Faraday conducted a series of experiments on coils in a magnetic field.
He found that if the magnetic flux density threading a coil were changed, a voltage was induced in the coil and a current flowed if the circuit was closed.
This was true however the change in flux is produced e,g,
1) relative movement between the coil and the magnetic field
2) a change the current flowing in a nearby coil
3) a change in the current flowing in the coil itself.