Electromagnetic devices

(1) Relays

A relay is a switch operated by an electromagnet, mainly used in controlling a large current by a smaller current, for example, the output of an electronic component such as a digital I.C. is connected to the coil leads and the contact leads are connected to a circuit with the 220 V mains. The basic structure of a relay is shown in Fig. 2.5.


(a) structure of a relay


(b) circuit
Fig. 2.5

When a controlling current, usually small, flows through the coil, the soft iron core is temporarily magnetized and attracts the iron armature. This operation will close the contacts and a larger current can flow through the contact leads. When the coil current disappears, the armature is and the will return to their original position. There is another kind of relay called reed relay.


(2) Loudspeakers

The commonest type of loudspeakers is the moving coil type shown in Fig 2.6:



(a) structure of a moving coil loudspeaker
(b) circuit symbol
Fig. 2.6

When a current, carrying the information of a sound, flows through the coil, the magnetic field induced interacts with the permanent magnet. The coil is then pulled in or pushed out and the amplitude of the movement will depend on the direction and the magnitude of the current respectively. The paper cone, which is firmly attached to the coil, passes this movement onto a large area of air. The change of air pressure will produce a corresponding to the original electric signals. Headphones and earpieces are some daily life examples of loudspeakers.


(3) Microphones

A microphone converts sound energy into electrical energy, exactly opposite to the operation of a . The structure of a moving coil microphone is shown in Fig. 2.7.


(a) structure of a moving coil microphone


(b) circuit symbol
Fig. 2.7

It consists of a small coil of many turns wound on a tube which is attached to a light disc - the diaphragm. When vibrating air, i.e. a sound, strikes the diaphragm, the coil moves in and out across a magnetic field. By the principle of electromagnetic induction, a varying e.m.f. having the same frequency of the sound is induced in the coil (typically a few mV).

There is another kind of small size and commonly used microphone called condenser mic.


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