Calibrating a voltmeter

Class practical

Using the specific thermal capacity of water to check the reading of a voltmeter.

Apparatus and materials

Stopwatch or stopclock

Thermometer

Immersion heater, 12 V

Measuring cylinder, 100 ml

DC ammeter (0-5 amp)

Plastic cup

Battery, 12 V

Retort stand and clamp

Glass rod stirrer

DC voltmeter (0 -15 volt)

Health & Safety and Technical notes

Do not use a mains immersion heater which would involve unsafe mains connections.

You can replace the battery by a 12 volt a.c. supply if appropriate a.c. meters are available.

Cheaper, cup-top immersion heaters are available. These remove the need for a separate support.

Procedure

a Place 200 ml (grams) of cold water in the plastic cup. Support the immersion heater in the cup so that it does not touch the base. Place the immersion heater in the water and connect it into the circuit shown.

b Record the temperature.

c Switch on the circuit and start the watch at the same instant. Stir the water and allow the current to flow for 2 minutes. Note the ammeter and voltmeter readings during this time.

d At the end of 2 minutes switch off the current, stir the water again, and note the maximum temperature. (A rise of about 7° C can be expected.)

Teaching notes

1 This experiment makes use of the principle of conservation of energy in order to calibrate a voltmeter, rather than an experiment to measure the thermal energy gained by the water.

2 Knowing that all energy is measured in the same unit, joules, then the energy transferred from the power supply can be equated to the gain in thermal energy of the water.

Electrical transfer of energy, E = potential difference x current x time = VIt

Thermal transfer of energy, E = mass x specific thermal capacity x temperature rise

VIt = m x specific thermal capacity x temperature rise

As all the values can be measured, you can compare the calculated value for V with the reading on the voltmeter. The specific thermal capacity of water is 4,200 J/kg °C.

3 If you are using d.c. meters, this experiment is best carried out using car batteries. The potential difference is ‘pure’ d.c. If rectified, unsmoothed d.c. is used from a power supply, then the calculated product of the readings on the voltmeter and ammeter will be 20% less than it should be. This is because the meters are measuring a time average not an r.m.s. average. If an a.c. supply and a.c. meters are used, r.m.s.values will be recorded, and the product will be accurate.

4 It may be helpful to remind students that the definition of potential difference is the energy supplied to each coulomb of charge flowing. Therefore the basic unit of the volt can be expressed as joules/coulomb.

5 In many textbooks, specific thermal capacity is also referred to as 'specific heat capacity'.

This experiment was safety-checked in January 2006