If a bar of soft iron is placed in a coil of wire and a current is made to flow round it the iron will become a magnet but remains so only while the current is flowing, and this forms an electromagnet. An electromagnet works best on a direct current but an alternating current can also be used to energize it.
A coil of wire with an air core, that is without either an iron or a steel core, becomes a magnet when a current is made to flow through it. If, now, one end of a bar of soft iron is slipped into the hole in the coil of wire and the current is turned on the iron bar, or core, will be drawn into it. This kind of a magnet is called a plunger electromagnet, or solenoid.
Radiation.—Whenever you light a match, or make a light by any other means, electric charges on the molecules of the substance which is heated vibrate violently to and fro and the minute surgings of the electric charges set up electro-magnetic waves in the ether which the eye can see and the brain can sense and this is what we call light.
When some substances are intensely heated, as for instance, the carbons of an arc lamp, waves are also sent out which are too short for the eye to see but which will nevertheless affect a photographic plate. These are called ultra-violet waves. The infra-red waves are too long for the eye to see but the nerves of our bodies sense them as heat.
Fig. 56a. SOME USEFUL ELECTRO-MECHANICAL DEVICES
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