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126
grade" temperature scale, and is the standard for
scientific work.
Certain properties of matter such as the expansion
of gases change with temperature according to known
laws. From these laws the absolute zero of temperature
has been found to correspond to minus 2/3 deg.
C, that is, 273 C. deg. below the temperature of melting
ice.
Before the Centigrade scale was established, another
arbitrary but less scientific temperature scale
had come into extensive use, in English speaking
countries. This was the Fahrenheit scale, and is still
the one in popular use in the United States. We
therefore may speak of temperature in either of these
scales, and it is fortunately easy to change from one
to another.
The melting point of ice corresponds to 32 degrees
on the Fahrenheit scale, and the boiling point of water
to 212 degrees. We may lay off the two scales side
by side as shown in Fig. 71. The figure, might in
fact, represent two ordinary mercurial thermometers,
one graduated in Centigrade, the other in Fahrenheit
Since a temperature of 32 deg. F. corresponds to 0
deg. C, and 212 deg. F corresponds to 100 deg. C, it
follows that 100 divisions on the Centigrade scale are
equivalent to 212 — 32 or 180 divisions on the Fahrenheit
scale. One degree Fahrenheit, therefore, equals
100
or 5/9 of a degree Centigrade, and we may change
180
from one scale to the other by the following simple
formula:
Fahrenheit to Centigrade: Substract 32 deg. and
multiply by 5/9.
Centigrade to Fahrenheit: Multiply by 9/5 and add
32 deg.
Examples:
100° F 1292° F.
—32 —32
68 X 5/9 = 37.7°C.
1000° C. X 9/5 = 1800
32
Forging- Stamping - Heat Tieating
1260 X 5/9 = 700° C.
-20° C. X 9/5 = -36
32
183:2° f - 4°F-
It is important to know how to make this conversion,
but where many conversions are to be made
it saves time to have a conversion table in which
corresponding temperatures in the two scales are
given in parallel columns. Many such tables have
been published. A convenient one for use in connection
with heat treating operations is given in
Fig. 72. This table gives equivalent Fahrenheit and
Centigrade temperatures at intervals of 5 degrees
Centigrade (9 deg. F.) over a range which covers
all ordinary metallurgical work. The columns have
been arranged for easy reading and the Fahrenheit
and Centigrade figures distinguished by different
tvpe. For many purposes it is satisfactory to know
the temperature to the nearest 5 deg. C. (that is,
within plus or minus 2y deg. C). For somewhat
greater accuracy, intermediate temperatures may be
figured closely enough by taking 2 deg. F. as the
equivalent of 1 deg. C, counting from the nearest even
5 deg. C. For example :
537° C. = 995 plus 4 = 999° F
539° C. = 1004 minus 2 = 1002° F.
This may be done mentally.
April, 1925
For complete accuracy, intermediate temperatures
may readily be interpolated on the basis that 5 deg.
C. = 9 deg. F. Thus, to change 537 deg. C. to Fahrenheit
:
535° C =995° F.
2° C. X 9/5 = 3.6
Or, 1000° F. to Centigrade
Answer 998.6° F.
995° F.
5 X 5/9
= 535° C.
= 2.8
Answer 537.8° C.
PART 2.
METHODS OF MEASURING TEMPERATURE
Gas Thermometer.
If the temperature of a certain quantity of a pure gas
is raised, and it is meanwhile kept at constant volume, the
pressure will increase in proportion to the absolute temperature.
p v = K T
Where p = pressure
v = volume
T = absolute temperature
K = a constant, depending upon the
nature of the gas
The absolute temperature T is equal to the Centrigrade
temperature plus 273 degrees.
By confining a gas in a container of known volume,
heating it and measuring the pressure, the temperature
scale may be extended as high as the material of the container,
and other limitations of the apparatus, will permit.
In this way, temperatures up to about 1500 deg. (J. have
been established with considerable accuracy. Such an apparatus
is called a "gas thermometer" or "gas pyrometer".
It is fragile and tedious to operate, and is used only as a
laboratory tool to standardize other and more convenient
means of temperature measurement.
Melting, Freezing, Boiling Point.
By the aid of a gas pyrometer the melting or boiling
temperatures of certain materials, such as pure metals or
salts, may be found, and these may then be used as standards
of temperature in calibrating pyrometers of various
types. This will be discussed further under "Calibration"
Expansion.
The commonest type of temperature measuring instrument
is the familiar mercurial thermometer. This consists
of a glass bulb containing mercury, connected to a
stem in which there is a fine capillary tube. The mercury
in the bulb expands when heated, forcing a column
of mercury up into the tube. The amount of mercury in
the tube varies directly with the temperature, and a scale
on the tube may therefore be marked off in degrees C
or F. Thermometers of this type have been developed for
commercial use up to about 550 deg. C, which is the