Relays and Loudspeakers
Two important uses of magnets are in relays and loudspeakers.
A relay is a magnetic switch. Suppose you want to switch on the light in your room. You use your finger to push a switch on the wall, and the light comes on.
There are situations where you do not want to use your finger to push the switch. One is when the switch is connected to a very high voltage. In this case, we would use a relay. (Another situation is when there are millions of these switches, as in a microchip. In this case, we use millions of transistors.)
A relay simply uses an electromagnet to attract a switch. In the picture below, when the electromagnet is on, the switch is attracted away from the contact point of the light bulb circuit, so the light bulb is off. When the electromagnet is off, a spring moves the iron to the contact point. The switch closes and the bulb lights up.
The simplest form of a loudspeaker consists of a paper cone. Wire is coiled round the narrow end of the cone. The wire is connected to a microphone or a recorder.
Voice signal arrives in the form of alternating current in the wire. This current oscillates in strength and direction in exactly the same way as the sound from the microphone. This current passes through the field of magnets that are placed around the coil.
When a current goes through a magnetic field, the field produces a force on the current. This is the electromagnetic effect. This force causes the paper cone to move.
The strength and direction of the force follows the same pattern as the current, which in turn follows the same pattern as the original sound. So the paper cone is displaced in the same way and therefore produce the same sound. By increasing the strength of the current, the amplitude of the vibrating cone can be increased to give a loud sound.