MEASURING ATMOSPHERIC PRESSURE 



53 



E. Adjusting screw which raises bridge C either up or down. The head 

 of this screw is seen in the back of all aneroids. 



F. Steel spring which slides in back of bridge C. 



G. Knife-edge (triangular or square steel rod). This passes through the 

 stud of the vacuum chamber and tends to open it by pulling strongly upwards." 



At this point it is interesting to note that the mechanism is already sen- 

 sitive to changes in atmospheric pressure. As the vacuum chamber is similar 



FIG. 45a. Before exhausting air. 



FIG. 456. After exhausting air. 



to a small circular metal box (closely resembling two lids of a tin can soldered 

 together at their edges) it will, when exhausted of air, collapse. If we pull 

 it from the bottom and also from the top, we pull it open, but when we let go 

 it collapses again. 



As the under side is secured to the base plate A and the upper side is 

 secured to the strong spring F, the action is the same as the illustration just 

 given with the two tin can lids, viz., the strong spring "opens" the vacuum 

 chamber and holds it open. 



ChaJn arbor 



Chain 



Floating link 



Long arm of bell crank 



Mainspring' 

 Coupling rod 

 Vacuum box 



\ 



' Regulator shaft 



FIG. 46 Interior movements of an Aneroid Barometer. 



