Electricity and Magnetism

Transformer: dependence on number of turns

Practical Activity for 14-16 PRACTICAL PHYISCS


An intriguing demonstration in which a lamp grows brighter and brighter with more turns on the secondary coil of a transformer.

Apparatus and Materials

  • Demountable (demonstration) transformer with a coil of 1200 turns for mains use
  • Lamp, 2.5 V in holder
  • Copper wire, insulated with bare ends, 4 m

Health & Safety and Technical Notes

The 1,200 turn coil must be designed for connection to the mains with either a permanently-connected mains lead or a fully insulated mains computer used with an IEC connector (as on computer equipment).

Read our standard health & safety guidance

The 1,200 turn coil may overheat if left connected too long.


  1. Place the 1200-turn coil on one limb of the laminated U-core of the demountable transformer. Connect this to the a.c. mains.
  2. Connect a long lead (4 metres of insulated copper wire) to the lamp.
  3. With the mains switched on, wind the lead turn by turn round the other leg of the U-core. As more and more turns are wound on, the lamp begins to glow and then to get brighter and brighter. At least 10 turns will be necessary for this.

Teaching Notes

  • This is an intriguing demonstration in which the lamp grows brighter and brighter as the turns increase. The demountable transformer comes with an iron yoke to complete the magnetic circuit, and when that is put in place the lamp glows even more brightly.
  • The transformer kit normally has a collection of coils which can be interchanged, and some extra equipment in order to do further demonstrations such as melting solder or the jumping ring. Read the instructions that come with the kit.
  • When the primary current changes, a potential difference is induced in each turn of the secondary coil on the core. As the secondary is wound, turn upon turn, more potential difference is induced, volt upon volt, so to speak.
  • The relationship Vs / Vp = Ns / Np can be measured with different pairs of coils in the transformer kit. Do remember that the current ratios are the inverse of the potential difference ratios (energy conservation demands that IV is a constant at input and output). It is easy to set up a system which will demand more current from the mains than the QuantityUnit{13}{A} it can supply.
  • Here, Vs and Vp are the potential differences on the secondary and the primary coils and Ns and Np are the number of turns on the secondary and primary coils.

This experiment was safety-tested in April 2006

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