📘 CodingMarble Learn

How to Use a Multimeter

A multimeter is one meter that can measure voltage (V), current (A) and resistance (Ω). Choose the dial mode first. Voltage is measured in parallel (across the part), current in series (in the wire), and resistance with the power off and the part out of the circuit. Start with the biggest range and go down.

🎬 Step-by-step story

  1. This is a multimeter. The dial chooses what to measure and the screen shows the answer. The dial is OFF now.
  2. Resistance first. Take the resistor out of the circuit, power off. Dial on Ω. The screen shows 60 Ω.
  3. Now voltage. The circuit is ON. Touch the two probes to the two ends of the resistor. The meter is in parallel. It reads 6 V.
  4. Now current. Open the wire and put the meter in the gap. The meter is in series, so all the current goes through it. It reads 0.1 A.
  5. A mistake to avoid. Amps mode placed across the resistor is a short circuit. The screen shows ERR and the meter can burn out.
  6. Free play. Change the dial and the connection. Try the wrong combinations and read the warning.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why must I switch off before measuring resistance?

The ohmmeter uses its own battery. Any other voltage in the circuit confuses it and gives a wrong or damaged reading. Watch the loose resistor in step 1.

Why is the voltmeter connected in parallel?

Voltage is the difference between two points. The probes must touch both points. See the meter across the resistor in step 2.

Why does the ammeter go in series?

Current is the flow through the wire, so the meter has to be in the path, in the gap. Step 3 shows the gap.

What happens if I use amps mode across a part?

The meter acts like a thick wire joining the two ends, so a huge current flows: a short circuit. Step 4 shows the ERR warning.

Can I try the combinations myself?

Yes. Use the buttons in free play and read the warning for every wrong combination.

The reading shows a minus sign. Is it wrong?

No. It only means red and black probes are on opposite sides. Swap them and the sign goes away.

What is a multimeter?

A multimeter is a hand-held meter with many jobs in one box. It can be an ammeter, a voltmeter and an ohmmeter. That is why it is called multi-meter.

There are two kinds. An analog multimeter has a needle that moves over a scale. A digital multimeter shows numbers on a screen. Digital ones are easier to read, so most people use them today.

Parts of a multimeter

Measuring voltage (V)

Voltage is the push between two points. So you touch the probes to two points, one on each side of the part. This is called connecting in parallel.

Why does this work? A voltmeter has a very large resistance, so almost no current goes into it and the circuit is not disturbed. For a battery or DC circuit, red probe goes to the + side and black to the − side. If the sign shows minus, you have only swapped the probes.

Do not put a voltmeter in series. Its big resistance would almost stop the current.

Measuring current (A)

Current is the flow through a wire. So the flow must go through the meter. Switch off, open the circuit at one point, and connect the meter in the gap. This is series connection. Move the red probe to the current socket first.

An ammeter has a very small resistance, so it hardly changes the current. But this is also why it is dangerous: if you put it across a battery or a part, a huge current flows. That is a short circuit. The fuse may blow or the meter may be damaged.

Measuring resistance (Ω)

In the resistance mode, the meter sends a tiny current from its own battery through the part and finds R = V ÷ I inside. So:

  1. Switch the circuit off.
  2. Take the part out of the circuit (or at least one end), or other paths will change the reading.
  3. Put the dial on Ω and touch the probes to its two ends.

If the part is a wire or fuse, the continuity (beep) setting tells quickly if it is joined well: a beep means a complete path. If the display shows OL (open line) the path is broken.

Choosing the range and reading the scale

If the dial has ranges (for example 2 V, 20 V, 200 V), start with the largest one. Then step down until the reading is large but not over. A smaller range gives more digits and so a better result. If the display shows 1 or OL, the value is above the range: go up one step.

On an analog meter, reading = (needle position ÷ total divisions) × full-scale value. Look straight at the needle, not from the side, so that the reading is not shifted (this error is called parallax).

For AC (the mains supply) use the AC symbol (a wavy line ~). For a battery or cell use DC (a straight line with dashes).

Safety rules

How an ammeter and voltmeter are made (idea)

Inside is a sensitive coil meter called a galvanometer (resistance G, full-scale current Ig). To make an ammeter, a small shunt resistor S is joined in parallel so that most current bypasses the coil: S = IgG ÷ (I − Ig). To make a voltmeter, a large resistor Rs is joined in series: Rs = V ÷ Ig − G. A multimeter switches between these inside the box when you turn the dial.

Try it

In the 3D: before you click, predict the screen. Use 6 V and 60 Ω: voltage should be 6 V, current should be 0.1 A. Check both. Then try Amps across the part and watch the warning.

At home (with an adult): take a 1.5 V AA cell. Set the digital meter to DC volts and touch the probes to both ends. You should read about 1.5 V (an old cell reads less). Then measure a pencil lead: touch the two ends in Ω mode. Does the resistance get larger when you move the probes further apart?

Key formulas and definitions

Worked examples

1. A multimeter across a resistor shows 8 V. The resistor is 40 Ω. What current flows through it?

I = V ÷ R = 8 ÷ 40 = 0.2 A.

2. You measure 4.5 V across a 150 Ω resistor. What current should an ammeter in series show?

I = 4.5 ÷ 150 = 0.03 A = 30 mA.

3. A resistance mode reading is 220 Ω. A 6 V battery is joined to it. Predict the current.

I = 6 ÷ 220 = 0.027 A, about 27 mA.

4. An analog voltmeter has full scale 10 V and 50 divisions. The needle stops at 32 divisions. What is the voltage?

Each division = 10 ÷ 50 = 0.2 V. Reading = 32 × 0.2 = 6.4 V.

5. A galvanometer has G = 100 Ω and Ig = 1 mA. What series resistor makes a 0 to 10 V voltmeter?

Rs = V ÷ Ig − G = 10 ÷ 0.001 − 100 = 10 000 − 100 = 9 900 Ω.

6. Same galvanometer (G = 100 Ω, Ig = 1 mA). What shunt makes a 0 to 1 A ammeter?

S = Ig × G ÷ (I − Ig) = 0.001 × 100 ÷ (1 − 0.001) = 0.1 ÷ 0.999 ≈ 0.1 Ω.

7. A 6 V battery drives a 30 Ω resistor. Your ammeter has its own resistance 0.5 Ω. What does it read, and what is the true current?

True current = 6 ÷ 30 = 0.2 A. With the meter in series, total R = 30.5 Ω, so reading = 6 ÷ 30.5 ≈ 0.197 A. The meter's own resistance changes the circuit slightly, so a good ammeter must have very low resistance.

Common mistakes

Practice quiz

1. A voltmeter is connected:
2. To measure current, the meter must be:
3. Which is the safe way to measure resistance?
4. An ideal ammeter has:
5. The display shows OL (or 1) on a range. What do you do?

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What does a multimeter measure?

Voltage (V), current (A) and resistance (Ω). Many also test continuity and diodes.

Why is a voltmeter connected in parallel?

Voltage is the difference between two points, so you touch both. Its large resistance means almost no current enters it, so the circuit does not change.

Why can an ammeter burn out if put across a battery?

It has almost no resistance, so a huge current flows (I = V ÷ tiny R). That heats and damages it and blows its fuse.

Where this is taught

China高一Compulsory 3 Ch.11 Circuits

Learn first

Learn next

Related lessons

All Physics lessons