A model moving-iron meter

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