Electricity and Magnetism

Oscilloscope and alternating voltage from transformer

Practical Activity for 14-16 PRACTICAL PHYISCS


A way of showing quantitatively the relationship between the e.m.f. and turns ratios of transformer coils.

Apparatus and Materials

  • Demountable (demonstration) transformer with coils of 12 000 and 300 turns
  • Leads, 4 mm, 2
  • Oscilloscope

Health & Safety and Technical Notes

The 12,000 coil must be designed for use on the mains, i.e. the mains lead must be permanently attached or connected via an IEC connector.

The mains lead to the 12,000 turn coil should be permanently connected; 4 mm leads connected to a 13 A mains plug are a serious hazard.

If the coil fitted with a mains connector has only 1,200 turns, reduce the secondary to 30 turns. Do not improvise mains connections with 4 mm leads.

Read our standard health & safety guidance


  1. Set up the demountable transformer with the 12,000-turn coil on one side of the U-core and the 300-turn coil on the other.
  2. Plug the 12,000-turn coil into the (230 V) mains. There will then be about 6 volts across the secondary coil when the core is completed.
  3. Connect the low-voltage output from the secondary of the demountable transformer to the Y-plates of the oscilloscope by direct connection to the input terminals.

Teaching Notes

  • The frequency of the AC mains can be calculated from the time-base reading.
  • The turns ratio and the voltage ratios can be compared.

This experiment was safety-tested in April 2006

  • A video showing how to use an oscilloscope:

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