SIMPLE NANOAMP METER
You may already own one
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
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
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
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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