122 C Barus — Counter- twisted Curl Aneroid. 



The small residue of viscosity left here is very probably not 

 even yet a true phenomenon, i. e. it is due to strictures which 

 prevent the free passage of air, and to friction between the con- 

 tiguous walls of the tube and wire ; but the improvement over 

 the preceding cases is obvious. 



8. — To enter into this question somewhat more fully and 

 from a different point of view, a wider tube was selected (the 

 walls of which were but 0*12 cm thick), flattened and coiled 

 without closing the section. Indeed a blunt edge was left and 

 the section was about 0-06 cm broad and 1 '6 cm high. The data 

 for this coil (IV) were as follows : 



Diam. of curl, 3-2 cm Pressure, 75'9 cm , Hg. Deflection, 11-1° 



Turns of curl, 4-5 69*5 10-3° 



Length of curl, 14-0 cm 57'8 8'8° 



41-3 6-6° 



36-8 5-9° 



0-0 zero. 0-0° 

 Pressure per degree, per turn, 32 - 5 cm . 

 Pressure per degree, per linear cm., 330 cm . 



In this curl there is no evidence of viscosity, but the relation 

 of pressure-difference and deflection is not quite linear, as was 

 the case with sharp-edged coils. The data are mean ratios. 

 The sensitiveness (330 cm , Hg, per degree per linear cm.) is of 

 low order, in spite of the thin walls (0 012 cm ) and broad tube. 

 §2. _ 



This curl was now replaced on the metallic mandrel, and the 

 edges hammered quite sharp from end to end. On removing 

 the curl from the mandrel I found that no air could be sucked 

 through it. The walls, therefore, overlapped each other, im- 

 perviously to air. When, however, the curl was somewhat 

 uncoiled in the hands, the air came through quite freely. This 

 suggested a novel method of making the curl aneroid, requiring 

 no inclosed wires, and partaking of other advantages, since the 

 uncoiling can be done with a suitable spring. In a counter- 

 twisted system of this kind : 



(1.) The sharp- edged coils can be opened by an amount com- 

 patible with the free access of air. Therefore this system is 

 adapted for extreme sensitiveness. 



(2.) The system is differential ; or, in other words, the differ- 

 ences of viscosity and of elasticity of curl and spring, and the 

 differences of the thermal variations of these quantities come 

 into play. Thus if a spring and curl could be made having 

 the same effective viscosity* and the same thermal coefficients 

 of viscosity and elasticity, respectively, the system would be 

 perfectly elastic and independent of temperature / or, 



* Depending therefore both on the material and on the lengths of the two 

 helices. 



