It
is very easy to detect the magnetic field by putting a compass needle near a wire. When a
current is passed through the wire the compass needle will turn. The greater the current in
the wire the more the compass needle moved. This experiment was first done by a man
called Oersted in 1819 who made the discovery by accident.
You can find the
direction of the magnetic field produced by a wire by putting a small compass near the wire
(Figure 1).
The direction of the magnetic field produced depends on the direction of
the current and the shape of the field depends on the arrangement of the wires.
The core of an electromagnet is made of
iron because this is easy to magnetise and demagnetise. Steel would not be suitable
because it stays magnetised for much longer and so the electromagnet could not be
switched on and off easily.
You can use a small compass to test the polarity of an
electromagnet. The following diagrams show the result of doing this for different current
directions, different shaped cores and different ways of winding the wire round the
core.