C. Barus — Counter-twisted Curl Aneroid. 



123 



Q-a 



(3.) If the adjusted curl aneroid, exhausted and therefore 

 twisted by external pressure, be untwisted to the same degree 

 by the spring, the curl is now nearly without strain, excepting 

 such of higher orders ; and the viscosity and thermal relations 

 of viscosity and rigidity of the counter-twisting spring alone 

 come into play. Hence a metal of low rigidity may be chosen 

 for making the curl, while the counter-twisting spring is 

 selected for high viscosity. One would use, for instance, hard 

 steel annealed at 400° or even a sufficiently thick quartz fiber. 



It is clear that the counter-spring must 

 be weak, so that it may make a number of 

 rotations for each rotation of the curl ; for 

 the resistance of the counter-spring is incre- 

 mented at the expense of the deflection of 

 the curl. Hence a long liber, or spiral 

 spring, or a watch spring is adapted. 

 Finally, to prevent knotting of the spring, 

 a weight is {temporarily) added to the sys- 

 tem, preferably suspended in a basin of 

 water to deaden vibrations. The curl ane- 

 roid has now taken the form of figure 2, 

 where ce is the curl, communicating at one 

 end, Jc, with the air pump. The other end 

 is closed and carries the horizontal, radial 

 stylus y, to which the stiff wire dd of the 

 spring bb, the mirror e and the damping 

 plate g are attached symmetrically to the 

 curl. The plate g is of lead, thus serving 

 as a weight (as well as a damper) to pull out 

 the spring bb. A dish h (shown with g in 

 cross section) containing a liquid, surrounds 

 g. The spring b is attached above to a 

 rigid clamp at a, by means of a stiff wire Z, 

 the clamp (not shown) being so constructed 

 that the spring can be both raised and 

 twisted. 



The weight g and the damping arrange- 

 ment are only permissible in calibration 

 work like the present. If the registry of 

 the aneroid is to be instantaneous, the mass 

 must be kept down to the lowest limit pos- 

 sible, and no ballast is to be attached. 

 Hence a delicate watch-spring, or a double Counter-twisted system 

 helical spring is preferably employed, and of curled Bourdon tube 

 a suitable method of damping vibrations 

 must be investigated. An elastic fiber fas- 

 tened at both ends, with the end of the helix attached near 

 the middle, is also contemplated. 



spring 

 nances. 



and appurte- 



