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current flowing in the circuit exceeds a certain amount. When
this happens we say the fuse “blows”. It is very important that
fuse wire of the correct amperage is used.
It is often necessary to increase or decrease the voltage of
the electricity supply. One reason for this is the economic
transmission of power from its source, the generating station, to
wherever it is required for use, which may be at a great distance.
It is cheaper and easier to carry a very high voltage but low
current, over long distances. It can be done using thinner
overhead conductor wires, with an air gap between them to act
as an insulator.
When the overhead cables reach the area where the
electricity is required for use, the voltage must then be carried by
much heavier conductors, which have to be well insulated and are
therefore more costly.

V

A transformer is used to increase or decrease the voltage of
an electric power supply. A transformer is a static machine, it has
no moving parts. It consists of two coils of wire that are wound
around a soft iron core. The coils are called windings, one is the
primary, or input winding, and the other is the secondary, or output
winding.
When a current is passed through the primary winding, a
magnetic field is set up around the iron core that induces a voltage
in the secondary winding. If the number of turns in the secondary
is greater than the number in the primary it is a step-up transformer
and the output voltage is greater than the input. A step-down
transformer reduces the input voltage.
Some complicated pieces of electrical apparatus, for example
radio and television receivers, do not use alternating but direct
current. As the domestic supply is alternating current, it is
therefore necessary to change it to direct current inside the
apparatus. This change, or conversion, is brought about by means
of a rectifier, sometimes called a diode.
A simple diode consists of a valve that has only two

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