Two concentric circular coils, one of small radius `r_(1)` and the other of large radius `r_(2)`, such that `r_(1)ltltr_(2)`, are placed co-axially with centres coinciding. Obtain the mutual inductance of the arrangement.

Two concentric circular coil,s one of small radius r_(1) and the other of large radius r_(2) , such that r_(1) lt lt r_(2) , are placed co-axially with centres coinciding. Obtain the mutual inductance of the arrangement.

A wire of length l is bent into a circular coil of one turn of radius R_(1) . Another wire of the same material and same area of cross-section and same lengths is bent into a circular coil of two turns of radius R_(2) . When the same current flows, through the two coils, the ratio of magnetic induction at the centres of the two coils is