If a bar magnet is inserted into a coil of wire a sensitive voltmeter will show a deflection but only while the magnet is moving. As soon as it stops the instrument reads zero. If the magnet is withdrawn quickly the meter deflection will be in the opposite direction – but again only until movement stops.
The quicker the movement, the higher the reading.
The fundamental principle which this illustrates is that of electromagnetic induction: any change of magnetic flux linking a conductor will induce a voltage in it.
Alternating flux is constantly changing, and so induces a corresponding alternating voltage. We use this principle in two ways:
The rate of change of an alternating voltage is its frequency, i.e. number of positive and negative pulsations, cycles per second, with the SI unit of Hertz (Hz). Just as moving the magnet quicker gave a higher reading, alternating a field at a higher frequency induces a higher voltage for the same field strength