Coupling factor:
If all the flux of a primary coil links with all the turns of a secondary then 100% coupling exists. Sometimes it is more convenient to use a coupling factor - k. Maximum Coupling (100%) is represented by a k value of 1.Thus if flux linkage is 97% the coupling factor is 0.97. Given that mutual coupling depends on k then so does the mutual inductance. The relationship is given by: M = k√ (L_{1} L_{2} )Where L1 and L2 are individual inductance's of the mutually coupled coils.The value of k depends on:• Purpose of coils involved• Relative positions of the coils• Frequency or rate of change of currentand can be as high as 0.98 or as low as 0.0001.