Can a clinical thermometer be used for measuring the temperature of boiling water? Or for measuring the temperature of ice?
Answer
No. The temperatures of boiling water and of ice are outside the range of a clinical thermometer.
What we want to measure
Its temperature
Inside a clinical thermometer’s range?
Melting ice
about 0 °C
No — far too low
Human body
about 37 °C
Yes
Boiling water
about 100 °C
No — far too high
Why it happens: a clinical thermometer is built only for human body temperature, which never normally goes below 35 °C or above 42 °C. Its scale therefore covers just that small stretch. Beyond those marks there is simply nothing printed to read — and a glass one could even crack in boiling water.
For ice and boiling water we must use a laboratory thermometer, whose range is –10 °C to 110 °C.
Q2.
How can we measure temperatures beyond the range of a clinical thermometer?
Answer
By using a laboratory thermometer.
The one usually kept in a school laboratory has a range of –10 °C to 110 °C.
That single stretch covers ice at 0 °C, room air, warm water, and water boiling at about 100 °C.
It is a long sealed glass tube with a bulb of liquid (alcohol coloured red, or mercury) at one end and a Celsius scale marked along the tube.
Tip: For temperatures far outside even this range — a furnace, or the inside of a star — scientists use other instruments altogether. The temperature at the core of the Sun is about 15 million degrees Celsius.
Q3.
How was fever detected before thermometers were developed?
Answer
By feeling the pulse rate of the person.
Fever affects the pulse rate — it usually makes the pulse faster.
This was known even in olden days in India, and a physician would judge illness by holding the patient’s wrist.
But this method has a weakness: apart from fever, some other situations also affect the pulse rate — running, fear, excitement, exercise. Hence pulse rate alone is not a reliable indicator of fever.
Why the thermometer won: the pulse is an indirect clue and gives no number for hotness. The thermometer measures the very quantity we want — the temperature — and gives it as a number in °C that anyone can check.