The mutual inductance of any two coaxial circular coils and the distribution of the magnetic induction line from the system;

Discussion on the mutual inductance M_(12 )=M_(21);

The approximate analytic formula of the mutual inductance between two loops with the same axis and size and that of their interaction;

Direct proof for the coefficient of mutual inductance M_(21)=M_(12);

There is only one coefficient of mutual inductance about two round roads L_1 and L_2,i.

A simple and convenient method of direct proof for the coefficient of mutual inductance is given.

The directives concerning the protection of telecommunication lines against harmful effects from electricity lines specified by Comite Consultatif Internationale de Telegraphique et Telephonique (CCITT) give basic representation of mutual inductance coefficient which contains infinite integral.

This paper deduces the general result that the mutual inductance coefficient can be ne- glected when two infinitely long and straight solenoids in connected series from the theoreti- cal calculation and corrects the mistakes which appeared in some papers when the problem was discussed.

By means of analyzing the magnetic field distributing principle of bearingless motor,and based on the relationship between magnetic flux and windings inductance parameters,the general calculating model of mutual-inductance parameters of four-pole bearingless motor with two-pole suspension system was derived.