Induced e.m.f.

This is the physics behind how electric generator work:  Move a magnet towards a coil of wire, and you create electricity in the wire.

induced e.m.f.

In the figure, there is a coil of wire with a few turns.  The two ends of the wire are connect to an galvanometer.  A galvanometer is a special type of ammeter where the needle is in the middle.  The needle can move left or right depending on the direction of the current.  (If you use a digital ammeter, then it can show positive or negative numbers for different directions of the current.)  

There is a magnet on one side of the coil.  At first, the galvanometer needle is at the centre, showing zero current.  If you now move the magnet towards the coil, the needle moves to one side.  (It can be left or right, depending on how you connected the wires to the galvanometer.)  This shows that there is a current.  This means that a voltage is induced by moving the magnet to the coil.

When you stop moving the magnet, the needle on the galvanometer returns to the middle.  This means that the current has stopped.

If you then move the magnet away from the coil, the needle moves in the opposite direction.  This means that the there is a current in the opposite direction.  When you stop moving the magnet, the needle returns to the centre.  This means that the current stops.

So if you move the magnet, whether left or right, you induce a voltage.  If you stop moving the magnet, no voltage is induced.

Furthermore, if you move the magnet faster, the needle will move further.  This means that the faster the magnet moves, the large is the voltage induced.  Michael Faraday summarised this result into a law:

"The induced voltage is proportional to the rate of change of flux linking the coil."

This is called the Faraday's law of electromagnetic induction.

Michael was thinking in terms of the magnetic flux lines around the magnet.  These lines are all around the magnet and some go through the coil - linking the magnet to the coil.  The flux lines move with the magnet.  These flux lines are closer together near the magnet, but further apart away from the magnet.

So when the magnet moves, the amount flux lines linking the coil changes.  So Michael was saying that this is what really matter, that the induced voltage depends directly on  how fast amount of flux linking the coil changes.