The Fahrenheit thermometer was made on the assumption that a mixture of ice and salt was the coldest temperature that could be reached, so this temperature of a certain proportion of ice and salt was marked zero.
Fig. 161. Comparison
of Centigrade
and Fahrenheit.
The hundred point was given to what was supposed to be the normal body temperature. The intervening spaces were marked into equal divisions, and these divisions were carried below 0 degree and above 100 degrees. The boiling temperature of water came at 212 degrees Fahrenheit, and the freezing point at 32 degrees. This makes 180 degrees difference between thawing and freezing and boiling. So 100 degrees Centigrade equal 180 degrees Fahrenheit. Therefore, 1 degree Centigrade equals 9/5 degrees Fahrenheit, and 1 degree Fahrenheit equals 5/9 degree Centigrade.
For example, if 40 degrees Centigrade is to be translated into Fahrenheit degrees, first multiply 40 by 9 = 360, then divide by 5 = 72, and add 32, because 0 degree Centigrade is the same as 32 degrees Fahrenheit, and the result is 104 degrees Fahrenheit equal 40 degrees Centigrade. If 41 degrees Fahrenheit is to be translated into Centigrade degrees, first subtract 32 from 41 = 9, then multiply by 5 = 45, and divide by 9, and the result is 5 degrees Centigrade = 41 degrees Fahrenheit. Fig. 161 is a diagram showing relative readings of Fahrenheit and Centigrade thermometers.
300. Oven Thermometer. Some oven thermometers depend on the expansion of metal to indicate the temperature. A hand on the clock-like face of these indicators shows the degree of heat. Few of these give the actual temperature, but they do indicate a slow, a moderate and a hot oven.
301. Maximum Thermometers. A maximum thermometer is one in which the mercury rises to register the maximum amount of heat to which it has been subjected. It stays at this height when the temperature falls, until it is shaken back.
It is sometimes used in ovens to ascertain the temperature they have reached before the oven door is opened.