3.1 MAGNETISM
Long ago, in the middle ages, it was found that a
mineral called ‘lodestone’, which is in fact an iron ore, attracted small iron
objects. So ‘magnetism’, which is a
natural phenomenon, was discovered. It
got its name from a district in Asia Minor called ‘Magnesia’ where lodestone
was found.
It was found too that an iron bar or needle, if rubbed
with lodestone, could also be made to attract small pieces of iron - that is,
the magnetism could be imparted from the lodestone to the iron. Such a needle, if placed on a wooden raft and
left to float in a bowl of water, always tended to lie in a rough North to
South direction. So we had the first
primitive compass.
3.2 ELECTROMAGNETISM
In 1820 Oersted discovered that an electric current
flowing in a wire caused a magnetic field around it. This can easily be detected by placing a
small compass near the wire and observing the movement of the needle when
current is switched on. This is shown in
Figure 3.1.
Figure 3.1 MAGNETIC FIELD AROUND A
CONDUCTOR
The effect can be intensified by
bending the wire into a loop. The
magnetic fields from each bit of the wire are brought together inside the loop,
where the magnetic field is concentrated and intensified.
WITHOUT IRON WITH IRON
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Figure 3.2 MAGNETIC FIELD AROUND A COILED CONDUCTOR
If now the wire is bent into
several loops, or a ‘helix’, as shown in Figure 3.2, the magnetic fields of
each ‘turn’ are superimposed, and the field down the middle is still further
intensified. The result is a ‘coil’
which, when current flows in it, produces an artificial magnet, called an
‘electromagnet’. Unlike a natural
magnet, whose magnetism is always present, an electromagnet can be switched on or off at will.
If iron is introduced inside the coil, the magnetic
strength is still further increased, and ‘permanent’ magnets can be made this
way. Very powerful electromagnets can be
built, which are widely used: they can actuate solenoids or valves directly;
they can drive any device needing a fore-and-aft motion; and they are used with
cranes in scrap-yards for picking up large weights of scrap-iron. On a smaller scale they are used to operate
relays and switching devices.
Although it may not at first seem so, solenoids and
other electromagnets operate just as well with alternating as with direct
current.
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