# The voltmeter as a cell (battery) counter

##### Class experiment

A voltmeter, by measuring the voltage across several 1.5 V cells in series, can 'count' the cells.

#### Apparatus and materials

For each student group:

Cells, 1.5 V, 3

Lamp with lamp holders, 3 (e.g. 1.25 V, 0.5 A)

Leads, 4 mm, 9

Voltmeter, 0 to 5 V, DC

Ammeter, 0 to 1 A, DC

#### Procedure

a Set up a circuit with three cells and three lamps all in series, as in the diagram. Switch on the lamps.

b Attach two long, flexible leads to the voltmeter.

c Connect the voltmeter leads across one cell, then across two cells, then across three. Record the reading of the voltmeter each time.

• How many cells are needed to light one lamp fully?
• How many cells to light two lamps fully? Three lamps fully?
• What does the voltmeter tell you? What does it count?
• Is the voltmeter connected in series with the lamps or in parallel?
• What does an ammeter count?

d Connect the voltmeter leads across one lamp, two lamps, and then three lamps.

#### Teaching notes

1 Students connect the voltmeter across each cell, and so measure the energy per coulomb transferred by each cell to the circuit.

They then connect the voltmeter across all three cells, and show that the total potential difference across three cells is three times that across one cell. The potential differences add up, and so does the energy transferred per coulomb. The voltmeter is a cell counter.

2 In part d, students connect the voltmeter across each of the lamps in turn, and then across all three lamps. This shows that the energy per coulomb transferred from the cells is equal to the energy per coulomb transferred from the lamps to the environment.

This is evidence for the conservation of energy.

This experiment was safety-checked in October 2006