HIGH PRESSURE BRIDGMAN". 



203 



In regard to the details of the results, for which the reader should 

 consult the original paper, Doctor Bridgman says : 



To sum up : Different metals show minor irregularities in behavior, but they 

 are alike in several general features which must be the first task of any theory 

 to explain. These general features are the approximate constancy of pressure 

 coefficient with temperature, and the accompanying constancy of temperature 

 coefficient with pressure ; contrasted with this the pronounced decrease of in- 

 stantaneous pressure coefficient with rising pressure. It has been obvious 

 enough that the data have presented no spectacular features, and I must con- 

 fess to a sense of disappointment that an extension of the pressure range to 

 at least fourfold that of previous measurements has brought out no striking 

 new facts to reward the extra effort. When the magnitude of the change of 

 volume produced by a pressure of 12,000 kilograms is considered, however, 

 it does seem that the results acquire a physical significance great enough to 

 justify the extension of the range. The volume of many of the metals at 0° C. 

 and 12,000 kilograms is less than the volume at atmosphere pressure and 0° 

 Abs. The resistance of most metals tends toward zero at s Abs., but under 

 great presstire at 0° C, at the same volume as at the absolute zero, the resistance 

 is only a few per cent less than under normal conditions. Any valid theory 

 must explain the surprisingly little effect of the element of volume alone apart 

 from the element of temperature. It is furthermore known that at very low 

 temperatures the connection between resistance and temperature changes its 

 character ; the relation ceases to be linear, and the resistance curve approaches 

 the origin tangentially to the temperature axis. Whether the abrupt dis- 

 continuity shown by several metals a few degrees above 0° Abs. is an effect 

 of a polymorphic transition does not yet seem to be settled. It is significant 

 that no trace of any such effect is to be found at room temperature as the 

 volume is decreased toward and beyond its value at 0° Abs. The question 

 whether there is a change in the character of the resistance curves as the 

 volume approaches that at 0° Abs. could not, of course, have been answered by 

 measurements over a small pressure range ; it is perhaps some justification of 

 the extension of range that this question can now be answered. > 



An estimation as to the comparative volumes at (12,000 kg., 0° C.) and 

 (0 kg., 0° Abs.) is given in the accompanying table. The values of compressi- 

 bility used in the computations have been taken from Richards, assuming con- 

 stancy over the pressure range, and the volume at 0° Abs. has been taken from 

 the data of Ch. Lindemann on linear expansion to 20° Abs. 



Comparison of changes of volume produced by temperature and pressure. 



In general, the data are given for. the change of resistance of 22 metals be- 

 tween 0° and 100° C. over a pressure range from to 12,000 kilograms. Three 

 of the metals are abnormal ; bismuth and antimony both have a positive pressure 



