Physics · Moving Charges And Magnetism · NEET
A voltmeter is connected across (in parallel with) a component, so it must NOT steal current from that component. Adding a large resistance in series makes the galvanometer's total resistance very high, so it draws almost no current and the circuit is barely disturbed. An ammeter goes in series in the main line, so it must have very LOW resistance - that is why the ammeter gets a small parallel shunt instead. Series-for-voltmeter, parallel-for-ammeter is the key rule.
R = V/Ig - G. Here V is the maximum voltage the voltmeter should read (its range), Ig is the current that gives full-scale deflection in the galvanometer, and G is the galvanometer's own resistance. The idea: at full scale the same current Ig flows through R and G in series, so V = Ig(R + G). Solve for R.
An ideal voltmeter has infinite resistance so it draws zero current. If it drew current, it would change the very voltage it is trying to measure (loading effect). NCERT says we keep the disturbance below one per cent, so R is made large - typically thousands of ohms - and G becomes tiny compared to R.
Usually very little. Since R is very large and G is small, R + G is almost equal to R. That is why NCERT writes R + G ≈ R (large). But in NEET numericals you must still subtract G to get the exact series resistance: R = V/Ig - G, not just V/Ig.
A higher range needs a larger series resistance. From R = V/Ig - G, if you want to read a bigger V (same galvanometer), R must increase. To make a multi-range voltmeter, use different series resistances, one for each range.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
R = V/Ig - G, where R is the high resistance added in series, V is the desired voltage range, Ig is the full-scale deflection current, and G is the galvanometer resistance.
In series. A high resistance in series makes the voltmeter's total resistance very high so it draws negligible current when connected across a component.
So it draws zero current and does not disturb the circuit it measures. This avoids the loading error and gives the true voltage across the component.
Voltmeter: add a HIGH resistance in SERIES (high total resistance). Ammeter: add a LOW resistance (shunt) in PARALLEL (low total resistance). They are opposite modifications.
R = V/Ig - G = 18/0.003 - 12 = 6000 - 12 = 5988 ohm in series. Notice G (12) is tiny compared to R, so the voltmeter resistance is almost all R.
