3.4.1 Measuring Current, Potential Difference and Resistance

3.4.1 Measuring Current, Potential Difference and Resistance: Choose and connect an ammeter, voltmeter and ohmmeter correctly, and find resistance from measured current and voltage.

Each meter is connected in a different way because it measures a different quantity.

Ammeter

Measures current in amperes and is connected in series so that the whole current passes through it.

Voltmeter

Measures potential difference in volts and is connected in parallel across the component.

Ohmmeter

Measures resistance in ohms and is used on a component that has been disconnected from the circuit.

Multimeter

One instrument with a dial or buttons can act as any of these three meters.

Digital and analogue

Digital meters show the value as numbers, while analogue meters use a pointer, so the scale must be read carefully.

Meter	Measures	Connection
Ammeter	Current (A)	In series
Voltmeter	Potential difference (V)	In parallel
Ohmmeter	Resistance (Ω)	Across a component removed from the circuit

Good meters change the circuit as little as possible.

Meter Properties

Ideal ammeter

Has very low resistance so that adding it does not reduce the current in the circuit.

Ideal voltmeter

Has very high resistance so that hardly any current is diverted through it.

Polarity

Connect the positive terminal of a meter towards the positive side of the supply.

Ranges

Start on the highest range and move down to avoid overloading the meter.

Zero error

Check that each meter reads zero before use; otherwise every reading will be wrong by the same amount.

Resistance can be calculated from two readings instead of being measured directly.

Connect the component in a circuit with an ammeter in series with it.

Connect a voltmeter in parallel across the component only.

Close the switch and read the current and the potential difference at the same moment.

Calculate the resistance using R = V ÷ I.

Repeat at several settings and average the results to improve reliability.

Question: An ammeter in series with a resistor reads 0.25 A and a voltmeter across the resistor reads 3.0 V. Calculate the resistance.

Write down I = 0.25 A and V = 3.0 V.
Use R = V ÷ I.
R = 3.0 ÷ 0.25 = 12 Ω.

Answer: The resistance is 12 Ω.

3.4.1 – Key Points for the Exam

Ammeter: A meter, connected in series, that measures current.
Voltmeter: A meter, connected in parallel, that measures potential difference.
Ohmmeter: A meter that measures the resistance of a component.
Multimeter: A meter that can be switched between measuring current, voltage and resistance.

Common Mistakes

Connecting a voltmeter in series, which blocks the current and gives a wrong reading.
Measuring resistance with the component still connected to a live circuit.
Reading the wrong scale on a multimeter, for example mA instead of A.
Using the highest range only, which gives readings too coarse to be useful.