How electrical energy is produced

There are several methods of producing electricity for pra­ctical purposes. The battery of a pocket torch maybe 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 ceils 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. Piezo-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 not suitable for power supply.

5. Electronic, characterized by the flow of electrons through evacuated or gas-filled tubes, and having the following forms:

a) Thermionic emission, in which the 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.

d) Field emission, in which electrons are drawn from the surface of a metal by the application of very powerful electric fields.