# Bicycle dynamo and oscilloscope

##### Class practical

Showing students that the e.m.f. (voltage) produced by a dynamo depends on the rate at which it turns.

#### Apparatus and materials

Bicycle 'dynamo' assembly

Lamp, 1.5 V in lamp holder

#### Health & Safety and Technical notes

The dynamo assembly consists of a simple bicycle generator, mounted and geared so that it can be driven both at speed and slowly – see the illustration. For convenience, a lamp holder may be fitted between the terminals.

A video available at the National STEM Centre eLibrary shows how to use an oscilloscope.

#### Procedure

a Connect the output from the generator to the oscilloscope input (Y-plates). The time base should initially be switched off and there should be maximum gain on the Y-amplifier. The spot should be in the centre of the screen.

b Connect the lamp across the output of the dynamo, in parallel with the C.R.O.

c Turn the handle with low speed gearing so that the up and down motion of the spot is clearly visible.

d Switch on the time base at slow speed and centre the trace with the X-shift. The gain should still be at maximum. The dynamo is again driven at the slow speed, and the spot will be seen to generate a wave-like trace.

e Gradually speed up the time base. Cut down the gain on the oscilloscope to about 2 volts/cm and drive the dynamo at the high speed. With the time base set at 10 ms/cm, a roughly sinusoidal wave-form will be seen.

#### Teaching notes

1 A bicycle 'dynamo' is one of the simplest of generators and is easily available. It also has the advantage that the armature/coil is stationary and the field moves relative to it, in accordance with standard practice in heavy engineering. The field is normally produced by an 8-pole circular magnet rotating between two coils producing alternating voltages.

2 Turning the 'dynamo' more quickly will increase the e.m.f.

3 A long-persistence screen would be an asset in this experiment, but is not essential. Alternatively, you could capture the trace using a datalogging system, and display it on a computer screen.

4 The wave form will not be sinusoidal; the bicycle 'dynamo' was designed for efficiency and not for teaching purposes. Other generators can be found which give a more nearly sinusoidal wave form, but there is greater value here in using a generator as familiar as the bicycle 'dynamo'.

This experiment was safety-tested in June 2007