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

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

time a conversion is necessary. If you concentrate

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 100°C and 212°F 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 0°C as

(absolute zero) is defined as the temperature at

the freezing point and 100°C 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 32°F as the

Two absolute temperature scales have been in

freezing point and 212°F 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

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