Question

In a series type ohmmeter the zero adjustment should be done by changing the

• A. Value of series resistance
• B. Value of shunt resistance connected across meter movement
• C. Both series as well as shunt resistance
• D. Battery voltage

Hint:

In a series ohmmeter, the scale is backward: "0 ohms" is at full-scale deflection (right side), and "Infinity" is at zero current (left side).

Answer 1

The Correct Option is B

1. Zero Adjustment in Series Ohmmeter: "Zero adjustment" refers to the process of making the needle point to "0 ohms" (full-scale deflection) when the test leads are shorted together.

2. Role of the Shunt Resistance ($R_2$): In a standard series ohmmeter, a variable resistor ($R_2$) is connected in shunt (parallel) with the meter movement.

• As the battery voltage decreases over time, the total current in the circuit drops.

• By adjusting the shunt resistor $R_2$, we change the proportion of current diverted through the meter.

• Increasing $R_2$ forces more current through the meter, allowing it to reach full-scale deflection (0 ohms) even with a slightly weaker battery.

3. Why not the series resistor?: While changing the main series resistance ($R_1$) could technically change the current, it would also significantly alter the "mid-scale" calibration of the meter, making the non-linear scale inaccurate across the rest of the range. Shunt adjustment is the standard method for compensation.