118 C. Barus — Counter-twisted Curl Aneroid. 



soldered to a suitable terminal tube to connect it with the air 

 pump. The accompanying figure 1 shows the curl ready for 

 experiment ; a is the terminal tube leading to the air pump, 

 bo the curl aneroid, c a mirror or other form of index, to regis- 

 ter the rotation of the lower end of the curl relatively to the 

 upper, when the air within is being exhausted. For conven- 

 ience in drawing only a few turns of tube are shown in the 

 figure. 



4. Mesults. — In the first experiments the object sought was 

 some guidance as to the effect of varying the diameter of the 

 curls. Copper tube, with walls 0'025 cm thick, was flattened 

 till the spindle-shaped section was about 0'5 cm high and 0'08 c,n 

 wide, then wound so as to make an open helical spring. The 

 diameter of the curl was successively reduced by winding on 

 smaller mandrels, and the corresponding sensitiveness was 

 found by comparing the curl with a mercury gauge when both 

 were joined to the receiver of a Sprengel pump. The index 

 at the bottom end of the centered curl moved over a graduated 

 circle about a foot in diameter and divided in quarter degrees. 



Table 1 contains an example of the results. The curl lengths 

 are approximate, due to unavoidable irregularities of winding. 

 In addition to the direct readings I will give the pressure 

 difference in centimeters of mercury per degree of deflection 

 (i. e. per degree of deviation between the terminal tangents of 

 the helix), and the same datum taken per turn of the curl and 

 per centimeter of the length measured along the turns of the 

 helix. In general pressure denotes the difference of pressure 

 between the outside and the inside of the tube and is always 

 given in centimeters of mercury. 



It appears therefore, that within the range of accuracy of 

 these experiments, the untwisting of the curl in degrees of arc 

 is very nearly proportional to the pressure difference in cm. of 

 mercury, remembering that the pressure on the outside of the 

 tube acts on the cistern of the barometer. In the second place, 

 the sensitiveness of the coil for a given length of tube is some- 

 what greater for small radii than for large radii ; thus for the 

 diameter 3'l cm , a degree of arc corresponds to 4'07 cm of mer- 

 cury, whereas for the diameter 2*0 cm , a degree corresponds to 

 3.5cm Q f m ercury. The changes, however, are small and irregu- 

 lar and possibly largely influenced by temperature, which was 

 not taken. At all events, the data for cm. of mercury per 

 degree of deflection, per turns of curl, increases so rapidly, that 

 in view of the objectionably great length of curls of small 

 diameter, they offer no advantage. As a rule the diameter 

 2'S cm will be adhered to in the present paper. 



this Journal, xxxii, p. 182, 1886); and with another result (this Journal, xxxiv, 

 p. L83, 1887), in which the limit of torsional resilience of soft iron was reached 

 whenever the obliquity of the external fiber of the wire exceeded about 0*003 

 radians. 



