THE MEASUREMENT OF TEMPERATURE 49 



absolute zero. Temperatures of absolute zero have never been 

 reached on the earth, but from the fact that all gases contract 

 one two-hundred-and-seventy-third of their volume at 0C. 

 for each Centigrade degree of further cooling, it is inferred 

 that absolute zero must be 273 degrees below the zero of the 

 Centigrade scale, or as we commonly express it 273C. A 

 third temperature scale called the absolute scale begins at the 

 absolute zero and has degrees of the same size as those in the 

 Centigrade scale. On this scale, therefore, the freezing point 

 of water is 273 above zero. 



49. Changing from Fahrenheit to Centigrade. With two 

 thermometer scales in common use, it frequently becomes 



FIG. 16. Thermostat. 



necessary to change temperatures from one scale to the other. 

 This is easily done when the relative size of the degrees is 

 considered. From the fact that there are 180 degrees between 

 freezing and boiling on the Fahrenheit scale and only 100 on 

 the Centigrade, we perceive that the ratio of the sizes of the 

 two degrees must be as 180 is to 100; that is, the Fahrenheit 

 degree is rive-ninths of a Centigrade degree. Therefore, to 

 change Fahrenheit degrees to Centigrade degrees, we multi- 

 ply by %, and to change Centigrade to Fahrenheit we multi- 

 ply by %. This, however, is done only when comparing the 

 degrees. In matters of temperature, we must make allowance 

 for the difference in the zeros of the two scales. Thus, when 



