HEAT AND EXPANSION 103 



This rule may be stated as follows : ' 



To convert Fahrenheit to Centigrade subtract 32 from the 

 Fahrenheit degree and divide by 1 .8, or take j of it. 



To convert Centigrade to Fahrenheit multiply the Centigrade 

 degree by 1 .8, or 5, and add 32. 



EXAMPLE 1. Convert 212 F. to Centigrade reading. 

 5 (212 - 32) _ (180) _ 900 _ 

 9 99 



or 2*12 - 32 = 180 



180 + 1.8 = 100 C. 



EXAMPLE 2. Convert 100 C. to Fahrenheit reading. 



9 (100) 900 



- + 32 = - - + 32 = 180 + 32 = 212 F. 

 5 5 



or 100 X 1.8 = 180 



180 + 32 = 212 F. 



104. Heat Units. The unit of heat that is used in the 

 industries and shops of England and America is the British 

 thermal unit (B. T. U.) It is the quantity of heat required to 

 raise 1 Ib. of water to a temperature of 1 F. Therefore, the 

 heat required to raise 5 Ibs. of water through 15 F. is 



5 X 15 = 75 B.T.U. 

 Similarly 72 Ibs. of water require, to raise its temperature 



72 X 1 A = 36 B.T.U. 



The unit that is used on the Continent and in scientific 

 circles in America is the metric system unit called a calorie. 



A calorie is the amount of heat necessary to raise 1 g. of 

 water 1 C, 



