If the current $30 \,A$ flowing in the primary coil is made zero in $0.1 \,s$,the $e.m.f.$ induced in the secondary coil is $1.5 \,V$. The mutual inductance between the coils is.....$H$.

An $e.m.f.$ of $100$ $mV$ is induced in a coil when the current in another nearby coil changes from $0$ to $10$ $A$ in $0.1$ $s$. The coefficient of mutual induction between the two coils is $...$ $mH$.

$A$ long straight wire is placed along the axis of a circular ring of radius $R$. The mutual inductance of this system is

$A$ small square loop of wire of side $l$ is placed inside a large circular loop of radius $r$. The loops are coplanar and their centers coincide. The mutual inductance of the system is proportional to

$A$ small square loop of wire of side $\ell$ is placed inside a large square loop of wire of side $L$ $(L \gg \ell)$. The loops are coplanar and their centers coincide. The value of the mutual inductance of the system is $\sqrt{x} \times 10^{-7} \text{ H}$, where $x = \dots$

Two circular coils have their centers at the same point. The mutual inductance between them will be maximum when their axes