The Ammeter

The ammeter measures the current flowing through a branch of a circuit. Since it must “count” all the charge passing through, it must be inserted in series with that branch: all the current to be measured must pass through it, with no way round.

Principle — Ideal ammeter

An ideal ammeter has zero internal resistance: inserting it into the circuit does not alter the current being measured in any way.

Real instruments, however, have a small internal resistance $r_A$. Being in series, this adds to the resistance of the branch and lowers the current slightly compared with the value there would be without the instrument: $i_\text{misurata} = \frac{\Delta V}{R + r_A} < \frac{\Delta V}{R}$

To avoid “disturbing” the measurement too much, the resistance of the instrument must be negligible compared with that of the branch: $\ev{r_A \ll R}$

Why in series

In series the current is the same in every element: the ammeter reads exactly the current of the branch. An ammeter should therefore be kept at low resistance — the opposite of what is needed for a voltmeter.

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Topics: Circuiti elettrici Concepts: Corrente elettrica

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