David Lylis wrote:
> With nothing left to lose, I took the gauge apart and
> sprayed the inside with carb cleaner and blew it dry.
> It is working again.
An ammeter is a strange sort of beast. In fact, there is only one kind of
meter, a galvanometer, and it responds to current. It is usually built to have
as low a resistance as possible, and to expect very low current. A voltmeter
is made by putting a known high resistance in series with the galvanometer, and
calibrating the markings such that a given voltage produces some particular
small current that produces a given deflection on the meter. End of story,
more or less. The resistance is chosen to be high so that very little current
is carried, thus not burning out the meter and also so that it draws no
significant extra current on the circuit it is measuring.
An ammeter is a step more complicated. It must carry all the current of the
circuit being measured, but it must do so without adding significant resistance
so as not to cause a voltage drop. But it can't be just a pure conductor
because that would mean there is nothing for a galvanometer to detect. So it
needs a voltage drop, but a very tiny one. To that end, it places a known,
very low but non-zero resistance wire in series with the circuit. All current
going into one side of the meter must pass through that wire to get out the
other side. The resistance of that wire produces a small voltage drop from one
end of the wire to the other. A voltmeter, i.e. a galvanometer in series with
a large resistor, is placed in parallel with that low-resistance wire so as to
measuree that voltage drop.
The point of all this description is that an ammeter could fail in two ways.
If one of the terminals was broken or making a poor connection, or if the
low-resistance wire was broken, it would look like an open circuit and pass no
current at all. With this failure mode nothing in that part of the wiring
would work at all because no current would get through.
The other failure mode is that it carries current just fine but doesn't seem to
measure anything. This could be because something, perhaps water, is providing
a short between the main terminals. It wouldn't do much for the overall
conductivity of the meter (which must be as high as possible, but still have
some known small resistance), but it might be enough to make the internal
galvanometer have no voltage drop to detect. However the same failure mode
could be caused by a broken wire within the voltmeter section, i.e. within the
galvanometer or its associated series reistor. Or instead of a broken wire it
could just be a poor connection in the voltmeter section. Just the act of
taking it apart and re-assembling it could fix a problem like this.
--
Jim Muller
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