Question

If in the experiment of Wheatstone’s bridge, the position of cells and galvanometer are interchanged, then the balance point will
A. change
B. remain unchanged
C. depend on the internal resistance of cell and resistance of galvanometer
D. none of these

Answer 1

Wheatstone bridge is an electrical circuit used to determine the resistance of an unknown resistor. It has four resistors and the ratio of two of the resistors is kept fixed. The other two resistors are balanced out of which one resistor is unknown. The unknown resistance is calculated using the null or balanced condition. So, to solve this problem, write the balanced condition of the Wheatstone bridge and then write the balanced condition after interchanging the position of cells and galvanometer. Compare these to conditions and find the answer.

Complete answer:

From the above figure, we can say at null condition or balanced condition of the bridge,
$\dfrac {P}{Q}= \dfrac {S}{R}$ …(1)
If we interchange the position of cell and galvanometer the circuit becomes as shown below.

After interchanging the positions of cell and galvanometer, we can say at null condition or balanced condition of the bridge,
$\dfrac {Q}{R}= \dfrac {P}{S}$
$\Rightarrow \dfrac {S}{R}= \dfrac {P}{Q}$ …(2)
From the equation. (1) and (2) we can say before and after changing the position of cells and galvanometer, the balanced point remains unchanged.

So, the correct answer is option B i.e. remain unchanged.

Note:
Wheatstone bridge gives a very precise measurement of resistance. However, students must remember that the Wheatstone bridge is a very sensitive device and the measurements may not be precise in an off-balance condition. If all the four resistors are not comparable then the sensitivity of the circuit reduces. At the balanced condition, the current flowing through the galvanometer is zero.