A bar magnet is suspended vertically from the free end of a helical spring, as shown in the figure below.
One pole of the magnet is situated in a coil. The coil is connected in series with a high-resistance voltmeter. The magnet is displaced vertically and then released. The variation with time 
of the reading 
of the voltmeter is shown in the graph below.
State Faraday’s law of electromagnetic induction.
Use Faraday’s law to explain why
there is a reading on the voltmeter,
this reading varies in magnitude,
the reading has both positive and negative values.
Use the graph above to determine the frequency 
of the oscillations of the magnet.
.
