C. Barns — Counter-twisted Curl Aneroid. 119 



Table 1. — Behavior of Curls of different diameters, for the same 

 length of tube coiled to a helix. 



Description of curl.* 



Diam. of carl 3-l cm 

 Turns of curl 21 

 Length of curl 18 cm 



Mercury 

 Mano- 

 meter. 



Inflection 



of 



curl. 



Cm. of Hg 



per degree 



of arc. 



Cm. of Hg 



per decree 



per turn. 



Cm. of Hg 



per degree 



per cm. 

 of diameter. 



cm 



°arc 









76'7 



18-7 



4-07 



85 



1-3 



67'9 



16-6 



4-07 







61-5 



15-0 



4-08 







54-3 



]3-2 









41-5 



10-1 









32-1 



7'8 









76-3 



21-1 



3-55 



87 



1-5 



70-5 



19-5 



355 







65-3 



18-0 



3-57 







59'8 



16-4 



3-63 







52-5 



14-4 









47-4 



13-0 









41-4 



11-3 









29-3 



8-0 









60-3 



17-4 



3-45 



121 



1-7 



52-0 



14-9 



3-49 







31-0 



8 9 









00 



o-o 







- 



Diam., 2-8 cra 



Turns, 24 



Length of curl 19'5 C 



Diam., 2-0 cm 

 Turns, 35 

 Length of curl 22' 



Practical!}-, in estimating the effect produced on thinning 

 the walls by etching, I may therefore either express the num- 

 ber of cm. of mercury which correspond to a degree of arc per 

 turn of the tube, in which case the same diameter should occur 

 throughout or be specified ; or I may make similar reference 

 to the diameter ; or finally (and probably best), I may state 

 the centimeters of mercury per degree of deflection, of the 

 coil per unit (cm.) of length of the tube used in winding the 

 curl. Lumping the results of Table 1, where the length of 

 tube used was about 210 cm throughout, it appears that in these 

 initial experiments a pressure of about 780 c,n of mercury would 

 produce 1° of deflection between the ends (tangents) of a tube 

 l cm long, and having the size and thickness specified. 



The endeavor was now made to dissolve down this curl, but 

 the action was carried too far and it was eventually lost in the 

 process. After about i of the weight had been etched off 

 however, 61 cm of mercury corresponded to a degree of twist 

 per turn for the diameter of curl 2*9 cm , or 550 cm of mercury 

 per linear cm. per degree, showing a decided gain over the 

 general dataf of Table 1. 



5. Another curl, ~No. Y, was now wound, etched off and 

 tested with the following results, after having been exhausted 

 for some time. The original weight was 39 s and after etching 



* The length of the curl and the length of the tube used in winding the curl 

 must be carefully distinguished. 



f A slight digression was made in coiling the next tube like a watchspring ; 

 but the experiment showed no special advantages. 



