Use of apparatus and techniques

What you'll learn

How to choose and read analogue and digital measuring instruments.
How to improve accuracy using repeats, timing strategies and alignment aids.
How to build and check practical circuits, wave experiments and oscilloscope measurements.
How to use ICT and ionising radiation apparatus safely and sensibly.

The big idea: apparatus is only as good as your technique

In OCR A Physics, practical work is not just “following instructions”. You need to show that you can choose suitable apparatus, use it safely, take measurements carefully, and judge whether the results are reliable.

Definition

Apparatus and technique

Apparatus means the equipment you use. A technique is the method used to obtain a valid measurement, such as reading a scale at eye level, timing many oscillations, or checking a circuit before switching on.

A good practical answer often combines three ideas:

What you measure: for example length, time, current or count rate.
How you measure it: for example with a ruler, light gate, multimeter or GM tube.
How you reduce uncertainty: for example repeats, averaging, alignment aids or background subtraction.

Definition

Resolution

The resolution of an instrument is the smallest change it can show, such as 1 mm on a ruler, 0.01 mm on a micrometer, or 0.01 V on a digital voltmeter.

Analogue apparatus: reading scales and interpolating

An analogue instrument has a continuous scale, usually with a pointer, liquid column or marked divisions. Examples include:

metre ruler or tape measure for length and distance
thermometer for temperature
pressure gauge or manometer for pressure
newton meter for force
protractor for angles
measuring cylinder, burette or pipette for volume

Definition

Interpolation

Interpolation means estimating a value between two scale markings. If a pointer lies between 4.6 N and 4.8 N, you estimate where it is between those marks rather than rounding to the nearest mark automatically.

For analogue scales, you should read with your eye perpendicular to the scale to avoid parallax error, where the reading changes because you are viewing from an angle.

Example

Estimating an analogue reading

A newton meter has scale markings every 0.2 N. The pointer is about 70% of the way from 4.6 N to 4.8 N.

The interval between the two markings is $0.2\ \text{N}$ .
The pointer is 70% of this interval above 4.6 N, so the extra amount is $0.70 \times 0.2\ \text{N} = 0.14\ \text{N}$ .
The estimated force is therefore $N4.6\ \text{N} + 0.14\ \text{N} = 4.74\ \text{N}$ , which should sensibly be recorded as about 4.7 N or 4.74 N depending on the scale quality.
A reasonable reading uncertainty is about half the smallest division, so $N\pm 0.1\ \text{N}$ .

Common Mistake

Parallax when reading scales

Do not read a thermometer, ruler, protractor or meter scale from the side. Put your eye level with the marker or meniscus and perpendicular to the scale.

Digital instruments and multimeters

A digital instrument gives a numerical reading directly. Examples include digital stopwatches, digital balances, electronic thermometers, data loggers and digital multimeters.

Definition

Digital multimeter

A digital multimeter is an instrument that can measure electrical quantities such as potential difference in volts, current in amperes and resistance in ohms, depending on the selected setting and connections.

Digital instruments avoid interpolation, but they still have uncertainty. A common starting point is the resolution: if a balance reads to 0.01 g, readings change in steps of 0.01 g.

When using a multimeter:

choose the correct quantity: current, voltage or resistance
choose a range larger than the expected value
connect current meters in series and voltage meters in parallel
check polarity if using a DC circuit
never measure resistance in a powered circuit

Tip

Choosing a meter range

Start on the highest suitable range if you are unsure. Then move down to get more decimal places without overloading the meter.

Increasing accuracy: repeats, alignment aids and better timing

Accuracy means closeness to the true value. Precision means how closely repeated readings agree with each other. A practical method can improve both.

Useful techniques include:

taking repeat readings and calculating a mean
timing many oscillations rather than one
using a fiducial marker, which is a fixed reference mark used to judge a position consistently
using a set square to make a perpendicular measurement
using a plumb line to define the vertical
using light gates instead of human reaction timing

Key Idea

Measure a bigger interval when possible

If the uncertainty in starting and stopping a timer is roughly fixed, timing a longer total interval makes the percentage uncertainty smaller.

Example

Timing multiple oscillations

A student times 20 oscillations of a pendulum as 31.6 s. The estimated timing uncertainty for the whole interval is ±0.2 s.

Calculate the period using total time divided by the number of oscillations:
$\frac{31.6\ \text{s}}{20} = 1.58\ \text{s}$ .
Divide the absolute timing uncertainty by the same number of oscillations:
$s\Delta T = \frac{0.2\ \text{s}}{20} = 0.01\ \text{s}$ .
The period is therefore $\pm 0.01\ \text{s}$ .
The percentage uncertainty is $0.011.58×100%≈0.63%\frac{0.01}{1.58} \times 100\% \approx 0.63\%$ .

A stopwatch is simple but includes reaction time. Light gates are better when an object passes a known point: the gate starts or stops timing automatically as a beam is blocked.

Calipers and micrometers for small distances

For small distances, a ruler is often not precise enough. You may use:

vernier calipers for external diameter, internal diameter or depth
micrometer screw gauges for very small thicknesses or diameters, such as a wire

The diagram shows the main parts you should recognise and the basic reading idea.

Labelled vernier caliper and micrometer screw gauge

Definition

Zero error

A zero error occurs when an instrument does not read zero when it should. You correct for it by subtracting the zero reading from the measured reading.

Example

Correcting a micrometer reading

A micrometer reads 2.87 mm when measuring a wire. When fully closed, it reads +0.03 mm instead of zero.

Identify the zero error: the micrometer is already reading 0.03 mm too high.
Correct the measurement by subtracting the positive zero error:
corrected diameter $2.87\ \text{mm} - 0.03\ \text{mm}$ .
The corrected diameter is $mm2.84\ \text{mm}$ .
In SI units, this is $2.84 \times 10^{-3}\ \text{m}$ .

Common Mistake

Crushing the object

With a micrometer, use the ratchet rather than forcing the thimble. Over-tightening can squash soft materials and give a diameter that is too small.

DC circuits: constructing, designing and checking

For circuit practicals, you must be able to build circuits from diagrams using cells, DC power supplies, resistors, lamps, switches, diodes, LEDs and meters. Some components have polarity, meaning they must be connected the correct way round.

The key meter rules are:

ammeter in series with the component
voltmeter in parallel across the component
positive meter terminal towards the positive side of the supply
switch open while constructing and checking

Labelled DC circuit showing ammeter in series, voltmeter in parallel and LED polarity

Common Mistake

Check before switching on

Before turning on a DC supply, check for short circuits, correct meter ranges, correct polarity for diodes and LEDs, and that the current limit or supply voltage is sensible.

Designing a circuit means thinking ahead: what is the independent variable, what is the dependent variable, and which quantities must be controlled? For example, in an I–V experiment you might vary the potential difference and measure the current through a component.

Signal generators and oscilloscopes

A signal generator produces an alternating electrical signal with adjustable frequency and amplitude. An oscilloscope displays potential difference against time.

Definition

Volts per division and time-base

The volts/division setting tells you the vertical scale of the oscilloscope screen. The time-base tells you the horizontal time scale, usually in seconds per division or milliseconds per division.

Oscilloscope screen showing amplitude, peak-to-peak voltage and period

Example

Finding amplitude and frequency from an oscilloscope trace

An oscilloscope trace has a peak-to-peak height of 4.0 divisions. The volts/division setting is 2.0 V/div. One full cycle takes 5.0 divisions, and the time-base is 5.0 ms/div.

Calculate the peak-to-peak potential difference:
$VV_{pp} = 4.0 \times 2.0\ \text{V} = 8.0\ \text{V}$ .
The amplitude is half the peak-to-peak value:
amplitude $\frac{8.0\ \text{V}}{2} = 4.0\ \text{V}$ .
Calculate the period:
$\times 5.0\ \text{ms} = 25\ \text{ms} = 2.5 \times 10^{-2}\ \text{s}$ .
Calculate frequency using $\frac{1}{T}$ :
$\frac{1}{2.5 \times 10^{-2}\ \text{s}} = 40\ \text{Hz}$ .

Generating and measuring waves

Different wave experiments use different sources and detectors:

sound waves: loudspeaker and microphone
water waves: ripple tank, lamp and screen
vibrations: vibration transducer or signal generator
microwaves or radio waves: transmitter and receiver

You often measure frequency $f$ , wavelength $λ\lambda$ and wave speed $v$ , linked by:

f\lambda

Example

Calculating wave speed in a ripple tank

A ripple tank source has frequency 12 Hz. The distance across 10 wavelengths is measured as 0.350 m.

Find one wavelength by dividing the total distance by 10:
$m\lambda = \frac{0.350\ \text{m}}{10} = 0.0350\ \text{m}$ .
Use the wave equation:
$f\lambda$ .
Substitute the values:
$12\ \text{Hz} \times 0.0350\ \text{m} = 0.420\ \text{m s}^{-1}$ .

Light, interference and diffraction

A laser or suitable light source can be used to investigate interference and diffraction. A laser is useful because it produces a narrow, intense beam with one main wavelength.

Good technique includes:

never looking directly into the laser beam
aligning the laser, slit or grating, and screen carefully
measuring across many fringes, then dividing by the number of fringe spacings
keeping the screen distance large enough for measurable fringe spacing

Common Mistake

Measuring one tiny fringe spacing

Do not measure just one fringe spacing if several are visible. Measure across many bright fringes and divide; this reduces percentage uncertainty.

ICT: sensors, data loggers, modelling and software

ICT can improve practical work when used thoughtfully. A data logger records readings from sensors such as light gates, temperature probes, pressure sensors, microphones or motion sensors.

Software may be used to:

collect many readings quickly
plot graphs
calculate gradients and intercepts
model a physical situation
process data using a spreadsheet

Tip

ICT is not automatically better

A sensor still needs calibration, an appropriate sampling rate and a suitable range. A beautiful graph from bad data is still bad evidence.

Ionising radiation and detectors

Ionising radiation has enough energy to remove electrons from atoms. Practical work may involve sealed sources and detectors such as a Geiger–Müller tube connected to a counter or ratemeter.

Safe technique follows three principles:

minimise time near the source
maximise distance from the source
use suitable shielding and handling tools

You also need to measure background radiation, which is the count recorded even when the source is not present.

Example

Correcting a radiation count rate

A detector records 246 counts in 60 s with a source present. Background radiation is measured as 36 counts in 60 s.

Calculate the source-plus-background count rate:
$s−1\frac{246}{60\ \text{s}} = 4.10\ \text{s}^{-1}$ .
Calculate the background count rate:
$s−1\frac{36}{60\ \text{s}} = 0.60\ \text{s}^{-1}$ .
Subtract the background rate:
corrected count rate $4.10\ \text{s}^{-1} - 0.60\ \text{s}^{-1} = 3.50\ \text{s}^{-1}$ .
Quote the corrected count rate as $s−13.5\ \text{s}^{-1}$ to a sensible number of significant figures.

Exam technique

In the exam

Name the apparatus and the technique: for example “use a light gate to avoid reaction-time uncertainty”, not just “measure the time”.
Link improvements to uncertainty: say whether you are reducing parallax, reaction time, zero error, random variation or percentage uncertainty.
For circuits and radiation, include safety checks: correct polarity/ranges for circuits, and time–distance–shielding plus background count for radiation.

Self review

Check yourself

Why does timing 20 oscillations usually give a better period than timing one oscillation?
How should an ammeter and voltmeter be connected in a DC circuit?
Why must background radiation be measured before using a radioactive source?

What you'll learn

How to choose and read analogue and digital measuring instruments.
How to improve accuracy using repeats, timing strategies and alignment aids.
How to build and check practical circuits, wave experiments and oscilloscope measurements.
How to use ICT and ionising radiation apparatus safely and sensibly.

The big idea: apparatus is only as good as your technique

Definition

Apparatus and technique

A good practical answer often combines three ideas:

What you measure: for example length, time, current or count rate.
How you measure it: for example with a ruler, light gate, multimeter or GM tube.
How you reduce uncertainty: for example repeats, averaging, alignment aids or background subtraction.

Definition

Resolution

The resolution of an instrument is the smallest change it can show, such as 1 mm on a ruler, 0.01 mm on a micrometer, or 0.01 V on a digital voltmeter.

Analogue apparatus: reading scales and interpolating

An analogue instrument has a continuous scale, usually with a pointer, liquid column or marked divisions. Examples include:

metre ruler or tape measure for length and distance
thermometer for temperature
pressure gauge or manometer for pressure
newton meter for force
protractor for angles
measuring cylinder, burette or pipette for volume

Definition

Interpolation

For analogue scales, you should read with your eye perpendicular to the scale to avoid parallax error, where the reading changes because you are viewing from an angle.

Example

Estimating an analogue reading

A newton meter has scale markings every 0.2 N. The pointer is about 70% of the way from 4.6 N to 4.8 N.

The interval between the two markings is $0.2\ \text{N}$ .
The pointer is 70% of this interval above 4.6 N, so the extra amount is $0.70 \times 0.2\ \text{N} = 0.14\ \text{N}$ .
The estimated force is therefore $N4.6\ \text{N} + 0.14\ \text{N} = 4.74\ \text{N}$ , which should sensibly be recorded as about 4.7 N or 4.74 N depending on the scale quality.
A reasonable reading uncertainty is about half the smallest division, so $N\pm 0.1\ \text{N}$ .

Common Mistake

Parallax when reading scales

Do not read a thermometer, ruler, protractor or meter scale from the side. Put your eye level with the marker or meniscus and perpendicular to the scale.

Digital instruments and multimeters

A digital instrument gives a numerical reading directly. Examples include digital stopwatches, digital balances, electronic thermometers, data loggers and digital multimeters.

Definition

Digital multimeter

Digital instruments avoid interpolation, but they still have uncertainty. A common starting point is the resolution: if a balance reads to 0.01 g, readings change in steps of 0.01 g.

When using a multimeter:

choose the correct quantity: current, voltage or resistance
choose a range larger than the expected value
connect current meters in series and voltage meters in parallel
check polarity if using a DC circuit
never measure resistance in a powered circuit

Tip

Choosing a meter range

Start on the highest suitable range if you are unsure. Then move down to get more decimal places without overloading the meter.

Increasing accuracy: repeats, alignment aids and better timing

Accuracy means closeness to the true value. Precision means how closely repeated readings agree with each other. A practical method can improve both.

Useful techniques include:

taking repeat readings and calculating a mean
timing many oscillations rather than one
using a fiducial marker, which is a fixed reference mark used to judge a position consistently
using a set square to make a perpendicular measurement
using a plumb line to define the vertical
using light gates instead of human reaction timing

Key Idea

Measure a bigger interval when possible

If the uncertainty in starting and stopping a timer is roughly fixed, timing a longer total interval makes the percentage uncertainty smaller.

Example

Timing multiple oscillations

A student times 20 oscillations of a pendulum as 31.6 s. The estimated timing uncertainty for the whole interval is ±0.2 s.

Calculate the period using total time divided by the number of oscillations:
$\frac{31.6\ \text{s}}{20} = 1.58\ \text{s}$ .
Divide the absolute timing uncertainty by the same number of oscillations:
$s\Delta T = \frac{0.2\ \text{s}}{20} = 0.01\ \text{s}$ .
The period is therefore $\pm 0.01\ \text{s}$ .
The percentage uncertainty is $0.011.58×100%≈0.63%\frac{0.01}{1.58} \times 100\% \approx 0.63\%$ .

A stopwatch is simple but includes reaction time. Light gates are better when an object passes a known point: the gate starts or stops timing automatically as a beam is blocked.

Calipers and micrometers for small distances

For small distances, a ruler is often not precise enough. You may use:

vernier calipers for external diameter, internal diameter or depth
micrometer screw gauges for very small thicknesses or diameters, such as a wire

The diagram shows the main parts you should recognise and the basic reading idea.

Labelled vernier caliper and micrometer screw gauge

Definition

Zero error

A zero error occurs when an instrument does not read zero when it should. You correct for it by subtracting the zero reading from the measured reading.

Example

Correcting a micrometer reading

A micrometer reads 2.87 mm when measuring a wire. When fully closed, it reads +0.03 mm instead of zero.

Identify the zero error: the micrometer is already reading 0.03 mm too high.
Correct the measurement by subtracting the positive zero error:
corrected diameter $2.87\ \text{mm} - 0.03\ \text{mm}$ .
The corrected diameter is $mm2.84\ \text{mm}$ .
In SI units, this is $2.84 \times 10^{-3}\ \text{m}$ .

Common Mistake

Crushing the object

With a micrometer, use the ratchet rather than forcing the thimble. Over-tightening can squash soft materials and give a diameter that is too small.

DC circuits: constructing, designing and checking

The key meter rules are:

ammeter in series with the component
voltmeter in parallel across the component
positive meter terminal towards the positive side of the supply
switch open while constructing and checking

Labelled DC circuit showing ammeter in series, voltmeter in parallel and LED polarity

Common Mistake

Check before switching on

Before turning on a DC supply, check for short circuits, correct meter ranges, correct polarity for diodes and LEDs, and that the current limit or supply voltage is sensible.

Signal generators and oscilloscopes

A signal generator produces an alternating electrical signal with adjustable frequency and amplitude. An oscilloscope displays potential difference against time.

Definition

Volts per division and time-base

Oscilloscope screen showing amplitude, peak-to-peak voltage and period

Example

Finding amplitude and frequency from an oscilloscope trace

An oscilloscope trace has a peak-to-peak height of 4.0 divisions. The volts/division setting is 2.0 V/div. One full cycle takes 5.0 divisions, and the time-base is 5.0 ms/div.

Calculate the peak-to-peak potential difference:
$VV_{pp} = 4.0 \times 2.0\ \text{V} = 8.0\ \text{V}$ .
The amplitude is half the peak-to-peak value:
amplitude $\frac{8.0\ \text{V}}{2} = 4.0\ \text{V}$ .
Calculate the period:
$\times 5.0\ \text{ms} = 25\ \text{ms} = 2.5 \times 10^{-2}\ \text{s}$ .
Calculate frequency using $\frac{1}{T}$ :
$\frac{1}{2.5 \times 10^{-2}\ \text{s}} = 40\ \text{Hz}$ .

Generating and measuring waves

Different wave experiments use different sources and detectors:

sound waves: loudspeaker and microphone
water waves: ripple tank, lamp and screen
vibrations: vibration transducer or signal generator
microwaves or radio waves: transmitter and receiver

You often measure frequency $f$ , wavelength $λ\lambda$ and wave speed $v$ , linked by:

f\lambda

Example

Calculating wave speed in a ripple tank

A ripple tank source has frequency 12 Hz. The distance across 10 wavelengths is measured as 0.350 m.

Find one wavelength by dividing the total distance by 10:
$m\lambda = \frac{0.350\ \text{m}}{10} = 0.0350\ \text{m}$ .
Use the wave equation:
$f\lambda$ .
Substitute the values:
$12\ \text{Hz} \times 0.0350\ \text{m} = 0.420\ \text{m s}^{-1}$ .

Light, interference and diffraction

A laser or suitable light source can be used to investigate interference and diffraction. A laser is useful because it produces a narrow, intense beam with one main wavelength.

Good technique includes:

never looking directly into the laser beam
aligning the laser, slit or grating, and screen carefully
measuring across many fringes, then dividing by the number of fringe spacings
keeping the screen distance large enough for measurable fringe spacing

Common Mistake

Measuring one tiny fringe spacing

Do not measure just one fringe spacing if several are visible. Measure across many bright fringes and divide; this reduces percentage uncertainty.

ICT: sensors, data loggers, modelling and software

ICT can improve practical work when used thoughtfully. A data logger records readings from sensors such as light gates, temperature probes, pressure sensors, microphones or motion sensors.

Software may be used to:

collect many readings quickly
plot graphs
calculate gradients and intercepts
model a physical situation
process data using a spreadsheet

Tip

ICT is not automatically better

A sensor still needs calibration, an appropriate sampling rate and a suitable range. A beautiful graph from bad data is still bad evidence.

Ionising radiation and detectors

Ionising radiation has enough energy to remove electrons from atoms. Practical work may involve sealed sources and detectors such as a Geiger–Müller tube connected to a counter or ratemeter.

Safe technique follows three principles:

minimise time near the source
maximise distance from the source
use suitable shielding and handling tools

You also need to measure background radiation, which is the count recorded even when the source is not present.

Example

Correcting a radiation count rate

A detector records 246 counts in 60 s with a source present. Background radiation is measured as 36 counts in 60 s.

Calculate the source-plus-background count rate:
$s−1\frac{246}{60\ \text{s}} = 4.10\ \text{s}^{-1}$ .
Calculate the background count rate:
$s−1\frac{36}{60\ \text{s}} = 0.60\ \text{s}^{-1}$ .
Subtract the background rate:
corrected count rate $4.10\ \text{s}^{-1} - 0.60\ \text{s}^{-1} = 3.50\ \text{s}^{-1}$ .
Quote the corrected count rate as $s−13.5\ \text{s}^{-1}$ to a sensible number of significant figures.

Exam technique

In the exam

Name the apparatus and the technique: for example “use a light gate to avoid reaction-time uncertainty”, not just “measure the time”.
Link improvements to uncertainty: say whether you are reducing parallax, reaction time, zero error, random variation or percentage uncertainty.
For circuits and radiation, include safety checks: correct polarity/ranges for circuits, and time–distance–shielding plus background count for radiation.

Self review

Check yourself

Why does timing 20 oscillations usually give a better period than timing one oscillation?
How should an ammeter and voltmeter be connected in a DC circuit?
Why must background radiation be measured before using a radioactive source?

What you'll learn

The big idea: apparatus is only as good as your technique

Analogue apparatus: reading scales and interpolating

Digital instruments and multimeters

Increasing accuracy: repeats, alignment aids and better timing

Calipers and micrometers for small distances

DC circuits: constructing, designing and checking

Signal generators and oscilloscopes

Generating and measuring waves

Light, interference and diffraction

ICT: sensors, data loggers, modelling and software

Ionising radiation and detectors

Review Use of apparatus and techniques by teaching Genie

Remember key concepts with flashcards

Use of apparatus and techniques

What you'll learn

The big idea: apparatus is only as good as your technique

Analogue apparatus: reading scales and interpolating

Digital instruments and multimeters

Increasing accuracy: repeats, alignment aids and better timing

Calipers and micrometers for small distances

DC circuits: constructing, designing and checking

Signal generators and oscilloscopes

Generating and measuring waves

Light, interference and diffraction

ICT: sensors, data loggers, modelling and software

Ionising radiation and detectors

Review Use of apparatus and techniques by teaching Genie

Remember key concepts with flashcards

Use of apparatus and techniques

What you'll learn

The big idea: apparatus is only as good as your technique

Analogue apparatus: reading scales and interpolating

Digital instruments and multimeters

Increasing accuracy: repeats, alignment aids and better timing

Calipers and micrometers for small distances

DC circuits: constructing, designing and checking

Signal generators and oscilloscopes

Generating and measuring waves

Light, interference and diffraction

ICT: sensors, data loggers, modelling and software

Ionising radiation and detectors

Review Use of apparatus and techniques by teaching Genie

Remember key concepts with flashcards

Use of apparatus and techniques Revision Guide

Use of apparatus and techniques

What you'll learn

The big idea: apparatus is only as good as your technique

Analogue apparatus: reading scales and interpolating

Digital instruments and multimeters

Increasing accuracy: repeats, alignment aids and better timing

Calipers and micrometers for small distances

DC circuits: constructing, designing and checking

Signal generators and oscilloscopes

Generating and measuring waves

Light, interference and diffraction

ICT: sensors, data loggers, modelling and software

Ionising radiation and detectors

Review Use of apparatus and techniques by teaching Genie

Remember key concepts with flashcards

Use of apparatus and techniques Revision Guide