Introduction
Temperature tells us how hot or cold an object is. We use our sense of touch to get a rough idea, but touch can be misleading. A thermometer gives us an accurate, numerical measure of temperature.
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Key Concepts and Definitions
Temperature is the degree of hotness or coldness of an object or a place. It is measured in degrees Celsius (°C) in everyday life and science.
Thermometer is an instrument used to measure temperature. It works on the principle that liquids expand when heated and contract when cooled.
Clinical thermometer is used to measure human body temperature. Its range is 35°C to 42°C. It has a kink near the bulb that prevents mercury from flowing back, so the reading stays after the thermometer is removed from the body.
Laboratory thermometer is used in experiments. Its range is typically -10°C to 110°C. It does not have a kink.
Digital thermometer uses electronic sensors and shows temperature on a display. It is safer than mercury thermometers.
Normal human body temperature is 37°C (98.6°F).
Celsius (°C) scale: water freezes at 0°C and boils at 100°C.
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How a Mercury Thermometer Works
Mercury inside the glass tube expands as temperature rises, moving up the narrow tube. The scale is read at the level where mercury stops. The kink in a clinical thermometer keeps the mercury from falling back until you shake it down.
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Reading a Thermometer — Step by Step
Reading a clinical thermometer
- Hold the thermometer at eye level.
- Look for the mercury thread in the tube.
- Read the number on the scale where the thread ends.
- If the thread stops between 37 and 38 and aligns with the second small mark (each small division = 0.1°C), the temperature is 37.2°C.
Measuring the temperature of a liquid in the lab
- Place the bulb of the laboratory thermometer fully in the liquid without touching the sides.
- Wait until the mercury level stops rising (about 1–2 minutes).
- Read the scale at eye level.
- Record the value with the unit °C.
Why does a nurse shake the clinical thermometer before use?
- Shaking breaks the mercury at the kink and pushes it back into the bulb.
- This resets the reading to below 35°C so the next reading is accurate.
Converting a reading to understand normal vs fever
- Normal body temperature = 37°C.
- A child shows 38.5°C on the thermometer.
- Since 38.5°C > 37°C, the child has a fever.
Comparing two thermometers
- A clinical thermometer reads 36.9°C for Person A and 38.1°C for Person B.
- 38.1 > 37, so Person B has a fever; Person A is normal.
Identifying the correct thermometer for a task
- To measure the temperature of boiling water (around 100°C): use a laboratory thermometer (range up to 110°C).
- A clinical thermometer (range 35–42°C) would break — never use it for this purpose.
Reading in the presence of uncertainty
- The smallest division on a thermometer is 0.1°C.
- If mercury is between 36 and 37 and at the 7th small mark, the reading = 36.7°C.
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Key Formula
- There is no calculation formula needed at this level, but remember:
- Each big division on a clinical thermometer = 1°C
- Each small division = 0.1°C
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Common mistakes
- Avoid these errors:
- Reading the thermometer while it is still in the mouth (tilting may cause errors).
- Not holding the thermometer at eye level — always read at eye level to avoid parallax error.
- Using a clinical thermometer for measuring boiling water — it will crack.
- Forgetting to shake the clinical thermometer before use, giving a falsely high reading.
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Summary
Temperature measures hotness or coldness. Thermometers use the expansion of mercury to measure temperature in °C. A clinical thermometer (35–42°C, with kink) is used for body temperature; a laboratory thermometer (-10 to 110°C, no kink) is used in experiments. Normal body temperature is 37°C. Always read a thermometer at eye level.