Class 6 Science Chapter 7: Temperature and its Measurement

A Boy Who Felt Feverish

Phiban lives in Shillong with her younger brother Lambok. One afternoon he came home from school and said he was feeling feverish. She put a hand to his forehead and thought he might indeed have a fever.

But She Did Not Stop There

Instead of settling it by hand, she fetched the thermometer from the almirah, cleaned the tip in soapy water, and took his temperature properly. To her relief, the reading was normal. She cleaned the tip a second time, dried it, and put it back.

Which Was the Real Answer?

Her hand said one thing and the thermometer said another. So the question is worth asking directly. Can it always be judged correctly, only by touching a person, that they have a fever?

You Already Know the Easy Cases

Some bodies are plainly hotter than others and experience tells us so. On a summer day, water straight out of the tap can be warmer than what a matka, the earthen pot, holds, or what comes out of a refrigerator. Touching both samples is enough to notice the difference. Nobody needs an instrument to know that a hot cup is hot.

The Real Question

So touch is not useless. The question is whether we can always rely upon it, which is a very different thing. One clean experiment will settle it.

Three Containers

Find three large containers and put a label on each: A, B and C. Warm water goes into the first, ordinary tap water into the second, and ice-cold water into the third. The middle one matters most, though it is the least interesting to look at.

Predict First, Then Observe

The activity has two parts, and the first is prediction. Before you touch anything, write down what you think your right hand will feel in B, and what your left hand will feel in B. Writing it down is the whole point.

Now Do It

Dip your right hand into A and the left one into C, and leave them there a minute or two. Then take both out and put them into B at the same moment. Notice what each hand reports.

Two Hands, Two Answers

Your right hand finds the water in B cool. Your left hand, at the same instant and in the same bowl, finds that same water warm. Neither hand has made a mistake and nothing about the water has changed. Compare what happened with what you predicted.

What Follows From That

The inference is unavoidable. Our sense of touch cannot be trusted every time to settle whether something is hot or cold. What a hand tells you depends on what that hand was doing a moment earlier. It reports a change, not a state.

So How Do We Find Out?

The same sort of problem you met with handspans, and the same sort of answer. A dependable measure of how hot or cold a body is goes by the name temperature. It does not change with whose hand is asking.

What Temperature Tells You

Of two bodies, the hotter one carries the higher temperature. Take the difference between their two figures and you learn how they stand against one another. So a comparison becomes a number rather than an opinion.

The Device

Whatever device gives you that measurement goes by the name thermometer. Phiban reached for one the moment her hand gave her an answer she was not prepared to trust, which is exactly the right instinct. A reading can be written down and compared later.

Two Kinds to Know

Two kinds are worth knowing. The clinical sort is what measures the body temperature of a person. The laboratory sort handles everything else, from warm water to ice.

Why Two and Not One

A human body stays within a narrow band of temperatures, so an instrument for bodies can be built for that band alone. Everything else ranges far wider, and needs an instrument to match. The next topics take them one at a time.

Scenario:

A class fills three bowls. One holds cold water, one warm, one at room temperature. Each child puts the left hand in the cold bowl and the right in the warm bowl, waits a minute, then puts both into the middle bowl. Every child reports the same odd thing. The same water feels warm to one hand and cool to the other. Rohit says one hand must be wrong. Their teacher asks which one. Nobody can say. The class measures the middle bowl with a thermometer and gets one number. Sana writes the conclusion. Touch reports a change rather than a temperature, so two hands can disagree about one bowl.

Analysis:

Both hands were working properly and still disagreed. Touch compares against what it was just feeling, which is why it cannot give a number.

Activity:

Try the three-bowl test at home. Write what each hand reports in the middle bowl, then measure it if you have a thermometer.

Title:

Make your two hands disagree

Safety:

The warm water must be comfortably warm and never hot. Ask an adult to check it first, and do not use ice straight from a freezer against bare skin.

What You Notice:

The two hands disagree, and most people predict that they will not. The disagreement fades after a few seconds as both hands settle, which is itself worth noticing. Nothing about the water in B changed at any point in the experiment.

Lambok complained of feeling feverish and his sister found, using a thermometer, that his temperature was perfectly normal.

A bad reading is worse than none at all, as Anna Mani put it, because it still looks like a fact.

Water from a matka, an earthen pot, can be cooler than water straight from the tap in summer.

Why we need a thermometer — Two hands, one bowl, two answers — If touch could settle it, both hands would report the same thing.

The two thermometers — Clinical for people, laboratory for everything else — One narrow range for bodies, one wide range for the rest.

The One You Have Seen

Somewhere at home there is a slim instrument with a tip at one end and a small screen at the other. It is used for measuring body temperature and it is called a clinical thermometer. The ones you meet today show the temperature in digits.

How It Works

These are digital clinical thermometers and they run on batteries. The temperature is worked out by heat sensors inside. They give you a reading once the instrument has been placed in contact with a person's body, which is the whole of the method.

The Scale It Uses

The scale these instruments generally work in is the Celsius scale, and its unit is the degree Celsius, written °C. Any reading you take will carry that symbol.

What Came Before

Earlier, mercury thermometers did this job. They worked, but mercury is a very poisonous substance, and clearing up safely after the glass breaks is no easy matter. That is a serious drawback in a household instrument.

Why the Change Stuck

Digital thermometers pose no such risk, and the numbers on the display are easier to read than a thin thread of liquid. Between safety and legibility the case was settled, and mercury thermometers are being replaced.

Reading From a Distance

During the COVID-19 pandemic you may have seen a third sort, pointed at a forehead from a step away. Those are the non-contact kind, otherwise known as infrared thermometers. They measure without touching, which reduces the risk of spreading disease.

Using One Properly

There are precautions, and they are not fussiness. Read the instruction manual first. Give the tip a wash in soap and water, both before you use it and after. Keep the digital part, such as the display, well away from the water while you do it.

And One More

Never grip the instrument at its tip. Your fingers are warm, and the tip is the part that senses temperature, so holding it there is a way of measuring your own hand instead of the patient.

Now Measure a Few People

Wash your hands and the tip. Take your own temperature, then ask some friends who are willing, and record each reading in a table with the name beside it. Clean the tip with soap and water between one person and the next.

The Number Everybody Quotes

A healthy human body is taken to sit at 37.0 °C, and that figure is called normal. Now look at your table. Did every single person come out at 37.0 °C? Almost certainly not, and several will have read a little above or a little below.

Why That Is Not a Problem

Not everybody lands on 37.0 °C, and nobody expected them to. The figure called normal is what a great many healthy people average out at. A perfectly healthy person may therefore sit slightly off it and be perfectly well.

What Moves It About

Body temperature is influenced by several things: a person's age, the time of the day, and how active they have been. Take your own temperature at different hours over a month, keep the record, and the variation will be there in your own notebook.

The Limits of a Human Body

There is a band all the same. Human temperature keeps, as a rule, inside the stretch from 35 °C to 42 °C. That narrow band is exactly why a clinical thermometer can be built to cover so little.

Measuring in the Armpit

With a small child, or with somebody elderly, the instrument may instead go into the armpit. A reading taken that way runs about 0.5 °C to 1 °C under the true figure, so it has to be read with that in mind.

What This Instrument Cannot Do

Ask whether a clinical thermometer could measure boiling water, or ice. It could not. Both lie far outside its range, and an instrument built for a body has no business elsewhere. That is what the next topics are for.

Scenario:

A class records body temperature with a digital clinical thermometer. Twenty-one readings come in between 36.2 and 37.1 degrees Celsius. Imran expects every healthy person to read exactly the same. Their teacher asks the class to look at the spread, which is less than one degree across twenty-one people. The class then takes three readings from one child and finds those differ slightly too. Nita works out what that means. A single reading is not a fixed fact but a measurement with a small range. A number counts as unusual only once it falls outside that range.

Analysis:

Twenty-one readings in a span under one degree is agreement, not disagreement. Knowing the normal spread is what makes an unusual reading recognisable.

Activity:

Take your temperature three times, a minute apart. Write the three readings and the difference between the highest and lowest.

Title:

Ten readings and one average

Safety:

Only measure people who agree to it, and never share a tip without washing it. Do not use the thermometer on anybody who is unwell without an adult present, and do not treat any reading as a diagnosis.

What You Notice:

Very few readings land on exactly 37.0 °C and almost none land on your own average either. Everybody in your list is nonetheless well. Taking your own temperature at breakfast and again in the evening shows the same person shifting over a single day.

Mercury is very poisonous and hard to deal with after a thermometer breaks, which is why the digital sort has taken over.

During the COVID-19 pandemic, infrared thermometers read temperatures from a distance without touching anybody.

An armpit reading understates the truth by roughly half a degree to a degree.

What normal really means — An average, not a rule — A great many healthy people average out at 37.0 °C, so individuals vary around it.

The human range — Thirty-five to forty-two — Human temperature keeps, as a rule, inside that stretch.

A Friend With a Different Number

Suppose a friend shows you her digital thermometer and it reads 98.6 for a perfectly healthy person. You learned that normal is 37.0. Both of you are right, and the difference is not in the person but in the scale. Two rulers can measure one table and give two numbers.

The One You Have Been Using

Clinical thermometers in India generally work in the Celsius scale, whose unit is the degree Celsius, written °C. Every reading you have taken so far has carried that symbol. Weather reports here use it too.

A Second Scale

A second scale exists, and it goes by the name Fahrenheit. Its unit is the degree Fahrenheit, denoted by °F. It is a different set of markings laid over exactly the same hotness.

The Same Person, Two Numbers

What comes out as 37.0 °C in Celsius comes out as 98.6 °F in Fahrenheit. Nothing about the body has changed. The two readings describe one state in two languages.

Where Fahrenheit Stands Now

It has not vanished, and you will meet thermometers that use it. But most scientific studies have left it behind. Think of it as a scale you should be able to read rather than one you work in. Being able to read a thermometer is not the same as choosing to use that scale.

And a Third, for Science

Scientific work uses a third, and it goes by the name Kelvin. Its unit is the kelvin, denoted by K, and that unit is the SI one for temperature.

Converting Into Kelvin

Going from one to the other is simple arithmetic. Temperature in the Kelvin scale equals temperature in the Celsius scale plus 273.15. So 0 °C is 273.15 K, and 37.0 °C is 310.15 K.

What That Addition Tells You

Notice that you only add. You never multiply. That means one step on the Celsius scale is the same size as one step on the Kelvin scale, and only the starting point differs between them.

Now the Writing Rules

These are small and they are easy to get wrong. Start with the names of the scales. Celsius scale, Fahrenheit scale and Kelvin scale all start with a capital letter, because each carries the name of a scientist.

The Units Are Different

The units behave differently. In degree Celsius and degree Fahrenheit, it is only the scientist's name that takes a capital; the word before it stays in small letters. So you write degree Celsius and never Degree celsius.

The Odd One Out

Write the unit kelvin in small letters throughout. Not Kelvin, just kelvin, even though the scale it belongs to is the Kelvin scale with a capital. The scale carries the man's name; the unit is an ordinary word.

But the Symbols Are Capitals

The symbols of all three units are capital letters: °C, °F and K. Note carefully that the degree sign is not written with K. Three hundred and ten kelvin is 310 K, never 310 °K.

Full Stops and Spaces

Keep full stops off a symbol, unless the sentence happens to finish there. And put a space in front of the unit when you set a temperature down. So 37.0 °C, with the gap, and not 37.0°C.

One Degree or Many

One last rule. Anything warmer than a single degree takes the plural when you write the unit out in full. Five degrees Celsius, not five degree Celsius.

Before There Were Thermometers

How was fever detected at all before these instruments existed? A fever does change the pulse of a person, and that much was known in olden days in India. But plenty of other things change a pulse as well, so counting it settles nothing about fever on its own. A thermometer settles in seconds what a pulse could only hint at.

Scenario:

Two pupils read a weather report from another country giving the day as 98 degrees. Arjun says that is impossible because water boils at 100. Meera checks the symbol and finds °F rather than °C. The class works out what 98 °F is in Celsius and gets about 37, which is an ordinary warm day. Arjun sees that the number was never the problem. Their teacher asks what the mistake would have cost. The class decides it could have been serious, since a nurse reading a body temperature of 37 as 37 °F would think somebody was freezing. They agree the symbol carries as much meaning as the digits.

Analysis:

98 is alarming in one scale and ordinary in another. Writing the symbol is not tidiness, it is half the information.

Activity:

Find three temperatures in °F from a newspaper or a forecast. Convert each to °C and write both with the correct symbol.

Title:

Three columns for one week

Safety:

This is a reading and writing activity, so there is nothing to handle. If you look up reports online, ask an adult before using a device.

What You Notice:

The Kelvin figures are large and change by exactly the same amounts as the Celsius ones, which is the addition rule showing itself. The mistakes you circle are almost always the same two: a missing space, or a degree sign attached to K.

Each of the three scales carries the name of a scientist who worked one of them out.

Before thermometers existed, fever in India was judged from the pulse rate, which several other things also change.

One healthy body reads 37.0 in Celsius and 98.6 in Fahrenheit, and nothing about it has changed.

The three scales — Celsius, Fahrenheit, Kelvin — Two take a degree sign. The third, the SI one, does not.

Capital letters — Small degree, capital name, small kelvin — degree Celsius, degree Fahrenheit, kelvin. But °C, °F and K are all capitals.

A Different Instrument Altogether

A clinical thermometer covers the narrow band a body lives in. Boiling water and ice lie far outside it. For everything else there is the laboratory thermometer, and your school laboratory almost certainly has one. It looks nothing like the one at home.

What It Is Made Of

What you have is a glass tube, long and narrow and of even width, closed off at the top. At one end sits a bulb, and inside the bulb is a liquid. Out beyond the bulb, in the tube itself, a narrow column of that liquid can be seen.

The Liquid Inside

The liquid used is generally alcohol, coloured red so that the thin thread shows up clearly, or else mercury. Either way, the point of it is the same: the column rises or falls as the temperature changes.

How You Read It

A Celsius scale runs up the side of the tube. Find where the top of the liquid column lands on that scale, and the mark it lands on is your reading. That is the whole of the method, and the rest is care.

First Find Its Range

Before using any thermometer, look at it properly. Note the lowest temperature it can measure, then the highest. Those two figures together are its range. For a common school thermometer the range runs from –10 °C to 110 °C.

Then Find Its Smallest Division

Now look between two of the bigger marks. How much temperature difference do they show? How many small divisions lie between them? Divide the first by the second and you have the smallest value the thermometer can read. Nothing finer than that can be claimed from it.

Working One Out

Suppose the difference between 0 °C and 10 °C is marked, with ten small divisions between them. Ten degrees over ten divisions gives one degree. So the smallest value that thermometer can read is 1 °C.

And Always Check Yours

Another laboratory may hold thermometers with a different range and a different smallest division. So give the instrument in your hand a careful look every time, rather than assuming. Two instruments on one shelf may differ.

Rule One: Clear of the Glass

Now the measuring itself. Dip the instrument so that the bulb goes properly under, but keep it clear of the base and the walls. The glass of the beaker is not what you came to measure.

Rule Two: Hold It Upright

The thermometer should be held vertically. It should not be tilted. A tilted tube puts the column at an angle to your eye and invites exactly the reading error you met with the scale in an earlier chapter.

Rule Three: Read It While It Is In

Watch the liquid column rise, and once it settles, note the figure. Do not dawdle over it, or the water itself will begin to cool. And notice what happens the instant you lift the instrument out: the column begins to fall.

Which Is Why the Rule Exists

That falling column is the whole reason. Take your reading with the instrument still under the water. A figure noted after lifting it out belongs to something that is already changing.

Rule Four: Eye in Line

Bring your eye level with the top of the liquid column before you read it. Look from above or below instead and the column appears to sit at a different mark, exactly as before.

When Careful People Disagree

Phiban and her classmates in Shillong measured the temperature of boiling water and wrote down 97.8, 98.0, 97.9, 98.0 and 98.1. Five readings of one beaker. The likely reason is that the correct way of reading was not followed by every student.

And What Stays Still

Two things are worth watching for. Water holds one temperature all the while it boils, and ice holds one all the while it melts. Read either of them twice, some minutes apart, and the figure does not move. That steadiness is itself a thing worth knowing about water.

Scenario:

Phiban's teacher sets up one beaker of boiling water and asks five pupils to read its temperature in turn. They write down 97.8, 98.0, 97.9, 98.0 and 98.1, all in degrees Celsius. Shemphang says the water must have been cooling between readings. Onestar says the thermometer is faulty and should be thrown out. Kloi says the differences are too small to matter and they should simply take the middle value. Bandarisha says she noticed one classmate lift the thermometer out before reading it and another bend down to look up at the column. The teacher writes on the board: the water was the same, the instrument was the same, so what was not?

Analysis:

Three explanations are on offer and only one of them survives what the class already knows about boiling water and about how a laboratory thermometer must be read. The spread is small, which is itself a clue to its cause.

Activity:

Measure the temperature of the same cup of warm water three times over: once correctly, once with the bulb resting on the bottom, and once by lifting the thermometer out first. Write all three readings down with their units and say which rule each wrong one broke.

Title:

Break each rule on purpose

Safety:

Warm, never hot, and never boiling. A laboratory thermometer is glass and will break if it strikes anything hard, so ask an adult to be present and never hold it by the bulb.

What You Notice:

Every wrong reading differs from the careful one, but only by a fraction of a degree, which is why such faults are so easy to miss. The lifted reading is the one that keeps changing as you look at it. Five careful readings, by contrast, agree.

Alcohol in a laboratory thermometer is coloured red so that the thin column can be seen easily.

Water holds one temperature all the while it boils, and ice holds one all the while it melts.

Anna Mani, known as the Weather Woman of India, designed and made a great many instruments for measuring the weather.

The four rules of reading — Clear of the glass, upright, still in, eye in line — Bulb touching nothing, thermometer vertical, read while immersed, eye level with the column.

Smallest value — Gap divided by divisions — Ten degrees between big marks, ten divisions, so one division reads 1 °C.