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necessary to use a capacitor whose value can be varied. An
example of this is the tuning control on a radio receiver, when it
is turned the value of the capacitor is changed.
The most common way of making electricity is by the
conversion of mechanical energy. A dynamo is a machine that
makes electricity when it is made to rotate at high speed. A
turbine that is rotated by steam pressure often drives it. This
machinery is then called a turbo-generator. A building which
houses a turbo-generator is called a power or generating station.
A station that uses the power of falling water is known as a
hydroelectric station. Most of the world’s electricity for domestic
and industrial use is made оr generated in such power stations.
Any material that will allow electricity to flow through it is
called a conductor. Most metals are good conductors. Any
material that will not permit the flow of electricity is known as an
insulator.
Conductor wires, or leads, in any piece of electrical
apparatus or installation must not be allowed to touch each other,
or any other metal part. If they do, a short circuit will result
which will cause failure of the apparatus and the possible dangers
of fire and electric shock.
Insulating materials are used to cover and protect conductor
wires and sometimes to separate them. Common insulating
materials include air, oil, glass, porcelain, mica, rubber and
plastics. Heavy power cables are sometimes given an extra cover,
known as a bonding or shield, made of metal for the further
protection of the insulation.
Two different kinds of electricity are used to operate
electronic apparatus.
One kind is called direct current, usually abbreviated to DC.
This is the kind of electricity we get from a battery, the flow of
current is in one direction only. The other kind is alternating
current, usually known as AC. This is the kind of electricity that
is generated in power stations for domestic and industrial use. It
is called alternating current because the direction of flow changes
very quickly. The number of changes of direction, called cycles,
in every second is known as the frequency.

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