Question

In a meter bridge the point D is a neutral point (Figure).

1. The meter bridge can have no other neutral point for this set of resistances.
2. When the jockey contacts a point on meter wire left of D, current flows to B from the wire.
3. When the jockey contacts a point on the meter wire to the right of D, current flows from B to the wire through galvanometer.
4. When R is increased, the neutral point shifts to left.

Options

• a and c

• a and d

• b and c

• c and d

Answer 1

a and c

Explanation:

In case of meter bridge, the resistance wire AC is 100 cm long. Varying the position of tapping point B, bridge is balanced. If in balanced position of bridge AB = l, BC = (100 – 1) so that Q/P = (100 – 1)/1. Also P/Q = R/S ⇒ S =(100 – 1)/1R

When there is no deflection in galvanometer there is no current across the galvanometer, then points B and D are at the same potential. That point at which the galvanometer shows no deflection is called the null point and then potential at B and neutral point D is the same. When the jockey contacts a point on the meter wire to the right of D, the potential drop across AD is more than the 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.