Reading a Thermometer

A thermometer tells the exact temperature — but only if you read it correctly. This 90-second narrated lesson covers the clinical thermometer (35–42°C for body temperature) and the lab thermometer (−10 to 110°C for experiments), and how to read them at eye level to avoid parallax error. Includes quiz.

Class 6 ScienceClass 6 / Grade 6Ages 8–11
Lesson
🩺 Reading a Thermometer
A thermometer tells the exact temperature.DoctorDifferent jobs need different thermometers.

We know temperature is measured in degrees Celsius — but how exactly? We use a thermometer. A thermometer is a thin glass tube with a liquid inside that expands when it gets warm. Different situations need different thermometers.

The clinical thermometer

A clinical thermometer is the small thermometer a doctor uses to check your body temperature. Its scale runs from 35°C to 42°C — just the range needed for human body temperature.

  • Normal body temperature: 37°C
  • Mild fever: 37.5°C – 38.5°C
  • High fever: above 39°C

Clinical thermometers have a small kink (constriction) in the tube near the bulb. This kink stops the mercury from falling back when you remove the thermometer from your body, so you have time to read it accurately.

The laboratory thermometer

A laboratory thermometer is used in science experiments. Its scale runs from −10°C to 110°C — wide enough to measure boiling water, freezing mixtures, and everything in between.

Lab thermometers do not have a kink — they are designed to read continuously changing temperatures. You must keep the thermometer in the substance while reading.

You should never use a lab thermometer to measure body temperature — the scale is too wide to be precise for the small range of human body temperatures.

How to read a thermometer correctly

  1. Hold it upright or lay it flat on a white surface.
  2. Find the mercury level — look for the top of the silver column.
  3. Read at eye level — position your eye directly across from the mercury top. Looking from above or below gives a slightly different reading.
  4. Read to the nearest scale mark — if mercury is between marks, estimate to 0.1°C.

What is parallax error?

Parallax error happens when you read a scale from an angle instead of straight on. The mercury top appears to be at a different mark depending on the angle of your eye.

To eliminate parallax error: always position your line of sight perpendicular to the thermometer scale — look straight across at the mercury level, not from above or below.

Frequently asked questions

Why does a clinical thermometer have a kink?

The kink (constriction) near the bulb prevents mercury from flowing back down when the thermometer is removed from the patient's body. This gives you time to read it accurately. You shake the mercury back down before the next use.

Can I use a lab thermometer to measure body temperature?

No. Lab thermometers have a wide scale (−10 to 110°C) and don't have a kink, so the mercury falls immediately when removed from the body. They are also not precise enough for the small differences in body temperature that matter medically.

What is mercury and why is it used?

Mercury is a silvery liquid metal that expands and contracts very consistently with temperature changes. It's easy to see against the glass and gives accurate readings. However, mercury is toxic, so many modern thermometers use coloured alcohol instead.

How do digital thermometers work?

Digital thermometers use a small electronic sensor called a thermistor. Its electrical resistance changes with temperature. A microchip converts this resistance reading into a temperature value shown on the display.

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