Electric Bell

When an electric current flows through a wire - especially one in the form of a coil - it becomes an electromagnet. It is a temporary magnet, as it can retain its newly-gained characteristics only as long as the current flows through it. However, it shows all the properties of a permanent bar magnet, including the two opposite poles - N and S - during this period.

This very nature of electromagnets has made them indispensable to modern life: we can turn them on and off, as we wish whenever we want. The simple electric bell is a case in point.

We want an electric bell to produce a ring, at the touch of a button, only once or twice, but not for ever. So, the bell does the job when we choose to send a current through it and afterwards the bell remains silent.

Electric Bell

When the switch 'D' is on, the circuit is complete and then 'A' becomes an electromagnet. Then it momentarily attracts 'B' towards it and the ball at the end of it, hits the bell, producing the sound.

As 'B' was attracted to 'A', the current through the circuit got cut off, that in turn made it losing its magnetism.

Then 'B' was released from 'A' to be back in contact with'C'.

The circuit is now complete again and the current starts flowing through it, repeating the above sequence of events. As a result, we get a continuous ringing, as long as the switch remains 'on'

The following animation shows the whole sequence in detail:


Please answer the following questions.

  1. How do you increase the strength of an electromagnet?
  2. Discuss the uses of electromagnets at industrial level.
  3. How do you destroy a magnet?
  4. Draw magnetic field lines around two bar magnets, when north poles face each other. Where would you expect a neutral point?
  5. It is more appropriate to call the north pole of a magnet, a north-seeking-pole. Explain it.
  6. Magnetic poles are not known to have an existence in isolation. How do you explain this from molecular point of view?
  7. Discuss the most significant parts of an electric bell.
  8. How do you determine the poles of an electromagnet without a compass?