46 
PHYSICS: C, BARUS 
work smoothly and with a minimum of friction. Such as exists may be 
eliminated by tapping b and c before each observation. Thrusts up to 15 to 
20 kg. may be easily applied by the thumb nut w. These stresses act in the 
direction ar and fd, colinearly and there are no couples endangering the ac- 
curacy of the elastic displacements of r. The stress is standardized in terms 
of the observed rotation of the thumb nut w. 
Figures 2, 3, 4 are details, showing the methods of clutching the rod r, 
sheathed in figure 2, shouldered in figure 3, soldered in a small cap m with 
fusible metal n, in figure 4. The thick ends are threaded to be received by 
A figure 1. 
3. Observations. Hard rubber. — As in the preceding paper (1. c.) if A is 
the longitudinal compression of the rod r in ^, 
where A iV is the displacement of the micrometer, at i = 45° to the rays, h 
the breadth of the ray parallelogram, and r the effective length of the contact 
lever. Furthermore since the modulus E for the length of rod L and section 
^ is E = {F / A) / {^x / L),F being the thrust, 
The ocular micrometer if used is to be standardized in terms of A iV by direct 
comparison; i.e., if the former datum is A e arbitrary scale parts, A e / A iV 
must be known. 
To graduate the spring S, the apparatus ABC, figure 1, was detached from 
the interferometer and the brick B fastened near the edge of a strong flat 
table, with its large face toward A lowermost and horizontal. The rectangle 
AC was thus vertical, A below C, just clearing the edge of the table. Weights 
from 1 to 9 kg. were now hung from A , compressing the spring S by measure- 
able amounts. In this way it was found that the stretch 0.7 mm. corre- 
sponded to 1 kg. Since the threads of w were 1.275 mm. apart, it follows that 
1 rotation of the thumb screw w corresponds to 1.82 kg. or to 1.78 X 10® 
dynes. In the interferometer, b = 9.3 cm., r = 11.0 cm. were directly 
measured. 
The test rod r was here of hard rubber of length L = 2.47 cm., diameter 
= 0.377 cm., and area A = 0.112 cm.^ Hence for n turns of the screw w, 
(equation (2), A iV in cm.) 
Ax = (r cost / b) A N 
(1) 
E = F Lb / A rcosi. AN 
(2) 
E 
4.69 X 10' » / A iV; 
(3) 
or if we express A iV in 10 ^ cm.. 
E = 4.69 X 1010 X n/AN 
(4) 
The fringes, found without difficulty, were small though here more than 
adequate for the purpose. Measurements were made in cycles, care being 
