C. Barus — Counter-twisted Curl Aneroid. 



117 



may either be compressed between steel rollers, then neatly 

 coiled on a steel mandrel at the lathe with the turns one or two 

 millimeters apart, so that the finished tube has the general form 

 of a curl, and finally dissolved to the required thinness of wall 

 in the nitric acid bath ; or the tube may first be dissolved to the 

 required lightness, then flattened and coiled as stated. The 

 second of these methods is the best, though it requires a long 

 tubular bath with a cistern at the top to avoid spilling the acid 

 during the more or less violent effervescence in solution. The 

 ends of the tube are stopped with corks, covered over with a 

 cement of resin and beeswax, and all imperfect parts of the 

 tube are similarly protected. The progress of solution is tested 

 by taking weights from time to time, and when the tube is so 

 light as to begin to float, it is suitably held down. A spring 

 balance arrangement attached to the tube and out of the reach 

 of the acid fumes, obviates the need of repeatedly removing the 

 tube from the bath. Holes are sometimes eaten through the 

 walls at defective places, probably from galvanic action. Should 

 this occur, the tube must be cleaned at once, and cut apart at 

 the defective place. 



The first of the methods sketched above 

 is more convenient, as the curl requires a 

 much smaller bath. Unfortunately solution 

 is most active at the edges ; the tube be- 

 comes very fragile and is in danger of As- 

 suring on continued use. 



Tubes rolled down and coiled under a 

 tense pull are apt to be quite closed. The 

 way in which this can be remedied by coun- 

 ter-twisting will be described below (§ 8, et 

 seq.), where closed tubes may even be an 

 advantage. But in this place it is best to 

 stretch two fine wires of brass or copper (say 

 0*02 cm in diameter) from end to end of the 

 tube, so that complete closure is avoided. 



An interesting structural result was ob- 

 served in this work. Open helices, with 

 the turns about 0'3 cm apart, after solution 

 became closed helices with the turns all but touching at the 

 edges, showing that the layers of the metal were subjected to 

 unequal strains with a maximum of deformation at the out- 

 side.* 



As a rule tubes must first be cleansed internally (to remove 

 pulverulent or fatty matter) with ammonia, hydrochloric acid 

 and water, and then thoroughly dried by a current of warm 

 air. The finished curl is closed at one end, with the other 



* Compare this with the gradual loss of explosive tendency of a Prince Rupert 

 drop, when the external layers are successively etched off (Barus and Strouhal, 



Simple curled Bourdon 

 tube. 



