linguistic:paraphrasing, information:classification
Coupling coefficient in the resonance state

In the transformer, only part of the flux generated by current through the primary coil is coupled to the secondary coil and vice versa. The part that couples is called mutual flux and the part that does not couple is called leakage flux. When the system is not in the resonance state, this leads to the open-circuit voltage appearing at the secondary being less than predicted by the turns ratio of the coils. The degree of coupling is captured by a parameter called coupling coefficient. The coupling coefficient, k, is defined as the ratio of transformer open-circuit voltage ratio to the ratio that would be obtained if all the flux coupled from one coil to the other. However, if it is not open circuit, the flux ratio will change. The value of k lies between 0 and ±1. Each coil inductance can be notionally divided into two parts in the proportions k:(1− k). These are respectively an inductance producing the mutual flux and an inductance producing the leakage flux.