Alternating Current
Electricity and Magnetism

Slow AC with a low frequency generator and a voltmeter

Practical Activity for 14-16 PRACTICAL PHYISCS


Displaying slow AC using a voltmeter. This is an impressive teacher demonstration if performed using large-sized meters.

Apparatus and Materials

  • Low frequency AC generator with battery – see technical note
  • Voltmeter (± 5 V), DC
  • Voltmeter, AC
  • Power supply, low voltage, variable (for motor)
  • Electric motor, small
  • Rubber band
  • Leads, 4 mm, 6
  • Oscilloscope

Health & Safety and Technical Notes

Read our standard health & safety guidance

A low frequency AC generator consists of a coil of resistive wire, with a rotating pair of contacts. A low-voltage DC supply is connected across the coil of the generator. The metal brushes rotate in contact with the coil and are connected to the AC output terminals, giving an alternating output.


  1. Connect the battery to the DC terminals of the generator, and the voltmeter to the AC terminals. Set the pointer of the voltmeter centrally.
  2. Turn the generator faster and faster, so that the amplitude of the pointer's movement gets less and less. Then replace the DC meter with an AC meter.
  3. Use a motor to drive the generator, using an elastic band as the driving belt. The low-voltage variable power supply can be used to drive the motor, first at low speed and then at high speed.
  4. Show how the output from the AC generator, driven by the small motor, can be shown on the oscilloscope.

Teaching Notes

The generator is turned slowly so that the meter's pointer can be seen oscillating in time to the motor rotation. When the motor turns the generator too quickly, the pointer on the meter is unable to keep up. This is the time to use an AC meter.

This experiment was safety-tested in October 2006

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