Practical Activity for 14-16
Showing how the brightness of a lamp increases when a dynamo powering it turns faster.
Apparatus and Materials
- Demonstration meter with dial (2.5–0–2.5 mA. DC)
- Lamp, 1.5 V in lamp holder
Health & Safety and Technical Notes
dynamo assembly consists of a simple bicycle generator, mounted and geared so that it can be driven both at speed and slowly see the illustrations. For convenience, a lampholder may be fitted between the terminals. (Strictly this is not a
dynamo because that term is reserved for a d.c. generator.)
- Connect the output from the generator to the demonstration meter.
- Turn the handle with low speed gearing so that the deflection is clearly visible. Note the effect of increasing the speed of rotation. Note how, at higher speeds, the pointer merely vibrates over a very small range about the zero position, since the output is a.c.
- Turn the dynamo at high speed using the other gears (with the meter disconnected). The output will be sufficient to light a 1.5 V lamp connected across it.
- 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, generating alternating voltages.
- Turning the dynamo more quickly will increase the e.m.f.
- The use of a demonstration meter is suggested so that it is clearly visible to the whole class. Afterwards, you may want to leave the dynamo at the side of the class for students to use for themselves. If you do this, confine this experiment merely to lighting the lamp (a sensitive meter may be damaged if the dynamo is turned very fast).
This experiment was safety-checked in April 2006