Measuring voltage with a voltmeter

The measuring instrument: the voltmeter

To measure an electrical voltage, a voltmeter is used. This instrument has two terminals, often labelled ‘V’ and ‘COM’ (or ‘+’ and ‘-’), which are connected to the two points in the circuit between which we wish to measure the voltage.

Shunt connection

Unlike the ammeter, which is connected ‘in series’ (the circuit must be opened to insert it), the voltmeter is connected in shunt (or ‘in parallel’) across the terminals of the circuit under test, without ever opening the circuit.

Steps for measuring the voltage across a light bulb:

1. Select the highest range if you do not know the expected value.
2. Connect one terminal of the voltmeter to one terminal of the lamp.
3. Connect the other terminal of the voltmeter to the other terminal of the lamp.
4. Read the displayed value, taking into account the unit of the selected range.

The multimeter

In practice, a multimeter set to the voltmeter position (symbol V with a straight line for direct current, or a waveform for alternating current) is often used.

Practical example

To check that a 4.5 V battery is not flat, connect the voltmeter directly to its two terminals (without switching on a lamp): if the display shows a value close to 4.5 V, the battery is in good condition; if it reads 2 V or less, the battery is flat.

Common pitfalls to avoid

• Never connect a voltmeter in series with the circuit: the reading will be incorrect and this may disrupt the circuit’s operation.
• Ensure you use the correct range: if the needle or display is saturated, select a higher range.
• Check that the multimeter is set to the ‘voltmeter’ position and not the ‘ammeter’ position, otherwise you risk damaging the device.