21 



II. Aneroid Barometers 



46. Figures 16 and 17 represent two of the more important types 

 of aneroid or holosteric barometers, showing principally the internal 

 mechanisms. The first is a more common form, but the second is, 

 in general, somewhat better. The essential feature is the same in 

 both instruments and consists of the small metallic box or cell, M, 

 the upper and lower walls of which are made of very thin circular 

 sheets of corrugated German silver, which are soldered together 

 on their outer edges, forming a very short cylinder. The air is 

 thoroughly exhausted from this cell through a tube at one side, 

 which, when the vacuum is as perfect as desired, is pinched tightly 

 together, cut off, and hermetically sealed with solder, producing the 

 projection seen at <?. The flexible corrugated surfaces, which tend to 

 be collapsed by the pressure of the outside air, are forcibly held 



Figure 16. — Aneroid barometer 



apart by the action of a strong steel spring R. As the pressure 

 of the air increases the spring is compressed and the corrugated 

 surfaces approach each other slightly, returning again or separat- 

 ing still farther with diminution of pressure. To measure the 

 changes in atmospheric pressure, it is only necessary to measure the 

 minute movements of this flexible cell. 



The two forms of aneroid figured herein differ simpl}^ in the 

 manner by which the minute alterations in the elastic yielding of 

 the spring are magnified and rendered measureable, 



4T. In the common aneroid a lever Z, attached directly to the 

 spring connects by a link m, with a very short arm of a sort of 

 bell-crank lever r, t^ having a horizontal axis on pivots at each end. 



The longer arm t of this bell-crank lever is connected by means of 

 a wire s with a very fine chain, the other end of which winds around 



