A model hot-wire ammeter
Practical Activity for 14-16
A model showing the principle of the hot-wire ammeter. This is a chance to see models of historic meters, produced to measure AC in the days before multimeters and other electronic wizardry.
Apparatus and Materials
- Eureka wire (1 metre, 28 SWG), bare
- Retort stands and bosses, 2
- Weight hanger with slotted weights, 10 g
- Lamp (12 V 24 W) in lamp holder
- Wooden rods fitted with 4 mm terminal, 2
- LV variable power supply
- Leads, 4 mm, 2
- Metre rule
Health & Safety and Technical Notes
- Stretch a metre of bare Eureka wire (28 SWG) between two retort stands.
- Hang the weight hanger with a single 10 g mass, making a 20 g mass, from the centre of the wire which is pulled taut.
- Connect the wire with the lamp and the variable power supply. Pass a current of about 2 A through the wire.
- Show the experiment first with DC and then with AC.
- In this meter, the heating effect and consequent expansion of the wire when an electric current is flowing, is used to measure the current itself. The energy supplied to the wire varies as I2R.
- If desired, a vertical rule may be placed near to the mass so that the movement is more apparent.
- You may prefer to hang the weight hanger from a thread and give the thread one turn round a steel knitting needle which is free to rotate, putting a drinking straw on the needle as an indicator. As the mass falls, the needle will rotate and the drinking straw pointer will move.
This experiment was safety-tested in June 2007