Whenever the strength of a magnetic field changes near a conductor, a voltage is induced across the conductor. The process is called electromagnetic induction and is the basis of transformer action. A transformer usually consists of two or more coils linked by the same magnetic field, one of the coils, the primary, being used by the application of an alternating voltage, to set up the changing magnetic field which then links the other coil or coils. The remaining coils are called secondary coils and the alternating voltage induced across them may be used to supply electrical circuits. The relative sizes of the voltage across the primary coil and the induced secondary voltages depend upon the ratio of the number of turns on the primary coil and the number of turns on the secondary coils and it is possible to have secondary voltages greater or smaller than the primary voltage causing the transformer action. The voltage applied to the transformer is, of course, alternating since otherwise the magnetic field is not continuously changing. A transformer is an extremely useful circuit component because of the job it does and because it is extremely efficient and easy to maintain. Transformers are widely used in the distribution of electricity nationwide enabling voltages to be generated, distributed and used by the consumer at different levels to obtain maximum efficiency and safety. In electronic circuits transformers are commonly used to reduce the input mains voltage to a safe level for the components in the circuit; they may also be used in association with cathode ray tubes to increase the available voltages to a level high enough to operate these tubes.
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