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A scientist named Michel Faraday discovered in the year 1831 that a change in the value of the magnetic flux linked with a conducting coil give rise to the induction of an electron motive force in the coil. The emf generated this way is called the induced emf. Fig. 1 illustrates simple experiment which demonstrates the electromagnetic induction. In this experiment, a sensitive galvenometer is connected in series with a conducting coil. In fig. 1(a), a bar magnet is held stationary with respect to the coil. No deflection of galvenometer is observed under this situation. Now if the bar magnet is quickly moved towards the coil, with one of its poles facing the coil, the galvenometer records a deflection, showing that some current flows through the coil. It is also seen that faster the magent is moved, greater is the deflection of the galvenometer. Similiar effects are also observed if the coil is moved keeping the magnet fixed. Again, deflection in opposite direction is observed. When the magnet is moved away, to that recorded when the magnet is moved towards the coil. If the galvenometer deflects on one side when a particular pole is facing the coil (say north pole), delection of the opposite sense is recorded if the other pole is made to face the coil; when the magnet is brought near the coil. Also whenever, the coil and the magnet are stationary with respect to each other, no deflection is observed.
The above results can be understood as follows. When the bar magnet is near the coil some of the magnetic lines of force are passing through the coil; that is "some magnetic flux is linked with the coil". Now when there is relative motion between the coil and the magnet, the amount of flux linked with the coil is changing. When the relative motion stops, there is no further chang in the amount of the flux linked. So we conclude that, "when there is a change in the flux linked with the coil, there is an emf generated in the coil". The observation that a faster motion of the magnet gives rise to a larger defiection, shows that the emf generated depends upon the rate of change of the flux linking the coil. The current resulting from this "induced emf" is called the "induced current". To cause the change in the flux linked with the coil, it is not necessary to have a bar magnet. If two coils S and P are placed near each other as shown in fig. 2; and a key connected in the circuit of the coil P is switched on and off so that current alternately flows and stops flowing through the coil P, the galvenmeter connected in the circuit with the coil S alternately
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