Temperature and its Measurement

Before thermometers were invented, doctors had to guess if someone had a fever by touching their forehead. Sometimes they got it wrong! A person could die from an undetected fever, or healthy people could be treated unnecessarily. Accurate measurement changed medicine forever.

Wrong measurements are worse than no measurements at all." — Anna Mani, Indian scientist and "Weather Woman of India".

This quote reminds us: if we can't trust our tool, we should use a different one. A broken clock shows two correct times per day, but a thermometer must be reliable every single time.

A clinical thermometer is specifically designed to measure human body temperature. Most modern ones are digital thermometers that use heat sensors (not mercury!) to detect temperature when placed under your tongue, in your ear, or in your armpit.

Normal body temperature: 37.0°C (or 98.6°F)

But did you know? Not everyone's "normal" is exactly 37.0°C. Children often run slightly hotter (37.2–37.5°C), while elderly people may be 0.5°C cooler. Your temperature also changes throughout the day—it's lowest in early morning and highest in late afternoon.

Why digital, not mercury? Mercury is toxic. If an old mercury thermometer breaks, the mercury vapours can poison you. Digital thermometers are safer, faster (they beep when ready), and easier to read.

Proper measurement steps:

1. Wash your hands and the thermometer tip with soap and water.
2. Reset the thermometer by pressing the reset button.
3. Place the thermometer under your tongue, close your mouth gently, and wait.
4. When it beeps or flashes, remove it and read the number on the display.
5. Clean it again before putting it away.

Temperature ranges you should know:

• Normal: 36.5–37.5°C
• Mild fever: 37.6–38.5°C
• High fever: 38.6–40.0°C
• Dangerous: Above 40.0°C (seek medical help immediately)

While clinical thermometers are limited to body temperature (usually 35–42°C), a laboratory thermometer can measure a much wider range. A typical lab thermometer can measure temperatures from –10°C to 110°C—perfect for boiling water, frozen ice, or chemical reactions!

How to read a lab thermometer:

1. Look at the scale marked along the glass tube.
2. Identify the bigger marks (these show 10°C intervals, e.g., 0, 10, 20, 30).
3. Count the small marks between them. Each small mark usually represents 1°C.
4. See where the liquid column stops—that's your reading.

Critical rule for accuracy: Keep your eye level with the liquid column. If you look from above or below, the reading will be wrong!

Why thermometers use alcohol or mercury: These liquids expand evenly as temperature rises. When the substance heats up, the liquid inside rises. When it cools down, the liquid falls. The scale on the outside tells you the temperature.

Proper measurement steps:

1. Immerse the bulb of the thermometer fully in the liquid you're measuring.
2. Keep the thermometer vertical (straight up, not tilted).
3. Wait for the liquid column to stop rising (don't wait too long, or the liquid will start falling as the water cools).
4. Read the temperature while the thermometer is still in the liquid.
5. If you take it out, the reading changes immediately—so always read while immersed!

Finding the "smallest value" the thermometer can measure: Some thermometers show changes of 1°C per small mark, others show 0.5°C. Always check the scale before using it! To calculate: if there are 10 small marks between 0°C and 10°C, then each mark = 1°C.

Why are there three temperature scales? Historical accident! Different scientists developed different systems in different countries.

The Celsius Scale (°C): Used everywhere in science. Water freezes at 0°C and boils at 100°C. This is the scale used in India and most of the world.

The Fahrenheit Scale (°F): Still used in the United States (and some older thermometers worldwide). Water freezes at 32°F and boils at 212°F. Normal body temperature is 98.6°F. This scale is dying out in science.

The Kelvin Scale (K): The SI unit (official scientific standard). Used in physics and chemistry. Water freezes at 273.15 K and boils at 373.15 K. There's no "degree" symbol—it's just "kelvin."

Conversion formula: Temperature in Kelvin = Temperature in Celsius + 273.15

Example: 37°C = 37 + 273.15 = 310.15 K

Why Kelvin? Because it starts at absolute zero (–273.15°C), the coldest temperature possible in the universe. At this point, molecules stop moving entirely. You can't get colder!

When you boil water, does the temperature keep rising forever? No! Something strange happens: the temperature stays at 100°C until all the water turns to steam. It's as if the temperature "pauses" during the change of state.

Similarly, when ice melts, the temperature stays at 0°C until all the ice becomes liquid water. These fixed points—0°C for freezing and 100°C for boiling—are why the Celsius scale was created! Scientists needed consistent reference points.

Why the pause in temperature? Energy goes into breaking apart molecular bonds and changing the state, not into making molecules move faster. Once all the substance has changed state, temperature starts rising again.

Have you noticed weather reports mention "maximum and minimum temperature"? These are measured by special thermometers hung on walls in weather stations.

Why does air temperature matter?

• It tells us what to wear.
• It helps farmers decide when to plant crops.
• It helps scientists study climate change.
• It helps meteorologists predict rain, snow, and storms.

Maximum temperature: The highest temperature in a 24-hour period, usually in the afternoon when the sun is strongest.

Minimum temperature: The lowest temperature in a 24-hour period, usually just before sunrise when heat has been lost all night.

Did you know? Anna Mani, an Indian scientist, invented many weather measurement instruments. Because of her work, India became independent in manufacturing weather instruments instead of relying on other countries. She was called the "Weather Woman of India"!