and the boiling point is divided into 100 degrees
on the Celsius scale and divided into 180 degrees
on the Fahrenheit scale.
Since the actual value of the interval between
the freezing point and the boiling point is
identical, the numerical readings on Celsius and
Fahrenheit thermometers have no absolute
significance and the size of the degree is arbitrarily
chosen for each scale. The relationship between
degrees Celsius and degrees Fahrenheit is given
by the formulas
and
Figure 4-17.--Your arm is a lever.
Many people have trouble remembering these
formulas, with the result that they either get them
mixed up or have to look them up in a book every
MEASUREMENT OF
time a conversion is necessary. If you concentrate
TEMPERATURE
on trying to remember the basic relationships
given by these formulas, you may find it easier
Temperature is measured by bringing a
to make conversions. The essential points to
measuring system (such as a thermometer) into
remember are these:
contact with the system to be measured. You will
often need to know the expansion of a liquid, the
1. Celsius degrees are larger than Fahrenheit
pressure of a gas, emf, electrical resistance, or
degrees. One Celsius degree is equal to 1.8
some other mechanical, electrical, or optical
Fahrenheit degrees, and each Fahrenheit degree
property that has a definite and known relation-
is only 5/9 of a Celsius degree.
ship to temperature.
2. The zero point on the Celsius scale
The measurement of a property other than
represents exactly the same temperature as the
temperature can take us only so far in the
32-degree point on the Fahrenheit scale.
measuring of temperature. In noting changes in
3. The temperatures 100C and 212F are
temperature, we must be able to assign a
identical.
numerical value to any given temperature. For this
we need temperature scales. The two most familiar
In some scientific and engineering work,
temperature scales are the Celsius scale and the
particularly where heat calculations are involved,
Fahrenheit scale.
an absolute temperature scale is used. The zero
The Celsius scale is often called the centigrade
point on an absolute temperature scale is the point
scale in the United States and Great Britain. By
called ABSOLUTE ZERO. Absolute zero is
international agreement, however, the name was
determined theoretically, rather than by actual
changed from centigrade to Celsius in honor of
measurement. Since the pressure of a gas at
constant volume is directly proportional to the
the eighteenth-century Swedish astronomer,
Anders Celsius. The symbol for a degree on this
temperature, we assume that the pressure of a gas
scale (no matter whether it is called Celsius or
is a valid measure of its temperature. On this
assumption, the lowest possible temperature
centigrade) is C. The Celsius scale takes 0C as
(absolute zero) is defined as the temperature at
the freezing point and 100C as the boiling point
which the pressure of a gas would be zero.
of pure water at standard sea level atmospheric
pressure. The Fahrenheit scale takes 32F as the
Two absolute temperature scales have been in
freezing point and 212F as the boiling point of
use for many years. The Rankine absolute scale
pure water at standard sea level atmospheric
is an extension of the Fahrenheit scale; it is
sometimes called the Fahrenheit absolute scale.
pressure. The interval between the freezing point
4-17
