EFFECT OF TEMPERATURE ON ELECTRICAL RESISTANCE

№ 6. EFFECT OF TEMPERATURE ON ELECTRICAL RESISTANCE

The flow of current through a conductor is always accompanied by the development of heat. The amount of the developed heat is proportional to the square of the current flowing in the conductor. It is found that the electrons meet more resistance when the conductor is hot than when it is cold. When we turn on the light and our electric lamp is burning the tungsten filament of the lamp has about 30 times the resistance more than it had when cold. There are some exceptions to the general rule. Take carbon, for example, with its temperature rising, its resistance decreases. Thus, tungsten is usually the very metal which is used as the filament of electric lamps.

Various high resistance alloys are commonly employed in the heating devices used in our everyday life.

№ 7. A FEW NOTIONS OF ELECTRICITY

Ohm’s law tells us that current equals voltage divided by resistance. The current and the voltage multiplied together determine the power, the rate at which the electrical energy is used up when it is converted to other forms of energy. A unit of power is a watt. To simplify the figures used for great power, abbreviations are used. One thousand watts is kilowatt and one million watts is megawatt. Very sensitive instruments can measure a milliwatt which is a thousandth part of a watt. A millionth part of a watt is a microwatt. Electrical appliances are described according to the amount of power which they consume, so we can talk of a 60 watt lamp.

№ 8. INSULATORS

A material which allows electricity to flow through it is called a conductor. Any material which will not permit the flow of electricity is known as an insulator .Conductor wires in any piece of electrical apparatus or installation must not be allowed to touch each other or any other metal part. If they do the short circuit will result which will cause failure of the device and the possible dangers of fire and electric shock Insulating materials are used to cover and protect conductor wires and sometimes to separate them. Common insulating materials include air, oil, glass, rubber and plastics. Heavy power cables are sometimes given an extra cover made of metal for the further protection of the insulation.

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