THE INTERFEROMETRY OF 



The pitch washer was therefore removed and replaced by one of tallow 

 slightly hardened with a little resin or wax, the two being melted together. 

 This functioned perfectly within 1,000 atmospheres and was easily inserted 

 in parts which could then be molded into a disk within the stuffing-box on 

 forcing the gland into it. 



The measurements given in table 17 were made in steps of 100 atmos- 

 pheres. AL denotes the elongation per atmosphere. 



TABLE 17. 



An example of the individual results may be given in case of series 9 : 



Pressure 100 200 300 400 500 600 700 700 600 500 400 300 200 looatm. 



Micrometer reading 1. 01 9.4 8.6 7.9 7.0 6.2 5.5 5.4 6.3 7.0 7.7 8.5 9.1 9.8 cm./io 3 



The elongation here is always greater when pressures increase, although 

 time is allowed for dissipation of temperature, than when they decrease. 

 Four reasons may be assigned for this result: (i) the temperature increase 

 on increasing pressure and vice versa; (2) permanent set imparted to the 

 tube; (3) elastic warping of the tube owing to the end-thrust of internal 

 pressures and consequent disadjustment of the interferometer; (4) vis- 

 cosity of steel. Probably all of these discrepancies are present. That there 

 was set I infer from the gradual displacement of the reading of the interfer- 

 ometer for 100 atmospheres at the beginning and end of a series, though this 

 may be due to new adjustments. The incidental displacements are particu- 

 larly shown in the values of AL when pressure increases and are specially 

 marked in series 7 and 8. They are also apparent in the change of form of 

 the ellipses. As sunlight was used in the above work the annoyances of a 

 flickering arc do not occur. The ellipses were not centered. 



To obtain some notion of the relation of these discrepancies we may pro- 

 ceed as follows : The difference between the elongation per atmosphere dur- 

 ing the phases of increasing and decreasing pressures in the four series 

 given is, respectively, 1.8, 3.1, 1.7, 0.8 cm./io 6 , or i.SXio" 6 cm. per at- 

 mosphere of compression. For a tube-length of 160.8 cm. and a coefficient 

 of expansion 12 X io" 6 this is equivalent to a rise of temperature 9.3 X io" 4 , or, 

 roughly, io" 3 C. per atmosphere of compression. 



Supposing the compressibility of the oil to be cfo=iooXio~ 6 per atmos- 

 phere per cubic centimeter and the mean pressure = 500 atmospheres, the 

 work done is pdv or 



5ooXio 6 XiooXio" 6 = 5Xio 4 ergs per atmosphere per cubic centimeter at 



500 atmospheres 



