How does the mutual inductance of a pair of coils change when the distance between the coils is increased?

In which of the cases shown in Fig. 6.70, will the mutual inductance of two coils be maximum, if the number of turns remains the same in each case?

In which of the cases shown in Fig. 6.70, will the mutual inductance of two coils be minimum, if the number of turns remains the same in each case?

The circuit arrangement given in Fig. 6.71 shows that when an a.c. passes through the coil $A$, the current starts flowing in the coil $B$. Name the underlying principle involved.

The circuit arrangement given in Fig. 6.71 shows that when an a.c. passes through the coil $A$, the current starts flowing in the coil $B$. Mention two factors on which the current produced in the coil $B$ depends.

Figure 6.72 given below shows an arrangement by which current flows through the bulb (X) connected with coil $B$, when a.c. is passed through coil $A$.

Explain the following observation:

Bulb lights up.

Figure 6.72 given below shows an arrangement by which current flows through the bulb (X) connected with coil $B$, when a.c. is passed through coil $A$.

Explain the following observation:

Bulb gets dimmer if the coil '$B$' is moved upwards.