22 



a small wheel or drum on the axis ff, upon which is mounted the hand 

 as seen. At h is shown a small spiral steel spring, like the hair- 

 spring of a watch, which serves to take up the slack in the loose con- 

 nections of the numerous joints, levers, and links. 



At r is shown, also, a small counterpoise weight attached to the 

 bell-crank lever to aid in securing a more stable position of the index 

 when the barometer is placed in different positions ; that is, whether 

 the dial is horizontal, or vertical, or turned to one side or the other. 

 The point of attachment of the link m to the bell-crank lever is 

 sometimes adjustable so that the movements of the hand can be made 



to correspond to the value 

 of the scale graduations. 

 The steel spring R is 

 also slightly adjustable by 

 means of a screw from the 

 underside threaded into 

 the part N. This permits 

 adjusting the hand to any 

 particular point of the 

 scale to give correct read- 

 ings. 



48. Effects of temyeror- 

 ture. — The steel spring 

 and the feebler elastic re- 

 action of the composition 

 metal of the vacuum 

 chamber are appreciably 

 weakened by increase of 

 temperature, so that in 

 some cases a rise of the 

 pressure may seem to oc- 

 3ur which is really caused 

 by the weakening of- the 

 spring. In some cases ef- 

 forts are made to com- 

 pensate for this by leav- 

 ing a small quantity of 

 air in the vacuum cham- 

 ber, which when heated 

 increases its pressure upward and tends to offset the weakening effect 

 upon the springs. A better plan is to make the lever I of two differ- 

 ent metals, viz, brass and iron, firmly brazed together. The differ- 

 ential expansion of these two metals with temperature changes pro- 

 duces flexure in the lever. By filing and adjusting the bimetallic 

 bar, the flexure due to temperature can be made very nearly to 

 balance the effect of temperature on the spring. The aneroid is then 

 said to be " compensated " and this word is often found on the dial. 

 In many cases this word is there when the compensation is very 

 imperfect. 



49. Defects. — The friction and looseness in the joints of the links 

 and the lack of perfect balance in the various parts give rise to con- 

 tinually changing errors in the reading of the aneroids. This will 

 be shown by tapping the aneroid from difference sides and holding 



Figure 17. — Goldschmidt's aneroid 



