A field theory of bimodal cavity spin inductors for ERP spectrometry is developed. All the leakage effects are taken into account in order to attain results which are useful for an optimum design of such devices. Furthermore, perturbations of the cavity shape are considered in order to study the behavior of external decoupling devices. A general equivalent network is derived and formulas are given to evaluate all its parameters for any particular case. All the requirements to be fulfilled in order to minimize leakage and its frequency dependence are discussed. The general results obtained can be useful for the designer, who will be able to quantitatively evaluate the characteristic of a particular resonator, to compare different solutions, and to optimize all the parameters involved in a particular design.