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You build a circuit, add an ammeter and a voltmeter, divide one reading by the other, and the resistance comes out wrong. The meters are not broken. Every real meter has resistance, and it changes the circuit it is measuring. This post shows how big that effect is, which way to connect the meters, and how to correct a reading.
Where does this appear in exams?
The JEE (Main) 2026 syllabus lists the moving coil galvanometer and its "conversion to ammeter and voltmeter" in Unit 13. The JEE (Advanced) 2026 brochure includes voltmeters and ammeters in its syllabus and lists "verification of Ohm's law using voltmeter and ammeter" among its practical topics. The CBSE Physics curriculum for 2026-27 includes the conversion experiments in its Class XII practicals. Check your exam year's documents.
The NCERT chapter Moving Charges and Magnetism explains the design goal: an ammeter uses a small shunt so that putting it in series hardly changes the current, and a voltmeter uses a large series resistance so that it draws little current. It adds, "Usually we like to keep the disturbance due to the measuring device below one per cent." Our post on the moving coil galvanometer covers how the meters are built. Here we look at what they do to a circuit.
How does an ammeter change the current?
An ammeter of resistance R_A in series adds R_A to the loop. Take a cell of EMF 3.0 V and internal resistance 1.0 Ω driving a 5.0 Ω resistor. The true current is 3.0 / 6.0 = 0.50 A. Insert an ammeter of 0.50 Ω and the current becomes 3.0 / 6.5 ≈ 0.46 A, about 7.7% low. A small ammeter resistance matters when the rest of the circuit is also small. Our post on EMF and internal resistance shows why the internal resistance belongs in the sum.
How does a voltmeter change the voltage?
A voltmeter of resistance R_V across a resistor R puts the two in parallel, giving RR_V / (R + R_V), which is less than R. Less resistance means a smaller share of the voltage, so the voltmeter reads low. The effect is small only when R_V is much bigger than R.
Which way should you wire a resistance measurement?
To find R from V/I, there are two standard circuits. Use an ammeter with R_A = 0.50 Ω and a voltmeter with R_V = 2000 Ω.
Circuit (a): voltmeter directly across R. The ammeter reads the current through R and the voltmeter together, so V/I gives R in parallel with R_V.
R = 10 Ω gives 9.95 Ω, an error of −0.5%.
R = 5000 Ω gives about 1429 Ω, an error of −71%.
Circuit (b): voltmeter across R and the ammeter. Now the voltmeter reads the drop across both, so V/I gives R + R_A.
R = 10 Ω gives 10.5 Ω, an error of +5%.
R = 5000 Ω gives 5000.5 Ω, an error of +0.01%.
The rule: use circuit (a) for small resistances and circuit (b) for large ones. The two errors are roughly equal when R² = R_A R_V, here R ≈ √(0.50 × 2000) ≈ 32 Ω.
Can you correct a reading?
Yes, if you know the meter resistances. With circuit (a), suppose the meters read 2.00 V and 0.201 A (sample numbers). The voltmeter itself takes 2.00 / 2000 = 0.001 A, so R carries 0.200 A and R = 2.00 / 0.200 = 10.0 Ω, not the 9.95 Ω that V/I gives. With circuit (b), subtract the ammeter: R = V/I − R_A.
Why do meter problems confuse students?
Engelhardt and Beichner tested high school and university students on DC circuits and found that they tended to focus on current and often gave voltage or resistance the properties of current. Meter questions punish exactly that. An ammeter measures current through itself and a voltmeter measures the potential difference across itself; keeping that straight is half the work.
Which mistakes cost marks?
Assuming meters are ideal when values are given. If R_A or R_V is stated, use it.
Swapping the meters. A voltmeter in series blocks most of the current; an ammeter in parallel nearly short-circuits the component.
Expecting a voltmeter on a cell to read the EMF. It reads the terminal voltage, and only approaches the EMF when it draws almost no current.
Picking the wrong circuit. Compare R with √(R_A R_V).
A practice routine
Day 1: Derive the measured value for circuits (a) and (b).
Day 2: Redo the four cases above with R_V = 10 000 Ω and find the new crossover.
Day 3: Solve two problems where a voltmeter is placed across a resistor in a series pair.
Day 4: Correct three sets of sample readings using the known meter resistances.
Day 5: Take a mixed test. Dunlosky and colleagues (2013) rated practice testing and spaced practice as the most useful study techniques.
When a circuit with meters will not come out right, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Next time you use a lab multimeter, look up its input resistance and decide which circuit suits the resistor you are measuring.
