Measuring the voltage

Voltage has its own instrument, and it is connected in the opposite way to the ammeter you met two pages ago.

Figure to be added

Figure 19.12 — a voltmeter: a meter with a needle over a scale marked in volts, with a large V on its face.

The instrument used to measure voltage is called a voltmeter. A digital multimeter can also be used to measure voltage.

Across the two points

To measure the potential difference between two points in a circuit, the two terminals of the voltmeter are connected to those two points. The circuit is not broken to fit it.

Figure to be added

Figure 19.13 — a voltmeter connected to a circuit of battery, switch and bulb, its two terminals joined across the bulb so that it reads the potential difference between the bulb's two ends.

The two meters, side by side

Ammeter — in the path

Break the wire and put the meter in the gap, so the whole current goes through it.

Voltmeter — across the gap

Leave the circuit intact and touch the two terminals to the two points, so it reads between them.

Through for current, across for voltage

An ammeter goes in series — the current must pass through it. A voltmeter goes across the component whose potential difference you want. Swapping them is the most common wiring mistake in this chapter.