CHAPTER V. 



DISPLACEMENT INTERFEROMETRY IN CONNECTION WITH U-TUBES. 



JAMIN'S INTERFEROMETER. 



56. Introduction. A variety of constants in physics may be found from 

 the relative heights of two communicating columns of liquid. This is, for 

 instance, the case in the classical experiment of Dulong and Petit on the 

 thermal expansion of liquids. Again, if one of the tubes is subject to a special 

 force acting in the direction of its axis, this force in its bearing on the liquid 

 may be evaluated from the resulting difference of heads of the columns. 

 Thus one tube may be surrounded by a magnetizing helix and the effect of 

 the axial magnetic field on the liquid in question (i.e., the susceptibility) 

 found from the displacement of its surface by the presence and absence of 

 the field, etc. It seemed to me worth while, therefore, to test whether it 

 would be possible to measure small displacements of this kind by passing the 

 two component beams of a displacement interferometer axially through the 

 two columns respectively, and to measure the differential effects in question 

 in terms of the resulting displacements of fringes. 



57. Apparatus. Michelson interferometer. The interferometer first used 

 was of the same form as that described above ( 2, fig. 3), B, figure 70, being 

 a heavy iron block, i foot in diameter and 1.5 inches thick, on which the 

 mirrors M, N (the latter and preferably both on micrometers) are securely 

 mounted with the usual direct rough and elastic fine adjustment for hori- 

 zontal and vertical axes. A beam of parallel white rays L arrives from a 

 collimator (not shown) and impinges on the half -silver plate H, to be reflected 

 and transmitted at a convenient angle 6 (about 60), thus furnishing the two 

 component beams which are to traverse the limbs of the U-tube. 



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