Temperature (1) - Oxford University Press

102
THERMAL T
EFFECTS
ice
point
0 °C
pure
melting
ice
5.03
� Finding the lower fixed point
steam
point
100 °C
boiling
water
� Finding the upper fixed point
Temperature (2)
Fixing a temperature scale
To create a temperature scale, two standard temperatures must be chosen
against which others can be judged. These fixed points need to be defined so
that they can be reproduced in laboratories anywhere in the world:
On the Celsius scale:
0 degrees Celsius (0 �C) is defined as the melting point of pure ice. This is
the lower fixed point, known as the ice point.
100 degrees Celsius (100 �C) is defined as the boiling point of pure water,
where the water is boiling under standard atmospheric pressure (101 325 Pa).
This is the upper fixed point, known as the steam point.
Putting a scale on an instrument, so that it gives accurate readings, is called
calibrating the instrument. The diagrams on the left show how the fixed
points can be used to calibrate an unmarked thermometer.
To find the 0 �C point, the unmarked thermometer is placed in pure, melting
ice, as in the upper diagram on the left. The ice needs to be pure because its
melting point is lowered if any impurities are present.
To find the 100 �C point, the thermometer is placed in steam above boiling
water, as in the lower diagram on the left. Boiling must take place under
standard atmospheric pressure because a change in pressure alters the boiling
point of the water. Impurities also affect the boiling point but, in most cases,
they do not affect the temperature of the steam just above the water.
ice
point
steam
point
0 °C if end of ’thread‘
is half way between
0 °C and 100 °C,
temperature is 50 °C
100 °C
Once the 0 �C and 100 �C points have been fixed, the rest of the scale is made
by dividing the distance between them into 100 equal divisions, or degrees
(‘centigrade’ means ‘one hundred divisions’). The idea can can be extended
to produce a scale going above 100 �C and below 0 �C, although for some
thermometers, additional fixed points are used (see the top of the next page).
Putting equal divisions on a thermometer defines the temperature scale for that
particular type of thermometer. For example, if the end of the ‘thread’ is exactly
half way between the ice points and the steam points, as in the diagram above,
then by definition, the temperature is exactly half way between 0 �C and 100 �C.
So the temperature is 50 �C. If a scale has equal divisions, it is described as a
linear scale.