When current flows through the primary coil winding, it will produce a magnetic
field. As this magnetic field builds up, it will move outward from the primary coil
winding. This is a moving magnetic field, and will therefore produce a voltage
when the moving field crosses the secondary winding. The voltage produced will
be proportional to the number of turns in the primary and secondary winding. In
the example above, the secondary coil has 10 times as many windings as the
primary coil. Therefore, the voltage produced will be (theroretically) 10 times as
high, or 1000 volts.
 

    When the magnetic field stops moving, a voltage is no longer being produced
in the secondary winding - and the coil has reached it's saturation point. As the
magnetic field is building in the primary winding, a current flow starts to develop
in the primary winding itself that opposes the current flowing through the primary
coil. This causes the magnetic field to build slowly. As has been indicated earlier,
the amount of voltage being produced in a winding is dependant on the speed of
the magnetic field. So how can the magnetic field be speeded up? The answer is
quite simple - switch off the current flow in the primary winding and the magnetic
field will collapse quickly producing a higher voltage in the secondary winding than
when the field expanded slowly.
 

    This is the basis for the automotive ignition system - allow a magnetic field to
build up - and then collapse it quickly to produce a high voltage! You might want
to think of the buildup and collapse of the field like blowing up and deflating a
balloon. It takes a long time to blow up a balloon, but if you stick a pin in it after
it is blown up - it collapses very rapidly!