334 
PHYSICS: C. BARUS 
DISPLACEMENT INTERFEROMETRY IN CONNECTION WITH 
U-TUBES 
By C. Barus 
DEPARTMENT OF PHYSICS. BROWN UNIVERSITY 
Communicated, March 7, 1917 
1. 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 mag- 
netizing 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 re- 
spectively, and to measure the differential effects in question in terms 
of the resulting displacements of fringes. 
2. Apparatus. — The interferometer used was of the same form as that 
before described (these Proceedings, 3, 1917, 117),^ in figure 1 being a 
heavy iron block, one 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 adjust- 
ment for horizontal 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. 
The vertical columns of this tube are shown at C and C (with acces- 
sory mirrors removed) and they are joined to the capillary tube p near 
the bottom of C and C, Details will be given in connection with figures 
2 and 3. 
The ray HM strikes a mirror symmetrically at 45° to the vertical 
below C, is thence reflected upward along the axis a striking another 
mirror above also symmetrically at 45° and parallel to the former, 
whence it is reflected to the opaque mirror M. The latter reflects the 
ray normally back so that it retraces its path as far as fi", by which 
plate it is now transmitted to be observed by the telescope at T. Sim- 
