A model transformer

Class practical

Shows the basic principle of any transformer: a change in current in the primary coil will induce an e.m.f. (voltage) in the secondary coil.

Apparatus and Materials

For each student group...

• Galvanometer, sensitive to e.g. 3.5–0–3.5 mA., 10 ohm resistance (see note below)
• C-cores, laminated iron, 2
• Copper wire, insulated with bare ends, 200 cm, 2 lengths
• Clip for C-cores
• Switch
• Cell, 1.5 V in holder
• Leads, 4 mm, 4
• Lamp in lampholder, either 1.25 V or 2.5 V, 2

• Power supply, low-voltage

Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).

Health & Safety and Technical Notes

If a zinc chloride cell is used, it will polarize in 60 s or less and must be left overnight to recover.

If an alkaline manganese cell is used, there is a danger of the cell overheating with a risk of explosion: complete the circuit for 30 s or less.

If a rechargeable cell (NiCd) is used, the wire will get very hot and the cell will be discharged in a few minutes: do the experiment as quickly as possible.

Read our standard health & safety guidance

C-cores should be stored in their original pairs and clipped together to ensure a good fit and to prevent grit and dirt damaging them.

The slightest gap will dramatically reduce the efficiency of the transformer.

Procedure

1. Wind 10 turns of insulated wire around one arm of a C-core. This forms the primary coil.
2. Wind 25 turns of insulated wire around one arm of the other C-core. This forms the secondary coil.
3. Connect the ends of the secondary coil to the galvanometer.
4. Connect the ends of the primary coil, via the switch, to the cell.
5. Close the switch. Bring the primary coil up to the secondary coil, as illustrated.
6. Clip the two C-cores together to form a transformer. Open and close the switch, and watch the galvanometer deflection changing.
7. Replace the galvanometer with a lamp, and operate the switch.
8. Connect the primary coil to the a.c. terminals of the power supply (2 V). Connect a second lamp in parallel with the coil, as shown. Switch on. Which lamp glows more brightly?

Teaching Notes

• Care should be taken not to leave the cells connected for any length of time, as the primary coil may become hot.
• The lamp connected across the power supply will only glow faintly, whereas the lamp across the secondary glows brightly as long as the length of wire in each coil is the same. To confirm that this is not due to a difference in the two lamps, these should be interchanged.
• Students could change the numbers of turns of wire on each coil; in this case, the total length should remain constant (so that its resistance does not change).

This experiment was safety-tested in June 2007