12 



Mr. Carl Barns on the 



and also to prevent the saponification of sperm-oil and to 

 eliminate condensable air, when the tube is in place. Mer- 

 cury and the fluid amalgam are to be introduced by aid of 

 the air-pump, care being taken to boil out all traces of air. 

 The mercury of the inner compartment is in galvanic con- 

 nexion with the platinum wire b, which in its turn connects 

 with the piezometer-tube. Hence b need not be insulated. 

 The amalgam is pierced by the terminal c, completely insu- 

 lated from the piezometer by the unbroken length of cold glass 

 tube surrounding the wire. To prevent the wire from being- 

 pulled out of place during the adjustments, c is sealed in the 

 inside of the insulator-tube as shown in the figure. This 

 terminal is in galvanic connexion with the barrel. It is seen 

 that the tube is virtually a galvanic cell, NaHg/glass/Hg, and 

 is reversible. 



Under the influence of hydrostatic pressure, the figure of 

 the glass tube is symmetrically reduced in size. Hence, with 

 regard to the conduction taking place across the walls of the 

 inner tube, the resistance would be increased, in consequence 

 of changes of dimensions, only by the amount in which the 

 length of the inner tube is diminished. In other words, if 

 r = s//27rHL, where s, I, R, L denote respectively specific re- 

 sistance, thickness, radius, and length of the thin tube, then 

 the dimensional effect of pressure is Sr = (s7/27rRL) x SL/L, 

 nearly. But all such values (8L/L being less than *1 per 

 cent, per 1000 atm.) are negligible in comparison with the 

 relatively large pressure-coefficients actually obtained (cf. 

 Tables I. to VIII.), and to be interpreted with reference to 

 s—s. 



Fig. 2. — Diagram of Tube in place. 



Fig. 2 (diagram) shows the resistance-tube in place. PP 

 is the tubular steel piezometer, surrounded by the vapour- 

 bath V V, and the cold-water jackets D D and D' D', and 

 filled with the oil, 0, by which the pressure is transmitted. 

 The inner compartment of the glass tube is at a a (mercury), 



