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alternating wave is T.
1 second, the frequency of the alternating wave is f = Нz.

2. How Electrical Energy is Produced

There are several methods of producing electricity for
practical purposes. The battery of a pocket torch may be contrasted
with the source of enormous energy represented by a larger power
station. Both are examples of the application of electrical energy to
a particular purpose, and in general the purpose determines the
nature of the method used to produce the energy. Practical
methods of producing electricity may be enumerated as follows:
1. Chemical, as represented by the various types of batteries or
primary cells in which the electricity is produced by purely
chemical actions.
2. Electromagnetic, forming the basis of operation of rotating
generators in which the electricity is produced by conductors
moving through a magnetic field. This is the method employed
in practice for generators of various sizes.
3. Thermo-electric, in which the heating of the junction between
two different metals produces a very small voltage, which may
be used for purposes of temperature measurement and as a
source of power.
4. Pieso-electric, in which a very small voltage is produced across
certain faces of a crystal by application of mechanical pressure.
This effect is used, for example, as a means of frequency
control in radio oscillators or for gramophone pick-ups, but it is
suitable for power supply.
5. Electronic, characterized by the flow of electrons through
evacuated or gas-fi1 led tubes, and having the following forms:
a) Thermionic emission. In which electrons are produced by
the heating of special materials.
b) Photo-electric emission, in which electrons are liberated at
the surface of certain substances by the action of light.
c) Secondary emission, in which electrons are driven from a
material by the impact of electrons or other particles on its
surface.

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