Now, note that the common form of this equation is written with a N in the left hand side equation, denoting the number of coils in the second system. The difference between the two forms is that the other way of writing it assumes a case of a coil, and defines \(\Phi\) to be the flux through one coil within the second system. While that is correct in the case of a coil, it is possible that writing it as the above, while defining \(\Phi\) to be the total flux through the second system generalises for immediate application to most questions.
It should also be noted that the mutual inductance is the same for both systems. 

6.1.1 Consider two single-turn co-planar, concentric coils of radii \(R_1\) and \(R_2\), with \(R_1>>R_2\). Determine the mutual inductance between the two loops. 

6.2 Self-Inductance

Note, however, that inductance can also be applied to the system providing the magnetic flux. This property is denoted by self-inductance, and we call circuit elements with high self-inductance inductors. Mathematically,