Soft iron retains very little magnetism and yet it can be magnetized to such an extent that it can be utilized in lifting large bodies. When a bar of soft iron, in the form of a horseshoe, is wrapped round with copper wire and a current of electricity is passed through the wire, the iron becomes a powerful magnet called an electromagnet (Fig. 72) and may be constructed to support a weight of many tons. By making one magnet fixed and another movable, and by causing one magnet to revolve within the lines of force of another, an attraction and repulsion of great intensity can be created, which will act as a great moving power.
The strength or lifting power of a magnet is measured with a lever and scales by noticing the number of pounds registered. The lifting weight is the pull exerted minus the weight of the magnet. The magnetic flow is proportional to the number of turns of wire of the conductor and the current flowing around the turns. The magnetic flux is inversely proportional to the resistance of the circuit. The total resistance is the sum of the resistance of the iron path and the air path.
An electric bell (Fig. 73) depends upon the properties of electricity and magnetism for its action. When the button of the bell is pressed by the finger, an electric circuit is completed. The current flows around the coils of an electromagnet, which attracts a bar of soft iron metal fastened to a lever, at the other end of which there is a hammer that strikes the bell. When the soft iron metal is attracted the current is broken; this causes the bar to go back. This backward movement of the bar starts the current again and the operation is repeated. These operations are repeated in rapid succession so long as the button is pressed.
Fig. 72. - Electromagnet.