-136 A. L. Day and J. K. Clement — Gas Thermometer. 



cylinder and will therefore show the line in a somewhat differ- 

 ent apparent position from that which would be produced by 

 the expansion alone. The drawing is purposely exaggerated 

 to show exactly the character of this optical error. It was our 

 habit in beginning these observations to select three appropri- 

 ate lines upon each end of the bar, and to make all the meas- 

 urements on these, whereupon it was found by a careful exam 

 mation of the results that the displacement of the three lines 

 after expansion differed systematically by a measurable amount 

 and in a manner which could not be accounted for by the 

 movement of the bar. This difference was very puzzling for 

 a long time, but was finally traced to the source described, and 

 this inference verified by actually moving the bar about in 

 the field in various ways without changing the temperature. 

 The consequence of this discovery was to compel the rejection 

 of all measurements made upon lines other than those imme- 

 diately adjacent to the fixed cross-hair in the center of the field. 

 The number of observations at each end was therefore reduced 

 to two, but the agreement of the results was very considerably 

 increased thereby. 



The third and most serious difficulty of all amounts to an 

 essential limitation of the material itself and is therefore not 

 dependent upon the method of measurement. It is the failure 

 of the bar to return to its initial length after heating. 



In this particular bar, 25 cm in length, we actually found dif- 

 ferences between the lengths before and after heating of 

 the order of magnitude of '02 mm , which varied from one series 

 of experiments to another according as the bar happened to 

 be cooled rapidly or slowly. This quantity is some fifty times 

 larger than the smallest magnitude we could measure, and 

 inasmuch as it depends only upon measurements at the tem- 

 perature of the room, is readily accessible. It is of course 

 immediately obvious that this constitutes a limitation upon 

 the accuracy of gas-thermometric measurements in a bulb of 

 this material, but in this very particular the behavior of pl'atin- 

 iridium is enormously more favorable than that of any of the 

 other materials (porcelain, fused silica) which have yet been 

 applied to this purpose. Although this limitation of platin- 

 iridium would not therefore alone be sufficient to deprive it 

 of continued usefulness for the gas-thermometer, yet when 

 combined with the contaminating action of the iridium which 

 distils out of the alloy at all temperatures above 900° in suf- 

 ficient quantities to eventually destroy the accuracy of the 

 thermoelement, it has led us to abandon the iridium alloy for 

 the future, and to substitute an alloy of rhodium. 



This study of the irregularities present or possible in the 

 expansion of the bulb was pursued much more persistently 

 than is usual in an investigation which is but incidental to a 



