The ammeter: measuring current

A specialised instrument

To measure the intensity of a current, an ammeter is used. This is an instrument that directly displays a value in amperes (A) or milliamperes (mA), depending on the range selected.

How do you connect it?

Essential rule: the ammeter must always be connected in series in the circuit, i.e. directly in the current path, never in parallel. You must therefore open the circuit at the desired point and insert the ammeter between the two wires.

You must also observe the correct polarity: terminal A (often marked ‘+’ or ‘mA’) must be connected to the side of the circuit where the current comes from the positive terminal of the generator, and the other terminal to the side of the negative terminal.

Beware of short circuits!

Never connect an ammeter directly to the terminals of a generator without a load: this would cause a dangerous short circuit, as the ammeter allows a very high current to flow with virtually zero resistance.

Choosing the correct range

Before taking a measurement, select a range higher than the expected current, then adjust it if necessary. A range that is too low may damage the meter.

Some orders of magnitude

Device or phenomenon	Approximate current
Small torch (LED)	0.02 A to 0.1 A
Mobile phone charger	1 A to 2 A
Electric oven	10 A to 15 A
Lightning	several tens of thousands of A

Pitfalls to avoid

A common mistake is to connect the ammeter in shunt, like a voltmeter: this distorts the measurement and can damage the device. Remember: ammeter = in series, voltmeter = in shunt.