THE DEW POINT 79 



e. Examine the fuse box in your house, and tell how it is 

 constructed. Find out how you could cut off the current 

 entirely if you wished to. 



EXERCISE 75 

 HOW HEATING THE AIR CHANGES ITS DENSITY 



Apparatus and Materials. Balances, flask holding about 100 to 

 200 cu. cm., string, weights or shot, Bunsen burner. 



a. By means of a string having a loop at one end suspend a 

 clean, dry flask from one arm of a balance. Into the pan 

 attached to the other arm put weights or shot, enough to bal- 

 ance the flask. Now remove the flask and heat it by turning 

 it rapidly in a Bunsen flame; then put it back in its place on the 

 balance. What is the result? 



Let the flask hang until it becomes cool ; what happens? What 

 evidence have you that hot air is lighter, that is, has a lower 

 density, than cool air? 



EXERCISE 76 

 THE DEW POINT 



Apparatus and Materials. Metal beaker or bright metal (aluminum 

 or tin) cup, thermometer, large bottle with small mouth, bits of ice. 



a. Use a simple dew-point apparatus, such as is shown in 

 Fig. 217, 267, of the text. The metal beaker may be a bright 

 aluminum or tin cup. Have the cup % full of water at the room 

 temperature; then add bits of ice or snow, stirring vigorously 

 with the thermometer until the first drops of dew are deposited 

 on the cup. Now get the thermometer reading at once, and 

 record it. To repeat the trial take out any ice that remains, 

 and let the water become just warm enough to permit the dew 

 to evaporate. Then add another bit of ice, and stir with the 



