PHYSICS: C. BARUS 
341 
vibration under constant force and a large logarithmic decrement. An ex- 
ample of the observations is given in the graphs figures 5 and 6, the reading 
being made every minute beginning with the equilibrium position (M in the 
neutral position). The ordinates of the graphs show the displacement of the 
attracted shot, m' , in cm. The periods of alteration were 10 minutes during 
the first 110 minutes; thereafter the period of the needle was exceeded. Cir- 
cles show the time of turning M, from one side of m to the other. The ap- 
proach to a limiting arc is regular during the first 100 minutes, after which 
some extraneous disturbance ^enters; but both for the longer and the shorter 
intervals of alternation, the tendency is toward the same limiting arc. At the 
end of the damped period there is a sort of fluttering. The excursions of the 
needle are so large that it was not thought necessary to read the fringes through- 
out. In fact, if a micrometer scale is put across the wide slit, there are two 
methods simultaneously available for finding Ax, the displacement of m due 
to M. For if / is the focal distance of the collimator and Ay the reading in the 
telescope of the collimator micrometer scale specified, 
Ax = cos i . lAN/b = Ay . l/2f 
But the reading in terms of AN is enormously more accurate than the reading of 
Ay, the shift of the slit image in the telescope. If the motion of the needle 
were less nearly dead-beat, the attracting force could be computed from the 
limiting distance between elongations, if the torsion coefficient of the quartz 
fiber and the logarithmic decrement were known. From static experiments 
made during hour intervals this double amplitude was found to be 0.116 cm. 
or a departure of the shot m at the end of the needle from its position of equi- 
librium of 0.058 cm. in response to the attraction of M. If / is the semilength 
of the needle (between centers of shots), the micrometer displacement AN 
and the displacements Ax of the mass m are given by the equation 
Ax = /ANcos i/b = 0.89 AN 
where b is the breadth of the ray parallelogram and i = 45 degrees the angle of 
incidence of the interferometer. Thus the micrometer displacement is of the 
same order as the displacement of m, and if the latter is 0.116 cm., we should 
have 
AN =1.3 cm. 
As the micrometer reads to 10~ 4 cm. 1/1300 part of the attraction between 
.Mand m = 0.61 gram could therefore be detected; i.e., the attraction of 0.73 
gram or per interference fringe well within one-third of this, for the given 
quartz fiber (which was not specially selected) and given distance R = 4 cm. 
This is equivalent to the attraction of 0.2 gram at a distance of 1 cm., per 
fringe. 
Apart from the measurement of the torsion coefficient of the fiber, there is 
however a real difficulty involved, and that is the occurrence of marked drift 
in the needle. It is only incidentally that the fringes are found at rest. The 
chief contributory cause of this is no doubt the occurrence of motion of air 
