44 
PHYSICS: C. BARUS 
INTERFERENTIAL CONTACT LEVER EXPERIMENTS RELATING 
TO THE ELASTICS OF SMALL BODIES"^ 
By Carl Barus 
Department of Physics, Brown University 
Communicated, December 27, 1918 
1. Introductory. — In a preceding paper I communicated a series of experi- 
ments on the traction modulus of small bodies, using an interferometer design, 
which worked admirably so far as the optic measurements were concerned. 
The mechanical part of the contrivance showed an apparent yield, the nature 
of which I was unable to detect, but which seemed in some way, to be associ- 
ated with the flexure of parts of this massive apparatus. In fact pulleys and 
weights were used for imparting stress. It may be argued that any contri- 
vance of this kind, however convenient in other respects, is dangerous because 
of the force couples introduced, even when the rigid parts of the apparatus 
are nearly 2 inches thick, as in the case in question. 
In the present apparatus all this is completely avoided by the use of push- 
ing springs to impart stress, and the interferential contact lever to measure 
strain. True, friction enters into functioning of such an apparatus to a 
menacing degree. It thus becomes an experimental question to determine 
in how far it can also be eliminated by judicious tapping, etc. Cf. § 6. 
2. Apparatus. — The simplest of the apparatus designed is shown in figure 
1. The rod to be tested, 1 to 3 cm. long, is at r held in a brass sheath loosely 
fitting it. See figure 2. This is screwed into the middle of the massive brass 
cross piece A. A little disc of glass has been attached at and the end, e, 
of the contact lever touches it to indicate the small elongations. The longi- 
tudinal displacements A a: of the pin e are observed by the interferometer, as 
explained in the preceding paper.^ 
is a cast iron brick, about 10 inches high, 2 inches thick, and 3.5 inches 
broad, provided with 2 horizontal J inch perforations, parallel to each other 
and normal to the large face. Through these pass the J inch brass rods hh 
and cc loosely, rigidly connecting the cross piece A with the similarly massive 
cross piece C (screws and nuts w, n). The rectangle is thus free to slide 
in B, except so far as it is limited by the contact of the rod r with the smooth 
face of the brick B. 
To apply stress, the system w, S, f, has been provided, consisting of the 
stiff open spring S encircling the brass rod df, firmly screwed into the brick 
B and d, but passing loosely through a perforation in the middle of C. The 
end near d of the rod df is threaded (20 threads to the inch), so as to admit of 
the compression of the spring S, by aid of the thumb nut w. S was a pre- 
cision spring, taken from an indicator apparatus and provided as usual with 
two end brass collars. It is essential that the sliding parts of the apparatus 
