19 Aug
2004
19 Aug
'04
12:21 p.m.
Tony Duell wrote:
'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
the counter-argument about the sensitivity of the meter, but I
didn't think of it until many years later.
--
John Honniball
coredump(a)gifford.co.uk