224 PROCEEDINGS OF THE AMERICAN ACADEMY. 



4 per cent, while in the loop zero-D it is 40 per cent, an increase of 

 tenfold for a doubling of the pressure range. The return path D-zero 

 was not described at the same time as the part zero-D, because an 

 explosion occurred when the maximum D was reached. It is, how- 

 ever, the return path described on another occasion when the initial 

 path zero-D was identical with the above. 



Other types of gauge have shown the same characteristics at high 

 pressures. Whatever the true explanation may be, it has been found 

 in every case that an elastic deformation gauge does show behavior 

 like the above. This type of gauge appears, then, to be unsuitable for 

 the accurate measurement of high pressures, and must be replaced 

 by some form not showing hysteresis; for even if this gauge were 

 readily reproducible, the fact that it shows hysteresis would make its 

 indications such a complicated function of pressure, both present and 

 past, that the meaning of the indications could not be conveniently 

 deciphered. 



Any scalar physical property when changed by a strain the same in 

 every direction, such as is produced by hydrostatic pressure in a per- 

 fectly homogeneous solid, or a liquid, may be expected to show no 

 hysteresis relative to the stress. Such a property, which has the ad- 

 vantage of being easily measured, is electrical resistance. This has 

 been proposed at least twice as a pressure indicator. 



Lisell 1 measured the resistance of a number of metals, drawn out 

 into wires, when subjected to hydrostatic pressures up to 3000 kgm. 

 Pressure was measured on an absolute gauge in which the pressure on 

 the freely moving piston was balanced by weights on a lever. Lisell 

 found no evidence of hysteresis, and proposed the measurement of 

 electrical resistance as a satisfactory means of measuring pressure. 

 The variation of resistance of metallic wires, however, was found by 

 Lisell to have the fatal disadvantage for the present purpose of being 

 so greatly influenced by slight impurities in the metal that specimens 

 of the same metal from different sources gave very different results. 

 This gauge, then, would not be reproducible, but each new specimen 

 of wire would have to be calibrated individually against some abso- 

 lute standard. In addition, the pressure coefficient is inconveniently 

 small, so that great care must be taken to avoid other effects in measur- 

 ing the slight change of resistance brought about by pressure. Lisell 

 claims as an advantage of this method that the heat of compression 

 of the metallic wires is smaller than for most substances. 



1 Lisell, Om Tryckets Inflytande pa, det Elektriska Ledningsmotstandet 

 hos Metaller, samt en ny Metod att Mata Hoga Tryck. Upsala, 1903. (C. J. 

 Lundstrom.) 



