REVERSED AND NON-REVERSED SPECTRA. 93 



increased. The mean elongation per atmosphere is io 6 AZ, = p7 cm. and the 

 bulk modulus may be computed as 



a 2 L 0.6384X161 



K = ~ = " - = IO 6 Xl.^ 



a 2 i-a 2 3AL (0.9044- 0.6384) X3XQ7 X io' 6 



In spite of the large difference of dimensions, this datum is of the same order 

 of value as the above (&=i.39Xio 6 ) for the thick tube, particularly as the 

 present AL, from the occurrence of flexure, is probably slightly large. 



A tube of this kind with well-sealed ends (brazed probably), quite straight, 

 and supported at different points of its length by wire pendula, should make 

 a good pressure-gage within at least 1,000 atmospheres. An individual read- 

 ing about io~ 4 cm. per atmosphere or over 3 interference rings would be uni- 

 formly available throughout. 



47. Conclusion. Thermodynamic application. The data given show that 

 an independent method of measuring the bulk modulus of metals is quite 

 within the province of the displacement interferometer. The annoyances 

 encountered, resulting from the viscosity of the metal or from warping, may 

 be considered eliminated in the mean of the pressure-increasing and pressure- 

 decreasing phase of the experiment. The tubes should be supported at vari- 

 ous points along their length. Even the temperature discrepancy, if sufficient 

 time is allowed between the successive steps of pressure, seems not to be of 

 serious effect on the mean data. 



For the measurement of pressure, how- | ~ -j 



ever, the device is promising. In such a P "^ Sp ^ 



case the tube section should be chosen to 

 correspond with the pressures to be meas- 

 ured. Within 1,000 atmospheres a steel 

 tube about i cm. in diameter, with walls 



/" "*j s~~\ 



about 0.75 mm. thick, gave fair results, ^ c) 



showing the evanescence of about 3 inter- 

 ference rings per atmosphere. Such a tube must be rigorously straight, well 

 supported, and if possible of non-expanding (temperature) steel. 



It is interesting to consider the case of the adiabatic expansion of liquids 

 in relation to such a gage. The available thermodynamic equation is 



a e 



where A0 is the temperature increment corresponding to the adiabatic com- 

 pression Ap at the temperature 6, in case of a liquid whose coefficient of ex- 

 pansion is a, density p, and specific heat C P ., J is the mechanical equivalent 

 of heat. In an apparatus like figure 62, in which P is the screw compressor 

 (with tinned or waxed screw 5) filled with the liquid in question, G the Bour- 

 don gage, pressures may be suddenly applied without leakage by turning the 



