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electrical circuit may be connected to the circuit in two ways. If
the supply is fed directly through each component in turn, they are
said to be wired in series. If, however, the supply is taken to each
one independently then they are wired in parallel.
Every component in series in a circuit will add to the total
resistance of the circuit and decrease the current flowing through
it, but every additional resistance in parallel will reduce the total
resistance of a circuit. A high-tension battery is made up of a
number of small batteries connected in series. Its voltage is equal
to the sum of the voltages of the smaller batteries.
A switch is used in almost every piece of electrical
apparatus. Its purpose is to make complete or to break an electrical
circuit. When a circuit is switched on it is a closed circuit and
current will flow through it. When it is switched off the circuit is
broken, it becomes an open circuit and the flow of current is
stopped. The main contacts of a switch are called poles. These are
connected when the switch is on.
There are very many different kinds of switches ranging
from a micro-switch, which is very sensitive, to the large and
complicated ones, which are used for high voltages and are
sometimes known as circuit breakers. Temperature or pressure
control or timing devices can operate switches automatically.
It is sometimes convenient or economical to operate
electronic apparatus from a distance. This is known as remote
control and it may be brought about by the use of a relay, which is
really an electromagnetic switch.
A relay consists of an electro-magnet which, when energized
attracts a soft iron armature. The armature is returned to its former
position by a spring when the electro-magnet is de-energized and
therefore there is no longer any attraction. The movement of the
armature opens or closes the contracts of the relay. In this way one
electrical circuit can be used to control another.
The telephone uses very little electric current. It consists of
two main parts, the microphone and the earphone.
The microphone contains a very thin and flexible metal disc
called a diaphragm. It also contains a magnet. When we speak we
cause the diaphragm to vibrate. The movements of the diaphragm

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