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Apparatus

1m length of constantan wire, a metre rule, a low voltage power pack, a variable resistor, a voltmeter, an ammeter, connecting leads, a switch, 2 crocodile clips, Sellotape.

Circuit diagram

Method

1. Set up the circuit, as shown above.
2. Adjust the variable resistor until the current on the Ammeter is 0.1 A. Record the current in a suitable table.
3. Read the corresponding value of voltage across the wire on the voltmeter and record in the table.
4. Turn the switch off between all readings to prevent the temperature of the wire rising.
5. Turn on again. Ensure that the current is still 0.1 A and repeat the voltage reading. Calculate the average voltage.
6. Repeat the procedure for six values of current up to 0.7 A.

Avoiding errors

The temperature of the wire must be kept constant.

Whenever a current flows through a conductor there is a heating effect.

Electrical energy is converted to heat energy.

To ensure the temperature of the wire does not increase, switch off between readings and keep the current as small as possible.

Read the ammeter and voltmeter accurately by reading the scale from directly above the pointer or use digital instruments.

Results

Graph