Page 884 - Basic Electrical Engineering

A is the area of cross-section of magnetic flux path,
l is the length of the flux path.

Figure 12.7 Transducers operating on the basis of variable inductance

Thus, the inductance of a coil can be varied by (i) changing the
permeability of the core material, i.e., by changing the position of the core

inside the coil; (ii) changing the number of turns through which current will
flow in the coil, and (iii) changing the ratio of A/l of the coil. These methods
are shown in Fig. 12.7. These are passive transducers requiring power supply

and the output is analog in nature.
Two coils having inductance L and L will have mutual inductance M as
1
2

where K is the coefficient of coupling