Since the moving tape is carrying a changing magnetic field with it, it induces a varying voltage across the head.
A head consists of a core of magnetic material arranged into a doughnut shape or toroid, into which a very narrow gap has been let.
A coil of wire wrapped around the core opposite the gap interfaces to the electrical side of the apparatus.
The basic head design is fully reversible - a variable magnetic field at the gap will induce an electric current in the coil, and an electric current in the coil will induce a magnetic field at the gap.
While a head is reversible in principle, and very often in practice, there are desirable characteristics that differ between the playback and recording phases.
Manufacturers must find a compromise between intended tape speeds and head gaps for this reason.
Sometimes this can be achieved by automatic circuitry - the actual mechanical azimuth adjustment being carried out by taking advantage of the piezo effect of certain types of crystal material.
The erase head is powered during recording from a high frequency source (usually the same oscillator that provides the AC bias).
This material features extremely good electro-acoustical properties, but wears away fairly rapidly with a consequent deterioration of performance.
Its two main disadvantages are that it is brittle and easily damaged, and that it has a much higher noise output due to the Barkhausen effect.