Given a sensitive galvanometer G and a collection of resistances of various values,a voltmeter can be constructed by
A)connecting a small resistance in series with G.
B)connecting a large resistance in series with G.
C)connecting a small resistance in parallel with G.
D)connecting a large resistance in parallel with G.
E)It is not possible to make a voltmeter with this equipment.

Question

Quizplus · Accepted Answer

A voltmeter is designed to measure potential difference across a circuit element. To achieve this, a high resistance should be used in series with the galvanometer so that it draws very little current from the circuit being measured. Connecting a small resistance in series with G will not achieve this objective as it will still draw significant current from the circuit being measured. Connecting a small resistance in parallel with G will cause the galvanometer to short-circuit and damage it. Connecting a large resistance in parallel with G will not help either as it will cause the voltage drop to be small across G compared to the voltage drop across the parallel resistor. Therefore, connecting a resistance in series with G is the best choice. It should be chosen such that when the galvanometer is connected in series with it, it can measure the voltage on the circuit without altering the circuit behavior. The best resistance to use is one with the same order of magnitude as the resistance in the circuit being measured so that the total resistance is significantly larger than G, but not much larger.

Given a Sensitive Galvanometer G and a Collection of Resistances