178 BRIDGMAN. 



of compressibility between the specimen and pure iron, independent 

 of any imperfections in the apparatus, which were thereby elimi- 

 nated. Except for the convenience of using small corrections, it would 

 have been quite possible to have made the envelope of some other 

 metal, such as brass. 



Besides the empirically determined correction just discussed a 

 number of other corrections have to be applied for the change of re- 

 sistance of the nichrome wire under pressure, for the difference of 

 compressibility between nichrome and iron, and for the temperature 

 effects. These corrections are essentially the same as those needed in 

 getting the absolute linear compressibility of iron, and will not be 

 separately discussed; they are small and involve no difficulty. The 

 data requisite for making these corrections were determined by direct 

 experiment. For those substances whose compressibility is near to 

 that of iron the percentage magnitude of the correction on the differ- 

 ence of compressibility may of course be greater than in the experi- 

 ment on the absolute linear compressibility of iron. The percentage 

 magnitude of the corrections is approximately inversely proportional 

 to the difference of compressibility. 



The total motion of the wire was in all cases small, being seldom 

 more than a small fraction of a millimeter. It will of course be under- 

 stood that in dealing with such small quantities every feature of the 

 design had to be carefully thought out, and that the apparatus as 

 finally used embodied many details whose best form was found only 

 after several trials, but which it will not be profitable to describe here. 

 Many changes were made in several different pieces of apparatus, and 

 readings were made continuously over a period of four months before 

 the design used for the final measurements here recorded was attained. 

 By far the most trouble was found with the lever apparatus. It is 

 sufficient to mention here that the feature requiring the greatest care 

 is the construction of the bearing points and seats of the lever. These 

 must be of the greatest geometrical perfection, and highly polished, 

 and will repay a considerable amount of labor. The radius of the 

 bearing point must not be too small or the point will break, hence the 

 necessity for a high polish. The result of failure to get proper bearing 

 points is irregularity in the readings; the observations skip erratically 

 about, or often show systematic departures from the correct course for 

 considerable successions of readings. 



The pressure was determined, as in all this work, from the changes 

 in the resistance of a coil of manganin wire, and the corrections have 

 been discussed in detail previously. 7 During the course of the work 



