Measuring resistance with a voltmeter and an ammeter

Class praticals

Determining resistance from measurements of potential difference (p.d.) and current.

Apparatus and Materials

• Ammeter, 0 to 1 A, DC
• Voltmeter, (0-15 V), DC
• Power supply, low voltage, DC
• Lamp (12 V, 6 W) in holder
• Resistor (approx 15 ohms, 10 watt)
• Various other components

Health & Safety and Technical Notes

Read our standard health & safety guidance

Remind the class that the lamp will get hot, so it should only be moved by handling the lamp holder.

Procedure

1. Set up the circuit shown. Turn the power supply up until the p.d. across the lamp is 12 V (the normal operating voltage).
2. Take readings of the p.d. and current.
3. Calculate the resistance of the lamp at its running temperature.

4. Now, for several different values of p.d., measure the current through the lamp. Plot a graph of your results; this graph is known as the voltage-current characteristic of the lamp.
5. Replace the lamp in the circuit with the resistor. Repeat the experiment and calculate its resistance. Take sufficient readings to allow you to plot the voltage-current characteristic.

Teaching Notes

• This series of experiments should give students practice in taking a pair of current and potential difference readings for various components so that the resistance of the component can be calculated from V/I = R.
• It can also be extended so that students plot the current/potential difference characteristics for components such as a carbon resistor, a diode, a light-emitting diode (LED), a thermistor, motor armature, electric fire element (12 V supply only!) and so on. Students will need to be able to select appropriate meters, as the current through some of these devices may be very small. Each member of the class could tackle one component and present their results to the class, or produce a wall display.
• Some things which appear not to obey Ohm's law might, in fact, do so; for example, the tungsten filament of a lamp. Tungsten's resistance increases as the lamp gets hotter, but if it could be maintained at a constant temperature then its resistance would be constant.
• For suggested graphs, see below

This experiment was safety-tested in January 2007