BRIDGMAN. — A SECONDARY MERCURY RESISTANCE GAUGE. 223 



The Bourdon gauge used consisted of hard drawn Shelby steel 

 tubing T 5 ^ in. (0.79 cm.) outside diameter and ^ in. (0.159 cm.) inside 

 diameter, wound into a helix of five turns of 5 in. (12.7 cm.) diameter. 

 The tube was not flattened into an elliptic cross section, as in the 

 ordinary Bourdon gauge, since to do this would have too greatly 

 decreased the strength. Even when the cross section is left round, 

 however, the tube unwinds upon the application of pressure, like the 

 ordinary Bourdon. The amount of unwinding was read directly by 

 observing the position of the free end with a microscope, a method of 

 reading which proved more satisfactory than any multiplying mechan- 

 ism. Thus gauge had been in use for upward of six months before 

 the readings shown in Figure 1 were made. The gauge had been so 

 thoroughly seasoned by the many applications of pressure in this in- 

 terval that the deflections on many subsequent occasions were found 

 to agree within the errors of reading. Initially, the gauge showed 

 some slight set under the maximum pressure, but after the first few 

 applications of pressure no further set appeared. Elastic after effects, 

 which might be expected to be troublesome over this wide pressure 

 range, could be noticed at every stage of the pressure variations, but 

 were too small to appear on the diagram. 



In Figure 1 the deflection of the free end (mm.) is plotted against 

 pressure in kgm., which was measured with a mercury resistance that 

 had been calibrated against an absolute standard, as will be described 

 later. The figure shows the effect of applying four cycles of pressure, 

 from zero by steps to the maximum and by steps back to zero, each 

 subsequent maximum being higher than the preceding. Pressure 

 was first applied in steps from zero to A, and then reduced to zero. 

 The return path coincides so closely with the initial path that the 

 difference cannot be shown on the diagram. Pressure was now in- 

 creased from zero to B and decreased to zero. The first part of the 

 path zero-B coincides exactly with the path from zero to A. The 

 return path B-zero is sensibly linear, but does not coincide with the 

 path zero-B. We have here, then, the beginning of departure from 

 linearity, and also the beginning of hysteresis. Two more loops, 

 zero-C-zero, and zero-D-zero, reaching to higher pressures, were now 

 described. The essential characteristics are the same, but departure 

 from linearity and hysteresis both increase rapidly with the rise of the 

 range. The return paths for these longer loops do not continue linear, 

 as for zero-B-zero, but they both start as straight lines and run for 

 about the same distance before beginning to curve down to meet the 

 origin. The increasing importance of hysteresis is shown by the fact 

 that the greatest error introduced by hysteresis in the loop zero-B is 



