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Measure current and voltage · Physics

Voltmeter

Voltmeter, with labels: scale in V, pointer, zero mark.

What it is for

A voltmeter measures the potential difference (p.d.) across a component, in volts (V). It is connected in parallel: one lead on each side of the component, outside the main loop.

Range
0 to 5 V (the Practical Test voltmeter)
Smallest division
0.1 V
Record to
the nearest 0.1 V, 3.2 V, or, estimating between marks, half a division, 3.25 V; one precision in a table

Why choose it

For the p.d. across a lamp, resistor or cell, and, with an ammeter, to find resistance. A digital voltmeter or multimeter shows the number directly, often to 0.01 V; with either kind, choose a range just above the largest p.d. you expect.

How to read it

012345Vlook straight at the pointer: 3.2 V
  1. Build the main circuit first, then connect the voltmeter across the component.
  2. Join its + terminal to the side nearer the + of the supply.
  3. With the supply off, check the pointer is on 0.
  4. Read the pointer against the 0.1 V marks and write the unit: 3.2 V.

Exam-style question

A lamp and a resistor are in series with a 4.5 V battery. A voltmeter across the lamp reads 3.2 V, and the current is 0.25 A.

  1. Calculate the p.d. across the resistor.
  2. Calculate the resistance of the lamp.
  3. Why would a voltmeter connected in series with the lamp be useless?
Show what a good answer says
  1. The p.d.s in series add up to the supply p.d.: 4.5 − 3.2 = 1.3 V.
  2. R = V ÷ I = 3.2 ÷ 0.25 = 12.8 Ω, about 13 Ω.
  3. A voltmeter has a very large resistance, so in series almost no current flows: the lamp goes out and the meter reads nearly the whole supply p.d.

Teaching points for checking your own answer, not official Cambridge marks.

Common errors

  • Connecting it in series instead of across the component: almost no current then flows.
  • Joining + and − the wrong way round, so the pointer is pushed below 0.
  • On a meter with two scales, reading the scale that does not match the terminal in use.
  • Reading the pointer from one side (parallax error), or leaving out the unit (V).

Where you use it here

No experiment on the site uses it yet; more are on the way.

From the Practical Test apparatus lists of the Cambridge IGCSE Biology 0610, Chemistry 0620 and Physics 0625 syllabuses for 2026–2028, and the apparatus the experiments on this site use.

More in “Measuring instruments”