ions drift across the classroom as the VDG was turned on and off, etc.

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