An "induction recorder" records by detecting and measuring the magnetic field produced by any common speaker.
Most speakers use an electromagic of fluctuating power to drive the mechanical sound-producing component of the speaker. The magnetic field produced is directly comparable to the sound output. In fact, at its basic level, this is very very similar to recording using an electrical circuit, except appropriate accomodations for signal gain and noise filtering are required.
As I have mentioned before, any common microphone works similarly to an induction microphone in that an oscillating magnet in the mouthpiece drives an electric circuit via induction.
