On 13 Mar 1997, Trevor Boicey wrote:
> Remember, an ammeter is simply a voltmeter in a special
> application. Current flowing across a resistance creates
> a voltage drop across the device.
Yes and no.
Yes: You are describing one type of ampere-meter. Which measures the
voltage drop (in the millivolt range) across a shunt resistor.
And no: The type of ampere-meter usually found in cars DOES NOT work this
way. Instead, ALL the current to/from the battery runs through the
instrument (except for the starter current). The magnetic field set up by
that current (a coil with one turn, if you like) directly influences a
small magnet connected to the needle itself. A very inexpensive, robust
and reliable type of instrument, somewhat inaccurate. Perfect for cars!
What type of instrument a TR4 would have I don't really know, although all
I would certainly place my bet on the latter type. But remember that
running all current on these cars does NOT involve man sized cables. The
max. current is well below 30 amps, and the original wiring is probably as
thin as they could get away with.
Egil
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