poles and repel each other.

Another important discovery of Ampere is that a circular current
behaves like a magnet; and it has been suggested by him that the
atoms are magnets because each has a circular current flowing
round it. A series of circular currents, such as the spiral S in
figure 33 gives, when connected to a battery C Z, is in fact a
skeleton ELECTRO-MAGNET having its north and south poles at the
extremities. If a rod or core of soft iron I be suspended by
fibres from a support, it will be sucked towards the middle of the
coil as into a vortex, by the circular magnetic lines of every
spire or turn of the coil. Such a combination is sometimes called
a solenoid, and is useful in practice.

When the core gains the interior of the coil it becomes a
veritable electromagnet, as found by Arago, having a north pole at
one end and a south pole at the other. Figure 34 illustrates a
common poker magnetised in the same way, and supporting nails at
both ends. The poker has become the core of the electromagnet. On
reversing the direction of the current through the spiral we
reverse the poles of the core, for the poker being of soft or
wrought iron, does not retain its magnetism like steel. If we stop
the current altogether it ceases to be a magnet, and the nails
will drop away from it.

Ampere's experiment in figure 32 has shown us that two currents,
more or less parallel, influence each other; but in 1831 Professor
Faraday of the Royal Institution, London, also found that when a
current is started and stopped in a wire, it induces a momentary
and opposite current in a parallel wire. Thus, if a current is