There are two long co-axial solenoids of same length $l$. The inner and outer coils have radii $r _ { 1 }$ and $r _ { 2 }$ and number of turns per unit length $\mathrm { n } _ { 1 }$ and $\mathrm { n } _ { 2 }$, respectively. The ratio of mutual inductance to the self-inductance of the inner-coil is: (1) $\frac { n _ { 1 } } { n _ { 2 } }$ (2) $\frac { n _ { 2 } } { n _ { 1 } } \cdot \frac { r _ { 1 } } { r _ { 2 } }$ (3) $\frac { n _ { 2 } } { n _ { 1 } } \cdot \frac { r _ { 2 } ^ { 2 } } { r _ { 1 } ^ { 2 } }$ (4) $\frac { n _ { 2 } } { n _ { 1 } }$
There are two long co-axial solenoids of same length $l$. The inner and outer coils have radii $r _ { 1 }$ and $r _ { 2 }$ and number of turns per unit length $\mathrm { n } _ { 1 }$ and $\mathrm { n } _ { 2 }$, respectively. The ratio of mutual inductance to the self-inductance of the inner-coil is:\\
(1) $\frac { n _ { 1 } } { n _ { 2 } }$\\
(2) $\frac { n _ { 2 } } { n _ { 1 } } \cdot \frac { r _ { 1 } } { r _ { 2 } }$\\
(3) $\frac { n _ { 2 } } { n _ { 1 } } \cdot \frac { r _ { 2 } ^ { 2 } } { r _ { 1 } ^ { 2 } }$\\
(4) $\frac { n _ { 2 } } { n _ { 1 } }$