Demonstration

A model showing the principle of the moving-iron meter.

Apparatus and Materials

• Transformer coil, 600 turn
• Power supply, low voltage, variable
• Rheostat
• Nails, 8 cm, 2, one of them beheaded
• Drinking straws, 2
• Pins, 2
• Leads, 4 mm, 3
• Rubber band

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Slip the two straws over the ends of the beheaded nail by about 0.5 cm. Insert the arrangement into the 600 turn coil and bend the straws as shown in the diagram. Secure the straws to the terminals of the coil using pins, as illustrated. Cut off the excess on one straw.

2. Secure the second nail to the bottom inside corner of the coil, at the same level as the nail held by the drinking straws.
3. Tip the whole coil until the beheaded nail swings to a position which brings it very close to the fixed nail.
4. Connect the coil in series with a rheostat to the DC output of the power supply. When the current is switched on, the nails repel each other and there is a deflection of the pointer. Increase the current to increase the deflection.
5. Repeat using the AC output of the power supply.

Teaching Notes

• Moving iron meters use either the magnetic attraction on a small piece of iron pulled into a solenoid carrying a current, or the repulsion between two pieces of soft iron side by side in a solenoid carrying the current to be measured. This is a model of the latter design.
• This can be turned into a better working model by holding one bar fixed, installing an axle and pointer for a second bar, with a hairspring against which the repulsion pushes the second bar around.

This experiment was safety-tested in January 2007

Demonstration

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

Read our standard health & safety guidance

Procedure

1. Stretch a metre of bare Eureka wire (28 SWG) between two retort stands.
2. 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.
3. Connect the wire with the lamp and the variable power supply. Pass a current of about 2 A through the wire.
4. Show the experiment first with DC and then with AC.

Teaching Notes

• 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