IB Physics HLTool 1 — Experimental TechniquesPractical Skillsthe right tool for the job~16 min read
Measuring Variables
Good physics starts with good measurements. Whatever you’re investigating — a mass, a time, a current, a temperature — the quality of your data depends on choosing the right instrument and reading it correctly. In the exam you’ll often be asked which piece of equipment is most appropriate for a task, and why. This page walks through every common measurement, the instrument to use, and the pitfalls to avoid.
📚 What you need to know
Each variable has an appropriate instrument — and you may need to choose the best one
Mass: digital balance (tare first); time: stopwatch (mind reaction time)
Length: ruler (to the nearest mm), tape measure, or micrometer for tiny distances
Volume: measuring cylinder (1 ml = 1 cm3)
Temperature: liquid thermometer or digital probe (higher precision)
Current: ammeter in series; potential difference: voltmeter in parallel
Meters can be analogue (needle) or digital (numerical readout), each with pros and cons
Watch for zero errors and parallax errors when reading scales
Mass, time and length
Mass is measured with a digital balance, usually accurate to two decimal places. Always tare (zero) the balance before weighing, and record mass in kilograms.
Time is measured with a stopwatch or stop-clock. The big pitfall here is human reaction time — the delay between seeing an event and pressing the button. For very short intervals (under a second), this can seriously affect your result, so use light gates or repeat and average where possible.
Length depends on the scale. A ruler measures to the nearest millimetre, a tape measure handles larger distances, and a micrometer is used for very small distances. The standard unit is the metre.
Lay the object against the ruler and read to the nearest mm. A pencil ending near the 8 cm mark reads about 8.0 cm.
Volume and temperature
Volume of a liquid is measured with a measuring cylinder. Cleverly, you can also find the volume of an irregular solid by measuring the change in water level when you submerge it. Remember: 1 ml = 1 cm3.
Temperature can be measured two ways, and choosing between them is a classic exam decision:
Liquid thermometers are cheap and robust but slower and less precise; digital probes give faster, more precise readings and suit automated data logging.
When an exam asks “which is more appropriate?”, it’s not asking which is better in general — it’s asking which suits this task. Need to log temperature every second for an hour? The digital probe wins on speed and precision. Just need a rough classroom reading on a budget? The liquid thermometer is fine. Always justify your choice by the task’s needs.
Current and potential difference
Two electrical measurements that students constantly mix up — so let’s make it stick. Current is measured with an ammeter, connected in series, because all the charge you want to count must flow through it. Potential difference is measured with a voltmeter, connected in parallel, because it compares the potential at two points across a component.
An ammeter goes in the main loop (series) so all the current passes through it. A voltmeter branches across a component (parallel) to compare two points.
Analogue or digital?
Both ammeters and voltmeters come in two types, and knowing the trade-offs is a common exam point:
Analogue (needle and scale): can be read continuously, but is subject to parallax error — always read perpendicular to the scale — and to zero errors.
Digital (numerical readout): gives a specific, precise value and can read very small quantities (mA, µV), but the display may flicker, forcing you to judge which value to record.
Two errors apply especially to analogue scales. A zero error means the needle doesn’t rest at zero to start with — check it first and subtract it from every reading. A parallax error comes from viewing the scale at an angle — always look straight on, perpendicular to the scale.
Other quantities
A few more instruments to know, each with its typical precision:
Quantity
Instrument
Notes
Force
Force meter (spring balance)
Measures directly in newtons
Angle
Protractor
Precision ±0.5°; align the baseline, read at eye level, use a consistent reference (e.g. the normal line)
Sound intensity
Sound level meter
Precision ±0.1 dB; hold at a fixed distance, avoid background noise
Light intensity
Light meter / photodiode / lux meter
Keep the detector perpendicular to the source; shield from stray light
WE 1
A student needs to measure the current through and potential difference across a single lamp. Describe how each meter should be connected, and explain why.
Step 1 — the ammeter
Connect the ammeter in series with the lamp, so all the current flows through it.
Step 2 — the voltmeter
Connect the voltmeter in parallel across the lamp, to measure the potential difference between the two points across it.
Ammeter in series (through); voltmeter in parallel (across)Link each connection to what it measures: current flows through (series), potential difference is across two points (parallel). That reasoning is what earns the marks.
WE 2
A student reads an analogue ammeter and gets inconsistent values. Suggest two sources of error and how to reduce each.
Error 1 — zero error
The needle may not start at zero. Check it before use and subtract any offset from every reading.
Error 2 — parallax error
Reading at an angle shifts the value. Read perpendicular to the scale, straight on.
Fix zero error by checking/subtracting; fix parallax by reading square-onThese two — zero error and parallax error — are the classic analogue-scale problems. Naming both, with a fix for each, is exactly what’s wanted.
⚛ Choosing & using an instrument
Match the instrument to the quantity and the scale of the task.
Check for zero error before starting; subtract if present.
Read perpendicular to analogue scales (avoid parallax).
Ammeter in series, voltmeter in parallel.
Justify digital vs analogue by precision, speed and the task’s needs.
💡 Top tips
Ammeter series, voltmeter parallel — current flows through, voltage is across.
Always tare a balance and check zero on meters first.
Read analogue scales perpendicular to avoid parallax.
Digital = precise & fast; analogue = continuous but error-prone.
Justify instrument choice by the task, not “which is best”.
⚠ Common mistakes
Swapping the connections — ammeter in parallel or voltmeter in series
Forgetting reaction time affects short time measurements
Ignoring zero errors and parallax on analogue scales
Saying digital is “always better” without linking to the task
Forgetting to tare the balance before weighing
Quick recap: Choose the instrument that fits the task: digital balance (mass, tare first), stopwatch (time, mind reaction time), ruler/micrometer (length), measuring cylinder (volume), thermometer or probe (temperature). Ammeter in series, voltmeter in parallel. Watch for zero errors and parallax, and justify digital vs analogue by precision and the job at hand.
You can now work safely and measure accurately — the foundations of every practical. The next skills build on this: turning raw measurements into reliable results by handling uncertainties and errors. Master those, and your data analysis will be watertight.
Measuring instruments and errors unclear?
Book a free meeting and we’ll drill instrument choice, the series-vs-parallel rule, and how to spot zero and parallax errors.
