142. Units of Heat and Temperature.—There are two common units for measuring heat: the Calorie and the British thermal unit. The calorie is the amount of heat required to raise the temperature of a gram of water one centigrade degree. The British thermal unit is the amount of heat required to raise the temperature of one pound of water one Fahrenheit degree. One of the units plainly belongs to the metric system, the other to the English.
An instrument for measuring temperature is called a thermometer. Various scales are placed upon thermometers. The two thermometer scales most commonly used in this country are the Centigrade and the Fahrenheit. The Fahrenheit thermometer scale has the temperature of melting ice marked 32°. The boiling point or steam temperature of pure water under standard conditions of atmospheric pressure is marked 212° and the space between these two fixed points is divided into 180 parts.
The centigrade thermometer scale has the same fixed points marked 0 and 100 and the space between divided into 100 parts. (See Fig. 123.) The centigrade scale is the one used by scientists everywhere.
Fig. 123.—Comparison of centigrade and Fahrenheit scales.
143. Comparison of Thermometer Scales.—It is often necessary to express in centigrade degrees a temperature for which the Fahrenheit reading is given or vice versa. Since there are 180 Fahrenheit degrees between the "fixed points" and 100 centigrade degrees, the Fahrenheit degrees are smaller than the centigrade, or 1°F. = 5/9°C. and 1°C. = 9/5°F. One must also take into account the fact that the melting point of ice on the Fahrenheit scale is marked 32°. Hence the following rule: To change a Fahrenheit reading to centigrade subtract 32 and take 5/9 of the remainder, while to change centigrade to Fahrenheit multiply the centigrade by 9/5 and add 32 to the product. These two rules are expressed by the following formulas.
(F.° - 32)5/9 = C.°, 9C.°/5 + 32° = F.°
Another method of changing from one thermometric scale to another is as follows:
A temperature of -40°F. is also represented by -40°C., therefore to change a Fahrenheit reading into centigrade, we add 40 to the given reading, then divide by 1.8 after which subtract 40. To change from a centigrade to Fahrenheit reading the only difference in this method is to multiply by 1.8 or
C. = (F. + 40)/1.8 - 40 and F. = 1.8(C. + 40) - 40.