fixed iron
Figure 7.15 Principle of moving-iron repulsion instrument
pointer coil pivot vane
Figure 7.16 Principle of moving-iron attraction instrument
of the bars is fixed to the coil, and the other is attached to the moving system of the instrument and is free to swing with it. When current flows in the coil, both bars are magnetised to an extent which depends on the current. Since the bars are magnetised by the same field, their magnetic polarities will be the same, with both north poles together at one end and both south poles together at the other. The repulsion which results moves the bar which is free to swing, and gives a deflection.
Moving-iron attraction instrument
The magnetising coil of this instrument is wound so that the central space will accept a soft-iron vane which is attracted to it (Figure 7.16). The vane is attached to the moving system, and thus indicates the strength of the magnetic field, and hence of the current producing it.
All moving-iron instruments have the major advantage of reading on both AC and DC supplies. A coil fed with an alternating current results in an alternating magnetic field, which continues to attract the soft-iron vane, or to reverse the magnetisation of the two soft-iron bars together, so that they continue to repel. These instruments are robust and comparatively cheap, but have a serious disadvantage in that their scales follow a ‘square law’ and are very cramped at the lower end. Little can be done to improve the lower end of the scale, but the rest of it can be made more linear by shaping the disc of the attraction type, or substituting shaped plates for soft-iron bars in the repulsion type. Other disadvantages are that moving-iron instruments are not
as accurate as permanent magnet moving-coil types (Section 11.6), and are affected by stray magnetic fields and by changes in the frequency of an AC supply.
7.9 Exercises
1 Describe the construction and operation of (a) an electric bell (b) an electric buzzer.
2 Describe the construction and operation of a single-stroke bell; indicate one application of such a bell.
3 Using a carefully drawn sketch, show the construction of a continuous-ringing bell. Explain how it functions, and where it could be used.
4 Describe one type of bell indicator. Draw a circuit diagram for a four-way indicator system, using one bell, an indicator board and four pushes.
5 Explain the construction and operation of a relay. How can this device allow heavy currents to be switched remotely, using small conductors to the remote position?
6 Suggest four applications for a relay or contactor, and explain each.
7 Describe, with the aid of a sketch, the construction and operation of a simple electromagnetic relay suitable for a bell circuit.
8 Make a neat, labelled sketch to show the construction and operation of one of the following:
(a) a telephone receiver (earpiece) (b) a buzzer.
9 With the aid of a sketch, give a brief description of the construction of a telephone receiver.
10 Describe with the aid of a sketch the construction and operation of a telephone microphone.
11 Draw a circuit showing two telephones connected together. Each telephone must have provision for calling the other, as well as the speaking circuit. Explain how the circuit functions.
12 Using a clearly labelled diagram, describe the construction and operation of a loudspeaker.
13 Describe the construction and operation of one type of moving-iron instrument. What are its advantages and disadvantages when compared with the permanent- magnet moving-coil instrument.
7.10 Multiple-choice exercises
7M1 The trembler bell relies for its operation on (a) the application of thyristors and triacs (b) an electromagnetic resetting system
(c) contact which open on operation, allowing the system to repeat its action (d) the presence of a permanent magnet to give polarisation
Applications of electromagnetism 135
7M2 The buzzer differs from the bell in construction because it (a) makes a different noise
(b) has no bell or striker and thus oscillates more rapidly (c) is very much smaller
(d) is of the electrical-reset type 7M3 A continuous-ringing bell is one which
(a) is designed to ring forever
(b) cannot be stopped once it has started to ring (c) depends for its operation on magnetic repulsion (d) once started, continues to ring until reset 7M4 A bell or buzzer indicator system is used to
(a) show from which position the bell or buzzer was operated (b) make an installation more complicated
(c) handle current levels too high for the bell or buzzer alone
(d) allows more bells or buzzers to be installed than is strictly necessary 7M5 A polarised bell is usually used for
(a) a house door bell (b) very cold situations (c) a telephone (d) a permanent magnet 7M6 A relay can be useful to
(a) promote an athletics meeting
(b) allow a set of heavy-current contacts to be closed by a small current (c) make a telephone easier to hear
(d) change an old cable for a new one
7M7 The microphone or transmitter of a telephone relies for its operation on (a) the British Telecom or Mercury networks
(b) a change in the resistance of carbon granules when subjected to a change in pressure
(c) the changing magnetic field produced by an electromagnet (d) shouting loudly into it
7M8 The receiver of a telephone depends for its operation on
(a) the changing magnetic field produced by coils wound on a permanent magnet
(b) a change in resistance of carbon granules when subject to a change in pressure
(c) the British Telecom or Mercury networks (d) the surrounding sound level being very low
7M9 A loudspeaker with a very small cone is best at reproducing (a) pop music (b) very low-frequency notes (c) classical music (d) high-frequency notes 7M10 The moving-iron repulsion instrument operates because
(b) two fixed irons are oppositely magnetised
(c) a fixed and a moving iron are magnetised with the same polarity (d) a soft-iron vane is attracted into a shaped coil
7M11 The moving-iron attraction instrument operates because (a) a soft-iron vane is attracted into a shaped coil (b) it has a needle moving over a scale
(c) two fixed irons are oppositely magnetised (d) the scale is nonlinear
7M12 A moving-iron instrument will operate when the supply is (a) alternating current only (b) direct current only (c) alternating or direct current (d) very large