**16 Analogue Recording and Reproduction**

**Formulae**

Magnetomotive force, m.m.f is given by

m.m.f. = *NI*

where *N* is the number of coil turns and *I* is the current.

The units of m.m.f are ampere turns/meter (At/m)

The intensity or field strength is the total force acting per unit length *I* of the magnetic circuit. It is called the magnetizing force *H.*

*H* = m.m.f./*l* = *NI/l*(AT/m)

The magnetizing force causes a flux **Ф** in the magnetic circuit.

The flux density *B* is the flux per unit area. The unit is the tesla (= webers/m^{2}).

**Ф** = m.m.f./*S*

where *S* is the reluctance of the circuit.

Figure 73 shows a typical *BH* loop (hysteresis loop).

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