Measuring the current

A current has a size as well as a direction, and it has its own unit and its own instrument.

The ampere

The ampere (A) — the unit in which the size of an electric current is measured.

The instrument

The instrument used to measure the size of an electric current is called an ammeter. A digital multimeter can also be used to measure current.

Figure to be added

Figure 19.10 — (a) an analogue ammeter with a needle and a scale marked in amperes; (b) a digital multimeter, which can be set to work as an ammeter.

It goes in the path, not across it

To measure the current flowing along a conductor, the ammeter must be connected into the circuit so that the whole of that current passes through the ammeter itself.

Figure to be added

Figure 19.11 — an ammeter connected into a circuit with a battery, a switch and a bulb, wired so that the current that flows through the bulb also flows through the ammeter.

An ammeter is inserted, not attached

To put an ammeter into a circuit you have to break the wire and put the meter in the gap. Hanging it across a component measures nothing useful — that is how a voltmeter is connected, and you meet it two pages from now.

Exercise 19.1

The task

Mark, with arrows, the currents flowing in the circuits shown below.

Figure to be added

Figure for exercise 19.1 — two circuit diagrams, each with a cell: the first with a single lamp in one loop, the second with two lamps on separate branches.

Start at the positive terminal

To mark a current, find the cell's positive terminal and draw the arrow leaving it, then follow the wire round. Where the circuit branches, the arrows branch with it — which is the whole subject of the parallel skill later in this chapter.