Question
In a meter bridge the point D is a neutral point (Fig).
✓
Answer
- The meter bridge can have no other neutral point for this set of resistances.
- When the jockey contacts a point on the meter wire to the right of D, current flows from B to the wire through galvanometer.
Key concept: Meter bridge: In case of meter bridge, the resistance wire AC is 100cm long. Varying the position of tapping point B, bridge is balanced. If in balanced position of bridge AB = l, BC = (100 - l) so that $\frac{\text{Q}}{\text{P}}=\frac{(100-\text{I})}{\text{I}}$. Also $\frac{\text{P}}{\text{Q}}=\frac{\text{R}}{\text{S}}>\text{S}=\frac{(100-\text{I})}{\text{I R}}$.
When there is no deflection in galvanometer there is no current across the galvanometer, then points B and D are at same potential. That point at which galvanometer shows no deflection is called null point, then potential at B and neutral point D are same. When the jockey contacts a point on the meter wire to the right of D, the potential drop across AD is more than potential drop across AB, which brings the potential of point D less than that of B, hence current flows from B to D in the galvanometer wire.
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