PHYSICS: C. BARUS 
333 
vertical axis, were provided. If one of these, t for instance, is placed in a defi- 
nite effective position, while t f is relatively far from its armature h' , the screw 
I' may be gradually pushed forward diminishing the distance to the minimum. 
The effect of this is further to rotate mm' , and if the turning of V is cautiously 
done the fringes may be passed from top to bottom of the telescopic field by 
V and restored to position by the micrometer. In this way it is possible to 
find the tension of ee' in which for one position of the commutator there is ex- 
cessive motion of fringes whereas for the other there is practically no motion. 
Adjusting the bifilar as to tension, there is one position or distance between 
h' and t' pretty sharply determinable, for which the fringe bands change to 
stationary ellipses in the absence of all current. This peculiar result is at first 
puzzling, but since it is quite synchronous with the period of the telescope 
(stationary ellipse), it is obvious that the motion of the objective is the cause of 
the phenomenon and that the fibers are now in unison with its period. For 
distances ti ', t', greater or smaller the ellipses soon return to bands. The effect 
of the alternating current on the stationary ellipse is very beautiful. It now 
oscillates very much like a smoke ring for one commutator position, whereas 
it passes in an accentuated way through all phases for the other. 
Utilizing the preceding adjustment giving ellipses and bands respectively in 
the two positions of the commutator, many experiments were made to detect a 
change of phase when a large inductance is placed on both sides of one of the 
telephones. But in none of the experiments thus far, was any difference dis- 
cernable to be attributed to the presence of the inductance. 
As a tuned system responding to definite periods only, the vibration inter- 
ferometer is quite sensitive, provided the average currents are of the order of 
several microamperes. Between the types of compound vibration curves 
corresponding to frequency ratios of 4/3, 3/2, 1/1, 2/3, 3/4 there is usually an 
unbroken band of fringes. If the ratio of periods remains fixed, the vibration 
curve of course remains fixed, which is the usual sharp acoustic criterion. 
When the ellipses (out of tune) change continuously between lines of different 
inclination, the passage in one direction is often gradual whereas in the re- 
verse or return direction it is almost sudden. Linear forms flop into linear 
forms as it were. No doubt this is related to the vibration of a bifilar sys- 
tem like the above, where the two ends are liable to vibrate alternately. 
When the average currents approach the order of 10~ 4 am., the bands became 
waves for all periods not excessively high or low. 
Washington, Carnegie Inst., Pub., No. 149, pt. 3, § 123-125, 1914; cf., Amer. J. Sri., New 
.Haven, 3, 1897, (107). 
