This device is clearly an AC generator. The emf is directly proportional to the angular velocity, the magnetic flux density,
and the area of the loop. The generator can be designed as a trade-off between the various parameters. If the frequency must
be constant, the loop must be rotated at a fixed angular velocity. Most generators operate on this or a very similar principle.
As an example, the constant flux density may be generated by a permanent magnet or by an electromagnet and a DC source.