The five most important sources of electricity for technicians are chemical action, heat, light, pressure, and magnetism.
1) CHEMICAL ACTION
In the electrical and electronics fields, battery is the backup source of electricity. Batteries produce electrical energy by a chemical action.
2) HEAT
Heat can be used to free electrons from some metals and from specially prepared surfaces. When some materials are heated to a high temperature, electrons are freed from their surfaces. Any nearby metallic surface, if positively charged, attracts these electrons and produces electron flow. The freeing of electrons by heat is called thermal emission.
3) LIGHT
Light striking the surface of certain materials can be used to free electrons. This is called photoemission. With a suitable collecting surface, useful electron flow can result. Photoemission is used in photoelectric devices and television camera tubes.
4) PRESSURE
Mechanical pressure on certain crystals can be used to produce electricity. The crystal cartridge of an inexpensive record player is a good example. The needle causes a changing pressure. The crystal produces a changing voltage in step with the grooves in the record.
5) MAGNETISM
The most common method of generating electrical power is by turning a coil of wire in a magnetic field. This is the method the power station uses to generate the electric power that is used in homes, business, and industry.