Temperature and its Measurement · Lesson 3 of 5
Reading a Laboratory Thermometer
“Learn how a liquid-column thermometer works and how its range and divisions determine what it can read.”
• Identify the bulb, liquid column, sealed tube and marked Celsius scale. • Find a thermometer’s lowest and highest readings and describe its range. • Calculate the value of one small division from labelled marks. • Choose a scale that can show a required temperature precisely. • Explain why a clinical thermometer is not suitable for very hot or cold samples.
A different instrument for a different job
A clinical thermometer is suited to body temperature, which occupies a fairly narrow range. To measure warm water, ice, and many other things, a laboratory thermometer can provide a much wider marked range. Before using one, look at the instrument itself: its smallest and largest marks and the spacing between its scale lines determine what readings it can show.
Follow the liquid column
A common laboratory thermometer is a long, narrow sealed glass tube. One end has a bulb containing liquid. A thin column of that liquid runs up the tube beside a Celsius scale. Its level rises or falls as the temperature changes. Read the scale mark level with the very top of the column. Some instruments use red-coloured alcohol so the column is easy to see; others contain mercury, which calls for extra care if glass breaks.
The interval from the lowest temperature marked on an instrument to the highest temperature marked on it.
The thermometer illustrated in the chapter runs from −10 °C to 110 °C. That does not mean every laboratory thermometer has this exact range. A thermometer in your school may have different endpoints or different small divisions. Check the actual markings every time rather than relying on the appearance of an example.
Problem
A thermometer has its lowest mark at −10 °C and its highest mark at 110 °C. State the range.
- 1.Locate the lowest labelled mark: −10 °C.
- 2.Locate the highest labelled mark: 110 °C.
- 3.State the interval from −10 °C to 110 °C; do not confuse the two endpoint readings with a single reading of the liquid.
Work out what one small division means
A labelled scale may show 0 °C and 10 °C with smaller marks between them. The temperature difference between those labelled marks is 10 °C. If there are ten equal spaces, each space represents 1 °C. Count the spaces, not merely the interior lines: ten equal spaces may have nine small interior marks. This reasoning is needed whenever you read an unlabelled position.
Problem
The marks 0 °C and 10 °C are separated by ten equal divisions. What is the smallest value shown?
- 1.The difference is 10 − 0 = 10 °C.
- 2.Divide by ten equal spaces: 10 °C ÷ 10 = 1 °C.
- 3.One small division represents 1 °C; a column ending six spaces above 0 °C reads 6 °C.
Problem
There are 20 equal spaces from 10 °C to 20 °C. Can the scale show 17.5 °C?
- 1.The labelled difference is 20 − 10 = 10 °C.
- 2.Each space represents 10 ÷ 20 = 0.5 °C.
- 3.From 10 °C to 17.5 °C is 7.5 °C, or fifteen spaces. The scale can show that position.
Problem
A thermometer has 50 divisions from 0 °C to 100 °C. What does each division indicate?
- 1.The labelled interval spans 100 − 0 = 100 °C.
- 2.Share this equally among 50 spaces: 100 ÷ 50 = 2.
- 3.Each division represents 2 °C; do not claim it can directly show every 0.5 °C.
To read or draw a value such as 22.5 °C, the chosen scale must include 22.5 °C in its range and have sufficiently small divisions, such as 0.5 °C. A large range alone does not promise enough detail. When comparing several pictured thermometers, inspect both endpoints and the spacing before selecting one.
A thermometer may include a temperature within its range but still lack divisions fine enough to read it exactly. Check both before recording a decimal value.
Handle the glass and liquid safely
Glass can break if the instrument strikes the beaker or another hard object. Do not hold a laboratory thermometer by its bulb. A clinical thermometer should not be used to measure boiling water or ice: these temperatures are outside its intended body-temperature range. Conversely, a laboratory liquid-column thermometer must usually be observed while its bulb is still in contact with what it measures; it is not a convenient substitute for a digital clinical reading after removal.
Quiz
Which part of a liquid-column thermometer contains the liquid at one end?
A scale runs from −10 °C to 110 °C. Which describes its range?
Ten equal spaces lie between 0 °C and 10 °C. Each represents:
Twenty equal spaces lie between 10 °C and 20 °C. Which reading can a marked position show?
Which feature must be checked to read 22.5 °C exactly from a marked scale?
Why is a body-temperature clinical thermometer unsuitable for boiling water?
Practice Problems
- Label the bulb, narrow liquid column, sealed tube and marked scale on a sketch of a laboratory thermometer.
- A thermometer’s endpoints are −20 °C and 120 °C. State its range.
- A scale shows 20 equal spaces between 20 °C and 30 °C. Find the value of one space and locate 27.5 °C.
- Explain why counting equal spaces is safer than counting only interior small lines.
- A thermometer has 50 divisions between 0 °C and 100 °C. Explain whether it can directly show 22.5 °C exactly.
- Compare a thermometer whose smallest division is 1 °C with one whose smallest division is 0.5 °C for reading 7.5 °C.
Key Takeaways
• The top of the liquid column aligns with a temperature mark. • Range runs from the lowest to the highest mark; it may differ among instruments. • Divide a labelled temperature interval by its number of equal spaces to find one division. • Check range and division size before claiming a precise reading. • Handle the glass gently and never hold the instrument by the bulb.