338 
PHYSICS: C. BARUS 
GRAVITATIONAL ATTRACTION IN CONNECTION WITH THE 
RECTANGULAR INTERFEROMETER 
By Carl Barus 
Department of Physics, Brown University 
Communicated September 14, 1918 
1. Introduction. — The ease with which the rectangular interferometer ad- 
mits of the measurement of small angles, induced me to adapt an apparatus 
with reference to it, for the measurement of the Newtonian constant. In addi- 
tion to the usual system suspended in air, I also tested a floating system. 
Accurate work is scarcely to be expected in this laboratory where temperature 
variations and the agitation of the room conflict with the condition of its ob- 
tainment. But the trial is nevertheless interesting and the final work may 
be done elsewhere. 
2. Apparatus. — There is room here only for the well known quartz fiber 
method of Boys, using a light needle, weighing when loaded but 1.9 grams. 
This was made of a rigid shaft of straw mm, figure 1, 26 cm. long, the ends be- 
ing split slightly into four symmetrical segments each, which receive the two 
shots m and m' additionally secured with a little wax. The two light mirrors 
b and c were differently mounted: b on a fine pin d, snugly fitting correspond- 
ing perforations in the straw, was thus capable of rotation around the vertical 
axis (d) and moving up and down slightly. The mirror c, however, was 
mounted on a thin elastic strip of aluminum, clasping the shaft as shown in 
section at e', c' . This is thus capable not only of rotating on a horizontal 
axis, but of being placed at different distances (moving right and left) from 
d to accommodate the rays of the interferometer, to which b and c are to be 
normal. 
The needle is swung from a quartz fiber q, and a strip or hanger of elastic 
aluminum, a (see a', q f ), which clasps the shaft. Hence the latter may also 
be moved endwise to be balanced and rotated around a horizontal axis, till 
b and c meet the rays normally. 
These operations are completed by trial before the needle is definitely hung, 
preferably in a broad beam of sunlight. No great accuracy is required. 
The shallow case is made of two plates A A, figures 2, 3 (side view), of plate 
glass about 32 cm. broad and 45 cm. high, spaced by two strips e,f, 3 cm. wide 
of wood 1.2 cm. thick, reaching from top to bottom. Six steel clips, b, b, h, h, 
(such as are used for binding pamphlets), held these strips in place. At the 
projecting tops of the wooden strips, two nipples s and / of \ inch gas pipe 
fixed normally to the strips, served for the hanging of the case and needle 
from a firm wall bracket. The bottom of the case is subsequently to be closed 
from below by a strip of felting gg. To diminish the space within the case, 
two thin cloth covered wooden boards w, w, are inserted from the top. 
