EXERCISE 7 

 TO STUDY THE CONSTRUCTION AND OPERATION OF A MERCURIAL BAROMETER 



Materials. Glass tube about one-quarter inch in diameter and 32 or 33 inches in length; a 

 pound of mercury ; a dish 2 to 3 inches in diameter. 



Directions. The teacher may construct a barometer before the class as follows : Cut a glass tube 

 about 32 or 33 inches long. One end of the tube should be closed, which may be done by heating 

 until it melts. Fill the tube with mercury. Closing the tube with a finger, invert it in a cup of 

 mercury. When the finger is removed, measure the height of the mercury in the tube above the 

 level of the mercury in the cup. What supports the mercury ? How could this apparatus be used 

 to measure the weight of the air in different places and at different times ? 



Barometers constructed hurriedly as above are not accurate because some air bubbles adhere to 

 the glass and finally get into the space above the mercury. In a well-constructed barometer this air 

 has been driven out. Such a one, with graduation showing the height of the column in inches or 

 centimeters, should be on the wall. It should be examined from day to day and a record kept. 



Additional problems. In what ways and for what reasons will the column of mercury change when the 

 barometer is taken high into the air or down into a deep mine ? 



If the column of mercury in a barometer is 30 inches high at sea level, how high will the column be 

 at an elevation which has half as great air pressure as at sea level ? 



Incline the barometer tube to one side. Then measure the length of the mercury column, and also the 

 vertical height of the upper end of the mercury above a point at the same level as that of the mercury in 

 the dish. Explain the results. 



[8] 



