MEASUREMENT OF TEMPERATURE 31 



Remember always to make all computations from 

 the same starting point on both thermometers. 



Range of mercury thermometer. For all ordinary 

 temperature measurements the mercury thermometer 

 is most satisfactory. However, mercury freezes at 

 39C. and boils at 360C. Alcohol thermometers are 

 serviceable for temperatures lower than the freezing 

 point of mercury, since the freezing point of alcohol 

 is 130 C. For both very high and very low tem- 

 peratures the gas thermometer is the standard. 



Unit of heat measure. To measure anything we must 

 have some measuring unit. For example, to measure 

 length, we have the foot or the yard as a unit. So to 

 measure the amount of heat a substance contains it is 

 necessary to have a unit of heat measure. The unit here 

 accepted is the heat required to raise the temperature of 

 one gram of water through one degree centigrade. This 

 unit is called the (gram) calorie. Thus, if 100 grams of 

 water has its temperature raised 5C. we say 500 calories 

 of heat have passed into the water ; or on the other hand, 

 if 100 grams of water drops 5C. in temperature, we say 

 500 calories of heat have passed out of the water. To 

 determine the number of calories of heat that have passed 

 into a given amount of water, multiply the number of 

 grams of water by the increase in degrees centigrade. 

 The result will be in calories. 



