Alternating Current
Electricity and Magnetism

Bicycle dynamo and oscilloscope

Practical Activity for 14-16 PRACTICAL PHYISCS

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

  • Oscilloscope


  • 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.

Read our standard health & safety guidance


Procedure

  1. 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.
  2. Connect the lamp across the output of the dynamo, in parallel with the C.R.O.
  3. Turn the handle with low speed gearing so that the up and down motion of the spot is clearly visible.
  4. 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.
  5. 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

  • 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.
  • Turning the dynamo more quickly will increase the e.m.f.
  • 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.
  • 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

  • A video showing how to use an oscilloscope:

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