CHAPTER III. 



ELONGATION OF METALLIC TUBES BY PRESSURE AND THE MEASUREMENT 

 OF THE BULK MODULUS BY DISPLACEMENT INTERFEROMETRY. 



41. General method and apparatus. About 25 years ago* I obtained satis- 

 factory results in the measurement of pressures of the order of 1,000 atmos- 

 pheres by the expansion of steel tubes of suitable thickness. The tube in 

 this case was inclosed in a snugly fitting glass tube filled with water and the 

 volume expansion measured by an attached capillary tube, the system being 

 submerged in water to obviate thermal discrepancies. The whole subject has 

 since been transformed by the famous experiments of Prof. P. W. Bridgman, 

 and I merely touch it here with the purpose of testing the optic apparatus 

 involved and with a view to the experiment explained in the final paragraph 

 of this paper. In the present experiments I shall attempt to measure the 

 increase of length of a steel tube due to internal pressure, by the displacement 

 interferometer. The experiments will lead to an independent method for the 

 measurement of the bulk modulus (Tait) and to a procedure for studying the 

 thermodynamics of the adiabatic expansion of liquids. 



The interferometer used was of the linear type (fig. 54). Here L is a weak 

 lens, about 2 meters in focal distance and 12 cm. in diameter, concentrating 

 a beam of sunlight on the slit 5. c is the objective of the collimator, being 

 a spectacle lens of about i meter focal distance. It is particularly advanta- 

 geous to have rays of slight obliquity here if a brilliant and wide spectrum is 

 to be seen in the telescope at T. H is the half-silvered plate of the inter- 

 ferometer, N and M (on a micrometer) are the opaque mirrors, each about 2 

 meters from H. The rays reaching the telescope T would therefore show 

 two white slit images from N and from M, which are to be placed in coin- 

 cidence both horizontally and vertically by the adjustment-screws on M and 



* Phil. Mag. (5), xxx, p. 338, 1890; Bull. U. S. Geol. Sur., No. 96, 1892; cf. Am. Acad. 

 Arts and Sciences, xxv, p. 93, 1890; for effect of pressure on electrical conductivity of 

 liquids, see Am. Journal, XL, p. 219, 1890, and on the mercury pressure-gage, Am. Journal, 

 p. 96, XLV, 1893. 



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