Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Slow AC with a low frequency generator and oscilloscope

Class practical

Displaying slow AC on an oscilloscope.

Apparatus and materials

For each student group


Low frequency AC generator with battery – see technical note

Leads, 4 mm, 4

Health & Safety and Technical notes

Low frequency AC generator iwth battery Photo courtesy of Mike Vetterlein

A low frequency AC generator consists of a coil of resistance wire with a rotating pair of contacts. A smooth 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. 

Two or three (fresh) 1.5 V cells will probably do, but it would be wise to disconnect them as soon as the experiment is over. Readings

The low frequency generator works well. If it gets dirty, a few drops of thin oil on the brushes will ensure continuous contact.

A signal generator capable of giving low frequency (0.1 Hz) output is a suitable alternative.

A video available at the National STEM Centre eLibrary shows how to use an oscilloscope. There is also a video showing how to use a signal generator.


a Set the AC-DC switch on the oscilloscope to DC, the time-base to off, and the Y-gain to about 1. 

b Connect 2 - 4 volts across the low frequency AC generator. Connect the output terminals to the input of the oscilloscope.

c Turn the generator by hand to show the spot moving up and down.

d Switch on the time-base to its slowest speed on range 1. Turn the generator as steadily as possible. Turn up the time-base speed and repeat this, increasing the rate at which the generator is turned. Oscilloscope Photo courtesy of Mike Vetterlein

Teaching notes

Rotating the generator at a steady speed results in a sinusoidal output voltage. (This is an example of the link between circular motion and simple harmonic motion.) 

This experiment was safety-checked in July 2007 

Related guidance

Explaining rms voltage and current

Related experiments

Slow AC with a capacitor

Slow AC with a low frequency generator and voltmeter