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electrodes; one is the anode and the other the cathode. When it is
used to rectify, it will pass current only during each half cycle of
the applied alternating current. The result is a pulsating current
flowing in one direction, an irregular, or uneven current, but a
direct current. It can be made regular and even by means of a
smoothing circuit.
A valve is a device that allows a flow in one direction only.
In a radio valve this is the flow of electrons from cathode to anode.
A simple thermionic valve is called a diode because it has two
electrodes: the cathode, which is negative, and the anode, which is
positive. It is called a thermionic valve because the cathode is in
the from of a filament and when it becomes hot, negative
electrones become free and are attracted towards the positive
anode.
There are many different kinds of valves in a radio receiver;
they are specially designed to perform different functions. A
triode is a valve with three electrodes, an anode, a cathode and a
control grid. A tetrode has four, and a pentode five electrodes.
A major development in the electronics industry during
recent years has been the gradual replacement of thermionic valves
by transistors. A transistorized circuit is simpler, smaller, and
technically stronger. Because of these advantages the transistor has
contributed greatly to rapid developments in other scientific fields,
particularly space travel.

VI

A transistor is a semiconductor. This means that it is not
entirely a conductor, nor is it an insulator. It is crystalline in
structure and has three electrodes, a base, an emitter and a
collector.
The main difference between a valve and a transistor is that
while a valve amplifies, or gives a voltage gain, a transistor gives
an increase in current. Transistors can be made from germanium,
selenium, silicon and other substances.
The various components that go to make up a complicated

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