Learning to use voltmeters

Demonstration

An introduction to what voltmeters measure and how they are connected in circuits.

Apparatus and materials

Cells, 1.5 V, with holders, 3

Lamps with holders, 3

Demonstration voltmeter (0-5 V)

Digital multimeter with multiple voltage ranges (OPTIONAL)

Digital and analogue voltmeters with varying ranges (OPTIONAL)

Health & Safety and Technical notes

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a 'short circuit'. This discharges the cell rapidly and can make it explode. The risk is reduced by using 'low power', zinc chloride cells not 'high power', alkaline manganese ones.

Procedure

a Connect three cells in series. (Don’t complete the circuit.)

b Attach two leads to the demonstration voltmeter of a different, distinctive colour, e.g. green.

c Connect the meter, reading 0-5 volts, first across one cell, then across two, then across three. Show that the meter reading increases in equal steps – the meter is ‘counting the cells’. (You might wish to mark the meter face to indicate ‘1 cell’, ‘2 cells’, ‘3 cells’.)

d Now connect three lamps in series. Connect one cell across the three lamps - the demonstration meter should read approximately ‘1 cell’. Repeat with two and three cells.

e Finally, with three cells and three lamps, make readings across one, two and then three lamps to show how the voltage of the cells is shared between the lamps when they are in series.

Teaching notes

1 As a first introduction to the voltmeter there is no need to define the volt. Instead use it like a ruler or a watch is first used - without defining the metre or the second. Eventually the volt will be used as a measure of the energy which a cell is able to provide. (A potential difference in volts is defined from the energy given to each coulomb of charge flowing between the two points in the circuit, where the voltmeter is connected.)

You could start by introducing the cell as a 'supplier of energy' to the circuit. This may help students to understand why the current is not used up and so is the same all round a series circuit. (The current transfers energy to the lamps and hence warms up the surroundings.) The energy transferred is what we pay for in electricity bills.

2 Students could carry out a similar experiment, but they are likely to be less confused by their observations if the experiment is performed as a demonstration. They could go on to practise using voltmeters to measure voltages in any circuits which they have previously studied.

3 How Science Works extension: Use this demonstration as an opportunity to raise some of the issues relating to the selection of appropriate equipment for practical work. Students may think that there is only one type of voltmeter. By demonstrating that there are voltmeters with different ranges, you can reinforce the importance of selecting appropriate equipment. With two or more meters on different ranges, take the same measurement: the pointer of a meter with a more sensitive scale will be deflected further.

In some experiments, students will not get a single, fixed reading on their voltmeter but will get a constantly fluctuating value. Deciding what the ‘right’ reading is provides an excellent opportunity to discuss:

• the relative merits of analogue and digital meters
• uncertainty in measurements
• how to select a meter with an appropriate range and sensitivity.

Point out that care needs to be taken to avoid meters being overloaded or damaged. If an electrical meter has more than one range, students should always select the highest range first and then more sensitive ranges, if appropriate. They should also select the lowest voltage on the power supply.

This experiment was safety-checked in December 2006