Electricity and Magnetism

A step-up transformer

Practical Activity for 14-16 PRACTICAL PHYISCS


Using lamps and meters to compare the voltages in the primary and secondary coils of a transformer.

Apparatus and Materials

  • Demonstration meters, with AC dials (1 A, 5 A, 5 V), 2
  • C-cores and clip, pair of
  • Lamps (2.5 V, 0.3 A) in holders, 2
  • Copper wire, insulated with bare ends, 200 cm, 2 lengths
  • Leads, 4 mm, 4
  • Power supply, low-voltage

Health & Safety and Technical Notes

If general-purpose low voltage power units are used, it is possible that students will increase the voltage applied to the primary above the suggested 2 V. In principle, the secondary voltage could become more than twice the primary one. In practice, the overload cut-out will operate before the secondary voltage becomes hazardous.

Read our standard health & safety guidance


  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. Clip the two C-cores together to form a transformer.
  4. Connect the ends of the secondary coil to a lamp.
  5. Connect the ends of the primary coil to the AC terminals of the power supply. Connect a second lamp in parallel.
  6. Switch on. Both lamps should light; the secondary lamp should be brighter.
  7. Insert the demonstration ammeter and voltmeter in the primary circuit, as shown. Note the readings.
  8. Insert the demonstration ammeter and voltmeter in the secondary circuit, as shown. Note the readings.

Teaching Notes

  • Although the primary current is less than 1 amp, it is advisable to use the 5-amp range of the ammeter. This avoids any marked reduction in input to the transformer (as the meter's resistance will be lower). You might disconnect the lamp from the secondary in order to observe the effect on the meters in the primary circuit. In this case, the 1-amp range of the meter should be used for convenience.
  • With the circuit in operation, you could unclip the two C-cores, and gently separate them. Emphasize that there is no electrical connection between the primary and secondary circuits. They are joined only by the magnetic field in the cores.
  • Step-up and step-down transformers are used in electricity distribution networks to change the voltage output from a power station (e.g. 25 kV) to that needed for high voltage transmission (e.g. 132 kV or 400 kV) and back down again for use in homes, factories and offices (e.g. 230 V). Higher voltages on the power lines make the transmission process much more efficient.

This experiment was safety-tested in July 2007

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