Opening
Your hand is a passionate witness and a poor instrument. A metal spoon and a wooden spoon that have sat in the same room are the same temperature, and the metal feels colder because it carries heat away from your skin faster. Feeling is real. It is not a measurement. Temperature is the measure of how hot or how cold something is.
The three bowls, and why we stopped trusting skin
If one hand has been in hot water and the other in cold water, and both then enter the same lukewarm bowl, the hands disagree. One says warm. One says cool. The water did not become two temperatures to settle an argument. The hands arrived with two histories. That is why a thermometer exists. It does not bring a history from the previous bowl.
Clinical thermometers
A clinical thermometer measures the temperature of the human body. The ones you should meet now are digital. They run on a small battery, show the number on a screen, and have replaced many glass thermometers that used mercury. Mercury is a silvery liquid metal, and it is poisonous if a glass thermometer breaks and the liquid escapes. If you ever meet a broken old one, do not touch the silver beads. Call an adult.
A healthy human temperature is close to thirty-seven degrees Celsius. On the Fahrenheit scale, still used in some homes for fever, that is about ninety-eight point six degrees Fahrenheit. Close to thirty-seven does not mean every healthy person reads exactly thirty-seven all day. Children, elders, the time of day, and a recent run can shift the number a little. What matters is a clear rise that stays, and an adult's judgment.
Digital clinical thermometers are built for a narrow job, roughly the range of the body, about thirty-five to forty-two degrees Celsius. They can notice a small fever. They cannot measure ice water or boiling tea. The range does not go there. Ice-cold water is near zero degrees Celsius, far below a clinical thermometer's world.
Wash the tip as the instructions and your teacher say, often with soap and water, before and after, so the thermometer does not carry a fever from one person to the next. Read the screen after the device says it has finished. Do not take a number from a person who has just drunk hot tea. You will be measuring the tea.
Laboratory thermometers
A laboratory thermometer measures the temperature of other things: water, soil in an experiment, a liquid you are warming. Its range is wide, often about minus ten degrees Celsius to one hundred ten degrees Celsius. It can face ice and hot water that a clinical thermometer must refuse.
The reading habit is different, and it is the heart of this chapter.
A laboratory thermometer must be read while its bulb is still in the liquid. Lift it out, and the liquid column inside falls as it cools, and the number you then read is a lie about the water. There is no kink holding the column up, and no screen remembering it. Older mercury clinical thermometers had a narrow kink so the column stayed put until you shook it down. Digital clinical thermometers remember on the screen. A laboratory thermometer does neither favour. You read it in place.
Further precautions. Hold it by the top, not by the bulb, or you measure your fingers. The bulb should be well inside the liquid, not touching the bottom or the sides of the beaker, or you measure the glass as well as the water. Keep it upright. Give it a little time to settle. Do not use it as a stirrer. It is glass, and glass breaks into a bad day.
Check yourself
What is the normal temperature of a healthy human body, in both scales you met?
Check yourself
Why must a laboratory thermometer be read while its bulb is still in the water?
Celsius, Fahrenheit, and kelvin
The Celsius scale is the one school science in India lives on. Water freezes near zero degrees Celsius and boils near one hundred, under ordinary conditions. The Fahrenheit scale is an older household scale for body temperature. You are not asked to convert every number. You are asked to recognise the pair: thirty-seven Celsius and ninety-eight point six Fahrenheit.
The SI unit of temperature is the kelvin. We do not say "degrees kelvin." We say "kelvin." A change of one degree Celsius is the same size as a change of one kelvin. The zeros are in different places. Zero degrees Celsius is about two hundred seventy-three kelvin. So a rough shift from Celsius to kelvin is to add two hundred seventy-three. Thirty-seven degrees Celsius is about three hundred ten kelvin. You will rarely need the arithmetic this year. You need the name, and the reason it exists: a scale that starts at the coldest possible story, so temperatures are not negative in that scale. That coldest start is called absolute zero. We will not camp there. We will only nod at it.
Turn this over
Two students measure the same beaker. One touches the bulb with their fingers while holding it in the water. One lets the bulb float against the hot base of the beaker. Whose reading describes the water?
PauseIf metal feels colder than wood in the same room, what exactly is your skin reporting, if it is not a lower temperature?