Updated February 11th 2009

How does an ammeter work?

First some words about electricity. If electricity is compared to water, then the current can be compared to the flow whereas voltage compares to the pressure. If the current increases, usually the voltage decreases and vice versa.
An ammeter could be called a current flow meter and the voltmeter could be an electricity pressure meter.

In an ammeter, the current passes through a set of coils, which causes rotation of a central armature. Attached on this armature, an indicator needle shows the amount of deflection against a graduated scale. Ammeters migh be designed to have a full-scale deflection as small as a few microamperes but as for automotive use, up to several amperes.
To extend the range of an ammeter, allowing it to register very large currents, a resistor of precisely determined value is placed in parallell with the coils of the meter. This resistor diverts much, or most, of the current so that the meter actually reads only a fraction of the current.
Ammeters might be used as voltmeters by placing a resistor in series with the meter coils. Then, even a very high voltage will cause a small deflection of the needle. Thre greatest accuracy is obtained when a sensitive ammeter is used with a large-valued resistor. Then, the current drawn from the circuit is minimized. Ammeters should never be connected across a source of a voltage without a series resistor because the meter mechanism might be damaged as a result.
Ammeters are available for measuring both ac and dc but for automotive used, only dc is used.

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