314 PKOCEEDINGS OF THE AMERICAN ACADEMY. 



pressure directl}^ inside the cylinder. This was done by Tammann 

 by connecting a Bourdon gauge directly to the cylinder. But it is 

 known that the errors of the Bourdon gauge become rapidly more 

 serious at higher pressures,* due to the increase of hysteresis, so that 

 this gauge could not be used for the pressures of this experiment. 

 Furthermore, no Bourdon gauge has up to the present been made of 

 sufficient sensitiveness which is capable of standing more than 6500 

 kgm. In the present work the pressure was measured inside the 

 cylinder by inserting directly into it a coil of manganin wire, which 

 had been already calibrated against an absolute gauge. This method 

 of measuring pressure has been fully described in a previous paper. ^ 

 It was necessary for the purposes of the present work, however, to 

 make a somewhat more careful determination of the temperature 

 coefficient than was done formerly, and this determination will be 

 described in detail later. The method has shown itself perfectly 

 satisfactory and reliable in every respect. One coil of wire has been 

 used almost continuously for over six months, and occasional calibra- 

 tions have shown no change. These calibrations were made by 

 measuring with the coil certain fixed temperature-pressure points, 

 such as the freezing pressure of mercury or of ice VI, at some fixed 

 temperature. 



The apparatus used in the present work is the same in most features 

 as that used in the former work, a detailed account of which has already 

 been given in the papers mentioned. Only the points in which this 

 has been changed will be mentioned here. It was a disadvantage of 

 the former method that the apparatus consisted of two parts; the 

 lower part, a cylinder containing the liquid to be measured, was placed 

 in a thermostat, and the upper part, a cylinder in which pressure was 

 produced, was exposed to the temperature of the room. When tem- 

 perature was changed in the thermostat below or pressure was changed 

 in the cylinder above, liquid passed from the one cylinder to the other, 

 experiencing in the transition a change of temperature, and so a 

 change of volume also. This change of volume accompanying a 

 known change of temperature varies in an unknown way with the 

 pressure, and to apply the correction it was necessary to make an 

 independent set of experiments. In the present form of apparatus 

 the difficulty was avoided by including everything in one cylinder. 

 This cylinder contained the liquid under investigation, the pressure 

 measuring coil, and the piston by which pressure was produced. It 



4 Bridgman, These Proceedings, 44, 201-217 (1909). 



5 Bridgman, These Proceedings, 47, 319-343 (1911). 



