MAGNETIC FIELD. RESISTANCE

№ 4. MAGNETIC FIELD

Electricity and magnetism are closely connected. Each may be used to produce each other. Around the poles of a magnet is an area called a magnetic field. In this area the magnet exerts its attraction. The directions of this attraction and the shape of the magnetic field are shown by lines of force. These lines of force can be seen if a small magnet is placed under a piece of paper onto which iron filings are sprinkled. When the electric current flows through a conductor, a magnetic field is set up around it. The conductor becomes a magnet. If a conductor is made to rotate in the magnetic field, a pressure of electricity called the voltage, is produced in it. This is the principle of production of electricity by a dynamo machine.

№ 5. RESISTANCE

The restriction to the flow, which is caused by the conductor, is known as its resistance. A unit of resistance is called an ohm. The resistance of a conductor will also vary according to the material of which it is made. Copper has much lower resistance than other common metals. It is a very good conductor. Current, voltage and resistance have a definite relationship to one another. Ohm's law tells us that current equals voltage divided by resistance.

Resistance tends to reduce the flow of electric current. Heat develops in a metallic conductor whenever current flows. The amount of heat developed when the current is flowing through the conductor is the measure of the ohm resistance of the conductor. When an electric current flows through a conductor there is a loss of energy as well as a loss of voltage. These loses are directly proportional to the amount of resistance. The larger the diameter of the wire, the smaller the resistance is and the current will flow through it. The resistance of the conductor varies with temperature. It is possible to find out the resistance of the conductor at any given temperature knowing its resistance at one temperature. Resistance is manifested whether the current is a direct or an alternating one.

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