C. Barus — Counter-twisted Curl Aneroid. 121 



Prom this it follows that the viscosity in question must be in 

 large measure apparent but not real. While Y b was preserved 

 for other purposes, Y a was now further etched off. In so 

 doing some of the turns had to be sacrificed, being eaten 

 through. The results in these cases were more irregular, and 

 apparent viscosity more marked. I will not therefore give 

 them in full. After the first solution, 9 turns of Y a showed 

 13 cm pressure per degree per turn, or 107 cm Hg per degree 

 per linear cm. After the second solution Y a showed 8'7 cm 

 pressure per degree per turn, or 77 cm pressure per degree per 

 linear cm. It was imposible to carry the solution further, 

 because the edges were too thin to withstand flexure without 

 Assuring ; but the walls were 0*01 cm in thickness, and might 

 easily have been reduced, cf. § 3. 



Summary. — Remembering, therefore, that if viscosity were 

 excluded, the curl would have been more sensitive, I may state 

 that it takes from 50 cm to 75 cm of flat copper tubing, the walls 

 being 0*01 cm thick, to make a curl aneroid such that a degree of 

 arc shall correspond to l cm of the barometer. If, therefore, the 

 reading be made with mirror and telescope, with the scale at a 

 distance of 286 cm from the mirror, 10 linear cm. of the scale cor- 

 respond to l cm of the barometer. If the tube be wound on a 

 mandrel a little over 2 8 cm in diameter, 7 or 8 turns would suf- 

 fice, and the length of the curl need therefore not exceed 8 cm . 

 Hence, 10 such curls joined in series end to end and suitably 

 supported, would show 100 cm at the telescopic scale per cm. of 

 the barometer, and the curl would be less than a meter long. 

 §12. 



Counter-twisted Systems. 

 7. — In the results thus far, the object has been merely to 

 exhibit the possibilities of the curl aneroid. If the apparatus 

 is to attain precision, the apparent viscosity must be brought 

 quite under control, and the effect of the temperature of the 

 medium evaluated and reduced to the smallest margin. I have 

 in a measure fulfilled both these conditions by using counter- 

 twisted systems in the way presently to be specified. As a first 

 step in this direction, I will cite some data obtained with curl 

 YIII, in which two fine brass wires (diameter 0*02 cm ) were 

 drawn through the tube before flattening and coiling. The 

 walls were # 013 cm thick, and all but 5*2 turns were lost during 

 solution. 



Diam. of 



curl, 



2-8 cm 



Pressure, 73*7 cm , 



Hg. 



Deflection, 



38-3° 



Turns, 





5-2 





63-2 







33-4° 



Length of curl, 



3.3cm 





48-5 







26-0° 











o-o 







0-6° 











00 







o-o° 





Pressure per 



degree, 



per turn, 



10-4 cm . 







Pressure per 



degree, 



per linear cm., 



91 cm . 





