19 Aug
2004
19 Aug
'04
5:50 p.m.
Tony Duell wrote:
Before the availability of cheap electronic meters it was sort-of true.
Using a moving-coil meter as a meter (as opposed to a null detector)
you'd want to have a deflection near FSD for accuracy. Which would mean a
current of 50uA if you were lucky.
Yes, electrometers (using special valves [1] in the input stage) existed
back then, but were expensive and touchy to use. The coming of the FET
helped a lot :-)
[1] It is one of the worse-kept secrets that the ME1400 electrometer
pentode is nothing other than a selected EF39 run with 4V on the heater
The real probklem is that schools are stuck in the past (the
potentiometer wire used to be the common way of measuring voltage that
draws a minimal current so it must always be), they perpertuate myths
(like the ones about the 0 current at balance) and teachers seem to be
incapable of thinking (I almost failed a physics exam for the above
arguments!)
-tony
'A potentiometer-wire voltmeter is better
than a DMM because the former
draws no curret at balance'. This is more subtle. While it's true that a
potentiometer-wire voltmeter dwraws no curret at balance, you can never
know it's exactly at balance. You are limited by the sensitivity of the
detector you're using. And when that detector is a moving coil/pointer
(not a mirror) type of meter with a sensitivity of, perhaps, 1uA if
you're lucky, then a 10M ohm DMM actually takes less current from, say, a
Weston standard cell.
I heard that argument for the superiority of the potentiometer at
school, too. It would've been in about 1979. Wish I'd thought of