Electric Current and Ohm's Law

Lessons

Senior 1 · Physics

Electric Current and Ohm's Law

5 lessons

Lesson 3 of 5

Measuring Current and Voltage

24 min

The phone repairer's first tool

At a phone-repair kiosk in any Rwandan market town, the first tool the technician picks up is not a screwdriver but a multimeter. Is the charger really giving $5\,\mathrm{V}$ ? Is current reaching the battery? Is there a break inside the cable? Each question is answered by measuring a current, a potential difference or a resistance, and each measurement must be connected the right way, or the reading is wrong and the meter may be damaged.

Ammeters and voltmeters in a circuit

Figure 3.1: The ammeter is part of the loop, so the lamp's current flows through it. The voltmeter is connected across the lamp, from one end to the other.

An ammeter measures the current THROUGH a component, so the current must flow through the ammeter as well: it is connected in series. To add it, you break the circuit and connect the ammeter into the gap. A good ammeter has a very low resistance, so it hardly changes the current it measures.

A voltmeter measures the potential difference ACROSS a component, the energy per coulomb transferred between its two ends, so it is connected to those two ends, in parallel. You do not break the circuit to add it. A good voltmeter has a very high resistance, so almost no current is diverted through it.

On both meters, the terminal marked + (usually red) must be joined to the side that leads back to the + terminal of the supply.

Definition

An ammeter is an instrument that measures electric current, in amperes; it is connected in series.

A voltmeter is an instrument that measures potential difference, in volts; it is connected in parallel with the component.

Warning

Ammeter in series, voltmeter in parallel: never the other way round

An ammeter connected in parallel across a component (or across the supply) provides an almost resistance-free path: a very large current flows through it, which can blow the meter's fuse or damage it.
A voltmeter connected in series blocks the current with its huge resistance: the lamp goes out, and the meter reads nearly the full supply voltage, which is misleading.

Worked example 1: meters around a series circuit

A $6.0\,\mathrm{V}$ battery is connected to two lamps in series. An ammeter between the battery and the first lamp reads $0.40\,\mathrm{A}$ , and a voltmeter across the first lamp reads $2.5\,\mathrm{V}$ . What would an ammeter between the two lamps read, and what is the p.d. across the second lamp?

In series the current is the same everywhere: the second ammeter also reads $0.40\,\mathrm{A}$ .

The p.d. of the battery is shared: $V_2 = 6.0 - 2.5 = 3.5\,\mathrm{V}$ .

Worked example 2: meters around a parallel circuit

Two lamps are connected in parallel to a $3.0\,\mathrm{V}$ cell pack. The ammeter next to the cells reads $0.90\,\mathrm{A}$ and the ammeter in the branch of the first lamp reads $0.35\,\mathrm{A}$ . Find the current in the second branch and the reading of a voltmeter across the second lamp.

I_2 = I - I_1 = 0.90 - 0.35 = 0.55\,\mathrm{A}

Each branch is connected across the cells, so the voltmeter reads $3.0\,\mathrm{V}$ .

Reading an analogue meter

Figure 3.2: (a) An ammeter with a 0 to 1 A scale. (b) A galvanometer with its zero in the centre of the scale.

To read a scale, first work out what one small division is worth. On the ammeter in Figure 3.2(a) the numbered marks go up in steps of $0.2\,\mathrm{A}$ , and there are 10 small divisions between them, so each small division is $\frac{0.2\,\mathrm{A}}{10} = 0.02\,\mathrm{A}$ . Look at the pointer from straight in front: many meters have a mirror strip under the scale, and when the pointer hides its own reflection your eye is in the right place. Looking from the side gives a parallax error.

Worked example 3: reading the scale

Read the ammeter in Figure 3.2(a).

The pointer is past the $0.6$ mark by 2 small divisions:

I = 0.6 + 2 \times 0.02 = 0.64\,\mathrm{A}

The galvanometer and the multimeter

A galvanometer is a very sensitive meter for detecting small currents, often millionths of an ampere. Its zero is in the middle of the scale, so the needle swings one way for a current in one direction and the other way for a current in the opposite direction, as in Figure 3.2(b). It shows WHETHER a current flows, and which way, rather than measuring large currents.

A multimeter combines an ammeter, a voltmeter and an ohmmeter (for resistance) in one instrument. You choose the quantity and the range with the dial, and plug the black probe into the COM socket and the red probe into the socket for that quantity.

Instrument	Measures	Unit	How it is connected	Its own resistance
ammeter	current	ampere (A)	in series	very low
voltmeter	potential difference	volt (V)	in parallel	very high
galvanometer	detects very small currents and their direction	(a scale of divisions, or microamperes)	in series	low
multimeter	current, p.d. or resistance, chosen with the dial	A, V or $\Omega$	in series for A; in parallel for V	depends on the setting

Important

Using a multimeter safely. Choose the range BEFORE connecting. If you do not know the size of the reading, start on the highest range and turn down. Never measure a current, or a resistance, with the probes across a mains socket; mains measurements are for trained electricians only.

Tip

Ammeter: along the line (series). Voltmeter: visits both ends (parallel).

Ammeters and voltmeters (Chris Gozzard, That Physics Guy). Watch where each meter is placed in the circuit diagram and why the ammeter goes in series and the voltmeter in parallel.

✦

Practice

Draw a circuit diagram of a cell, a switch and a resistor, with an ammeter measuring the current in the resistor and a voltmeter measuring the p.d. across it.
Explain why an ammeter must have a very low resistance and a voltmeter a very high resistance.
On a voltmeter scale, the numbered marks are 0, 1, 2, 3, 4 and 5 V, with 5 small divisions between each pair. The pointer is 3 small divisions past the 2 V mark. What is the reading?
Two lamps are in series with a $9.0\,\mathrm{V}$ battery. The p.d. across one lamp is $5.5\,\mathrm{V}$ . What is the p.d. across the other?
In a parallel circuit, the main ammeter reads $1.2\,\mathrm{A}$ and the two branch ammeters read $0.45\,\mathrm{A}$ and $x$ . Find $x$ .
How does a galvanometer differ from an ordinary ammeter? Give one use.
Mutesi connects a voltmeter in series with a lamp and a cell. The lamp does not light, and the voltmeter reads almost the full p.d. of the cell. Explain both observations.

Show answersHide answers

One loop: cell, switch, ammeter, resistor; the voltmeter connected in a separate loop from one end of the resistor to the other.
An ammeter is in series, so its resistance adds to the circuit: a low resistance keeps the current almost unchanged. A voltmeter is in parallel: a high resistance draws almost no current, so it does not change the p.d. it measures.
Each small division is $\frac{1\,\mathrm{V}}{5} = 0.2\,\mathrm{V}$ , so the reading is $2 + 3 \times 0.2 = 2.6\,\mathrm{V}$ .
$9.0 - 5.5 = 3.5\,\mathrm{V}$
$x = 1.2 - 0.45 = 0.75\,\mathrm{A}$
Its zero is in the centre, it detects very small currents, and it shows the direction of the current. Use: to check whether a tiny current flows, and which way.
The voltmeter's very high resistance allows almost no current, so the lamp cannot light. With almost no current, there is almost no p.d. across the lamp, so nearly all of the cell's p.d. appears across the voltmeter.

Key points

Ammeters measure current and go in series (very low resistance); voltmeters measure p.d. and go in parallel (very high resistance).
Work out the value of one small division before reading a scale, and look straight at the pointer to avoid parallax.
A galvanometer detects small currents and their direction; a multimeter measures current, p.d. or resistance, chosen with its dial.

Sources and credits

21.4: DC Voltmeters and Ammeters, from College Physics (1st edition) by Paul Peter Urone and Roger Hinrichs, OpenStax, hosted by LibreTexts, licensed under CC BY 4.0. Adapted by Ganzaa.
Photo: 2017 Cyfrowy miernik uniwersalny.jpg by Jacek Halicki, Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: Ammeters and Voltmeters - IGCSE Physics, Chris Gozzard (That Physics Guy), embedded from YouTube.

The circuit diagram, the meter scales, the worked examples, the comparison table and the practice exercises are written by Ganzaa.

1The phone repairer's first tool

2Ammeters and voltmeters in a circuit

3Reading an analogue meter

4The galvanometer and the multimeter

The phone repairer's first tool

Ammeters and voltmeters in a circuit

Reading an analogue meter

The galvanometer and the multimeter