Use a multimeter to measure voltage, current and resistance, connecting it correctly and choosing a suitable range

What this dot point is asking

SEAB wants you to use a multimeter to measure voltage, current and resistance, connecting it correctly for each and choosing a suitable range. The central insight is that the three measurements need three different connections: voltage is measured in parallel, current in series, and resistance on a component that is disconnected from any power, with the right range selected so the reading is accurate.

The answer

What a multimeter does

A multimeter is a single instrument that can measure voltage, current and resistance, selected by a rotary switch. Two leads (red for positive, black for the common or negative) plug into the right sockets and touch the circuit at the test points. Choosing the wrong function or connection gives a useless reading or can damage the meter, so the connection rules matter.

Measuring voltage

To measure the potential difference across a component:

• Set the meter to a voltage (V) range, choosing direct (DC) or alternating (AC) as needed.
• Connect the meter in parallel, across the two ends of the component, with the circuit powered.
• The meter has a very high resistance, so it draws almost no current and does not change the voltage it is reading.

Measuring current

To measure the current through a part of the circuit:

• Set the meter to a current (A) range.
• Break the circuit at the point of interest and connect the meter in series, so the current flows through the meter.
• The meter has a very low resistance, so it does not reduce the current being measured.

Because the circuit must be broken to insert the meter, current measurement takes more care than voltage measurement.

Measuring resistance

To measure a resistance:

• Set the meter to an ohms ($\Omega$) range.
• Disconnect the component from the circuit (or switch off and isolate the circuit), because the ohms range uses the meter's own internal battery to pass a small current and measure the result. External power would give a false reading and could damage the meter.
• Connect the meter across the component and read the resistance.

A related setting is the continuity test, which beeps when there is a low-resistance connection, useful for checking that two points are joined or a wire is unbroken.

Choosing the range

If the meter is not auto-ranging, choose a range just above the value you expect. Too low a range over-reads or shows an overload; too high a range loses precision. If unsure, start on the highest range and step down until you get a clear reading.

Examples in context

Example 1. Checking a battery under load. Measuring a battery's voltage with nothing connected can look healthy, but measuring it in parallel while it powers a load reveals whether the voltage sags. The high-resistance voltage measurement does not load the battery itself, so it shows the true voltage the rest of the circuit sees, a routine diagnostic.

Example 2. Finding a break in a wire. Using the continuity setting, touching the two ends of a suspect wire should give a beep if the wire is intact. No beep means a break. This quick test, done with the circuit unpowered, finds broken connections in seconds and is one of the most used features in fault finding.

Try this

• Cue. State how a multimeter is connected to measure current and why. In series, so the current to be measured flows through the meter; its very low resistance means it does not reduce that current.

• Cue. Explain why a resistor must be disconnected before measuring it on the ohms range. The ohms range uses the meter's own internal battery; external power would give a false reading and could damage the meter.

• Cue. Describe what the continuity setting is used for. It checks whether two points are joined by a low-resistance path, beeping when they are, which helps find broken wires and confirm connections.