Question

Figure (i) below shows a Wheatstone's bridge in which $P,Q,R$ and $S$ are fixed resistances, $G$ is a galvanometer and $B$ is a battery. For this particular case, the galvanometer shows zero deflection. Now, only the positions of $B$ and $G$ are interchanged, as shown in figure (ii). The new deflection of the galvanometer

• A. is to the left
• B. is to the right
• C. is zero
• D. depends on the values of $P,Q,R$ and $S$

Answer 1

Answer: (C)

In case (i), galvanometer shows zero deflection.
$\therefore V_B=V_D$
$\Rightarrow \frac{P}{S}=\frac{Q}{R}$ .....(i)
When battery $B$ and galvanometer $G$ are interchanged, position of galvanometer is as shown below,

Now, ratio of resistances across galvanometer is
$\frac{S}{P}$ and $\frac{R}{Q}$
As from Eq. (i), $\frac{S}{P}=\frac{R}{Q}$
Hence, galvanometer still shows zero deflection because Wheatstone's bridge is balanced.