326 
PHYSICS: C. BARUS 
Proc. N. a. S. 
^ The birefringence, y-a, of jadeite is uniformly given in the Hterature as 0.029, 
following apparently the early determination of Levy and Lacroix {Les Mineraux des 
Roches, 1888 (266)). This value is certainly too high for pure jadeite, and the value 
0.012, determined by Merwin on an analysed and crystallographically described crystal 
of pure jadeite, should replace it. 
' Merwin, in Washington, Proc. U. S. Nat. Mus., 60, Art. 14, 1922 (9). 
8 Wright and Larsen, Amer. J. Sci., 37, 1909 (33). 
ON A COMPARISON OF THE RELATIVE SENSITIVENESS OF 
TELEPHONES'" 
By Carl Barus 
Department of Physics, Brown University 
Communicated, September 20, 1922 
In the earlier papers I reduced the fringe deflections s, of the inter- 
ferometer U-gauge, produced by telephone currents of different effective 
strength by the simple relation sr = const., where for a fixed effective 
voltage, the resistance r in the telephone circuit is very large compared 
with the inductance, etc. The telephone mouthpiece is connected with 
one or both shanks of the U-gauge by a quill tube, simple or branched, 
respectively, each branch containing a pinhole probe suitably directed. 
Thus 5 measures the acoustic pressure evoked when the telephone sounds. 
The expression given is applicable only within the narrow range of 5 
values and low frequencies for which it was used. For wider ranges an 
exponential relation, which may be written 
log 5 = log^o — r/fQ 
where 5o and Tq are constant, fits the observations as accurately as the 
fringes can be read. To give examples chosen at random for a single 
telephone, branched connection, salient and reentrant pinholes (5o = 399 
fringes, fo = 6380 ohms) 
r X 10-3 4 5 6 7 8 ohms 
^ observed 95 65 45 32 22 fringes 
5 computed 94 65 46 32 22 fringes 
and the same for low ranges of s{so = 190, Tq = 8620) 
r X 10-3 8 9 11 14 18 ohms 
J observed 22 17 10 5 1 fringes 
5 computed 22 17 10 5 2 fringes 
