SWITCHES. SIMPLE CAPACITOR. D.C. AND A.C

№ 9. SWITCHES

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. They are connected when the switch is on. There are very many different kinds of switches. Switches can be operated automatically by temperature or pressure control or timing devices. In some pieces of electrical apparatus it is sometimes necessary to store an electric charge and hold it for some time. This is done by means of a capacitor, also called a condenser.

№ 10. SIMPLE CAPACITOR

A simple capacitor is made of two metal plates called electrodes, which are separated by an insulating material such as air, paper, etc., called the dielectric. The unit of capacitance is called a farad. This is a relatively large unit. Many capacitors used in electric apparatus are rated in micro farads, millionth part of a farad. It is sometimes necessary to use a capacitor whose value can be varied. An example of this is the turning control of a radio receiver. When it is turned the value of the capacitor is changed. There can be used two different kinds of electricity. One kind, is called direct current usually abbreviated to D.C. The other kind is alternating current, usually known as A.C.

№11. D.C. AND A.C

One kind of electricity is called direct current, usually abbreviated to D.C. This is the kind of electricity we get from a battery, the flow of current is in one direction only. The other kind is alternating current, usually known us A.C. This is the kind of electricity which is generated in power stations for domestic and industrial use. It is called alternating current because the direction of flow changes very quickly. The number of changes of direction, called cycles, in every second is known as frequency. The frequency of alternating current in most parts of Europe is 50 cycles. Very high frequencies are used in radio communication, sometimes several million cycles.

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