Choose the meter job before connecting it
Current passes through its meter; potential difference and resistance are measured across two terminals under their stated circuit conditions.
Continue where you stopped
The core idea
Electricity · Lesson 2 of 6 · about 25–35 min
What you need to understand
Current passes through its meter; potential difference and resistance are measured across two terminals under their stated circuit conditions.
Definitions
- current
- the rate at which electric charge flowsFor example: An ammeter measures current in amperes.
- voltage
- the electrical energy transferred for each unit of charge between two pointsFor example: A voltmeter measures voltage across a lamp.
- multimeter
- a meter that can be set to measure current, voltage or resistanceFor example: The mode and connections must be changed to match the quantity.
- resistance
- how strongly a component opposes currentFor example: Adding a series resistor increases total resistance.
Key idea
- LookMeasure the current through and potential difference across a lamp, then measure the lamp's resistance.
- ThinkCurrent passes through its meter; potential difference and resistance are measured across two terminals under their stated circuit conditions.
- DoChoose and connect the meter for the required quantity, then interpret the reading with its correct unit.
Explanation
To measure current, select current mode and suitable terminals and insert the meter in series so the measured current passes through it. To measure potential difference, select voltage mode and connect one probe to each side of the component.
For resistance, open and disconnect the supply and isolate the component from other circuit paths. Select resistance mode, connect across the isolated component and read the value in ohms. Begin with a safe suitable range and keep the switch open while changing connections.
Pause and say it: Current passes through its meter; potential difference and resistance are measured across two terminals under their stated circuit conditions.
Common mistake
Tempting wrong idea: An ammeter is connected across a component.
Why it fails: An ammeter measures only the current that flows through it. Placed across a component, it is not in that component's current path, so it cannot measure that current. It must be inserted into the circuit in series.
Use this instead: An ammeter is inserted in series so the current being measured passes through it.
Practical work
What to show: Choose and connect the meter for the required quantity, then interpret the reading with its correct unit.
Before you finish: Resistance is not measured on a powered component in this G1 procedure.
Practical: Series and parallel circuit comparison
How do current and voltage measurements differ between series and parallel arrangements?
Safety: Use low-voltage cells only; open the switch before changing connections and never short-circuit the supply.
- Change
- series or parallel arrangement
- Measure
- current and voltage readings
- Keep the same
- same supply, same components, same meter ranges
View the apparatus, method and sample results
Apparatus
cells in holder, two identical lamps or resistors, switch, multimeters, connecting leads.
Method
- With the switch open, build the series circuit from its diagram.
- Set the multimeter to current mode and connect it in series; begin on the highest suitable range.
- Measure voltage across each component using voltage mode in parallel.
- Open the switch, rebuild the components in parallel and repeat.
- Repeat readings and check that branch and total values are consistent.
| Arrangement | Position | Current / A | Voltage / V |
|---|---|---|---|
| series | component 1 | 0.20 | 1.5 |
| series | component 2 | 0.20 | 1.5 |
| parallel | branch 1 | 0.30 | 3.0 |
| parallel | branch 2 | 0.30 | 3.0 |
Limitation: Real component resistance may change as lamps warm.
Improvement: Use fixed resistors and take readings promptly after closing the switch.
6. Worked Examples
Modelled example 1
Measure three different quantities
Problem
Describe how to measure the current through, potential difference across and resistance of one lamp.
Study the worked solution
Match the meter to the quantity
Method
Use current mode in series and voltage mode across the operating lamp.
Reason
Current must pass through its meter; potential difference compares two terminals.
Working
Open the switch while changing mode, terminals, range and connections.
Measure resistance safely
Method
Disconnect the supply, isolate the lamp, select resistance mode and connect across it.
Reason
An external supply or another circuit path makes this resistance procedure invalid and may damage the meter.
Working
Record current in A, potential difference in V and resistance in Ω.
Guided practice 2
Correct two meter connections
Problem
A learner puts an ammeter across a lamp and a voltmeter in the circuit path. Describe both corrections before the low-voltage supply is switched on.
Match quantity, mode and connection
Hints
Hint 1: current
The measured current must pass through its meter.
Hint 2: potential difference
This measurement compares the lamp’s two terminals.
View solution step by step
Reconnect with the switch open
Method
Insert the ammeter in series with the lamp and connect the voltmeter across the lamp.
Reason
Current passes through an ammeter; potential difference is measured between two points.
Working
Select the correct modes, terminals and safe ranges before closing the switch.
Challenge 3
Change from live voltage to resistance
Problem
A learner has measured 3.0 V across a resistor in an operating circuit and now needs its resistance. State the safe changes required and the unit for a resistance reading of 220.
Change the circuit and meter together
Hints
Hint 1: remove the supply
Resistance mode is used only after the external supply is disconnected.
Hint 2: isolate and reconnect
Separate the component from other paths, then connect the meter across it.
View solution step by step
Prepare an isolated component
Method
Disconnect the supply, isolate the resistor, select resistance mode and connect across it.
Reason
The powered circuit and other paths would invalidate this resistance procedure.
Working
Record the reading as 220 Ω.
Guided practice
Try it with support
You need the current through a lamp and the potential difference across it. Where should each meter be connected?
- Current must pass through its meter.
- Potential difference compares the lamp's two terminals.
Check the guided answer
Answer: Connect current mode in series with the lamp and voltage mode in parallel across the lamp.
Check: Never connect current mode directly across the supply.
Practise and continue
Practise this
Describe the safe multimeter changes needed to measure a resistor's resistance after measuring circuit current.
Need a hint?
- Think about the supply, mode, leads and component connection.
Check your answer
Answer: Open and disconnect the supply, isolate the resistor, move the leads if required, select resistance mode and a suitable range, then connect across the resistor.
Check: Resistance is not measured on a powered component in this G1 procedure.
Think like a scientist
A meter displays 0.24 while in the A range and later 5.8 while in the V range. State both readings with units and the required connection for each.
Check the reasoning
The readings are 0.24 A with the meter in series and 5.8 V with the meter connected across the component.
Remember: A number without the correct unit and connection does not identify a valid measurement.
One-minute check
- Hide the page and explain choose the meter job before connecting it in your own words.
- Give a new example that is different from the worked example.
- Correct this common mistake: “An ammeter is connected across a component.”