THERMOMETERS

1
There are many different types of thermometers including liquid-in-glass, thermocouple,
digital thermometers etc.

These thermometers use many different physical properties in order to measure temperature.

laboratory thermometer
1. Liquid-in-glass thermometers
clinical thermometer

Laboratory
thermometer

These types of thermometers are called liquid-in-glass because they use a liquid (alcohol or
mercury) to show change in temperature.

Q1. How is the mercury used to measure temperature in these thermometers?

(Physical property of liquid-in-glass thermometers that is used to measure temperature).
(Hint: Remember experiment to verify that liquids expand when heated?)

These thermometers use expansion and contraction of

mercury/alcohol to measure temperature.
Q2. Give 3 reasons why these thermometers only use mercury or alcohol and no other liquid
such as water?

Mercury/alcohol expand linearly/uniformly.

They have low specific heat capacity (sensitive to heat gain/loss).
They do not stick to the walls of the capillary.
They have extensive temperatures in which they are in liquid form.

Q3. There are two types of liquid-in-glass thermometers. Draw both thermometers and
clearly label stem, bulb, mercury, capillary tube, and constriction/kink.

2
 Q4: What is the purpose of the white/yellow colour on the stem of these thermometers?

(It makes it easier to see the mercury level & the scale hence make) it
makes it easier to take the readings
Q5: why is it that a clinical thermometer has a pear-shaped cross section?
To magnify the mercury level & the scale for easy taking of the
readings.
Q6: What are clinical thermometers used for?
To measure human body temperature
Q7. Why is it that a clinical thermometer only ranges from 35˚C to 42˚C?
Because human body temperature is always within this range of
temperature
Q8: What is the purpose of a constriction in a clinical thermometer?
To prevent the mercury from flowing back to the bulb
Q9. State three differences between these two liquid-in-glass thermometers

Laboratory thermometer Clinical thermometer

Has no constriction Has a constriction

Has a larger range Has a smaller range
Has a round shape cross section Has a pear shape cross section

Q10. Between a laboratory and clinical thermometer, which one is more sensitive?
Clinical thermometer is more sensitive

Calibrating a thermometer (How to make scale on an unmarked

thermometer)

After a thermometer has been made, the manufacturing company has to make scale on that
thermometer.
How do they decide where to place eg 0ºC, 20ºC, 37ºC , 100ºC etc?

3
 Step 1: Determining the Lower fixed point (0°C)

Insert the thermometer in pure melting ice.

The mercury level will drop down in the
bore (capillary tube).
Where it stops, mark that position zero
degrees Celsius (lower fixed point).

Q11. Why should the ice used in determining the lower fixed point be:

a) Pure?
Pure ice has no impurities such as salts, chlorine etc. Impurities would alter
the melting point of the ice.
b) Melting?
To ensure that it is at zero degrees Celsius. If the ice is not melting then it
might not be at zero degrees Celsius.
Step 2: Determining the Upper fixed point (100°C)

Insert the thermometer in pure boiling water.

The mercury level will rise up in the bore
(capillary tube).
Where it stops, mark that position one
hundred degrees Celsius (upper fixed point).

Q12. Why should the water used in determining the upper fixed point be pure and boiling?
Water has to be pure:
Because impurities will alter the boiling point of the water
Water has to be boiling: 4
Because boiling point of pure water is 100 degrees Celsius.
 Now we know where the zero and 100 degrees Celcius are. Our job in step 3 is now
to complete the rest of the scale.
Measure the lengths between the 2 fixed points.
Step 3: Completing the rest of the scale:

20cm

0˚C 100˚C

C : cm
100 : 20 From this ration, it means every 1 C is represented by a length of
1 : 0.2 0.2cm. So we do 0.2cm divisions from 0 C to 100 C, each one
representing a temperature change of 1 C

Sensitivity & range of liquid-in-glass thermometer

A B

a) B has a larger capillary tube than A

Thermometer A is more sensitive than

thermometer B

A B

What about A has a smaller bulb than B

sensitivity here?
Thermometer A has a larger range than
b)
thermometer B

5
 A

B
c)
A has a longer capillary tube than B

Thermometer A has a larger

range than thermometer B

E F

d)

G has a thicker glass bulb than H

H G

Thermometer H is more sensitive than

thermometer G.

6
 2. Thermocouple thermometer

galvanometer calibrated in ˚C

copper wire copper wire

iron wire
cold junction always kept
at 0˚C
hot junction used to
Measure temperature
Of an object

This is another type of thermometer used to measure temperature.

It is very popular where temperatures measured are very high (100°C +) such as metal
smelting factories.

Q13. What physical property does a thermocouple use to measure temperature?

This thermometer has some advantages over a liquid-in-glass thermometer.

➢ It can measure rapidly changing temperature as it responds very quickly to

temperature change.
➢ It can measure very high temperatures (e.g 1500°C) as is usually the case in blast
furnaces such as that at the copper-nickel mine of Selebi-Phikwe
➢ The electric current that it produces can be used to operate other small electrical
appliances.

7
8
9