What involves converting mechanical energy into electrical energy through relative motion in a magnetic field?

The process of converting mechanical energy into electrical energy through relative motion in a magnetic field is fundamentally described by electromagnetic induction. This concept entails that when a conductor, such as a wire, moves through a magnetic field or when the magnetic field around a conductor changes, an electromotive force (EMF) is induced in the conductor. This induced EMF generates an electric current if the circuit is closed.

The principle of electromagnetic induction is outlined in Faraday's law, which quantifies the induced EMF in relation to the rate of change of magnetic flux. However, it is the broader concept of electromagnetic induction that embodies the entire process of generating electricity from mechanical movement in a magnetic field.

In electric motors, mechanical energy is created from electrical energy rather than the other way around. Galvanometry, on the other hand, involves measuring the current through a conductor, which does not directly relate to the transformation of mechanical energy into electrical energy.

Thus, electromagnetic induction is the correct choice, as it fundamentally captures the mechanism of energy conversion through the interaction of mechanical motion and magnetic fields.