332 
PHYSICS: C. BARUS 
be coupled with the primary current. A full account of the changes of 
phase and amplitude in any transformer system may then be obtained 
from the fringe ellipses usually seen in the interferometer. 
Z. Apparatus. Wide Bifilar. — This is in large measure a modification 
of the apparatus described heretofore^ except that special attachments 
have been added for sharply reaching the resonance tension of the 
wire. The latter is shown d,t d e e' d' in figure 1 (front elevation), being 
the thinnest steel music wire, about 0.023 cm. in diameter. Its ends 
are wound around the stiff screws h\ provided with locknuts, and 
rotating in horizontal short strong rods a, a', attached to stout stand- 
ards (not shown) fixed to the bed plate ^, ^ of the interferometer. 
The wire dd' passed around the grooved pulleys w\ and above the 
grooved pulley x, carried in a fork and screw stem y. The latter may 
be raised or lowered by the nut u, which rests upon the massive car- 
riage BB, supported by the slides A A' of the apparatus. Provision 
must be made (slotted sheath and pins, not shown) to prevent y from 
turning on its axis. Tension is roughly given to the wire at the screws 
b, h\ and the fine adjustment is thereafter made at the nut u. This 
worked very satisfactorily. 
The vibrator proper cc' is attached at the middle of the wires d, d\ 
and carries the parallel auxiliary mirrors m, m' , of the quadratic inter- 
ferometer. A thin steel umbrella rib seemed well adapted to fulfill the 
requirements of cc' though a light soft iron tube would have been 
preferable. 
The telephones T and T' are adjustable on special standards, attached 
to the bed plate (carriage BB) and placed horizontally, one in front 
and the other toward the rear of the vibrator cc' . It is desirable that 
one be adjustable on a micrometer screw and spring, so that the distance 
of the poles of both from cc' may be regulated. 
The achromatic fringes in the fine slit image of the telescope field of 
the interferometer must be observed with a vibration telescope, and it 
was found desirable to control the latter by a special electromagnet. 
Figure 2 is a diagram of the parts of the apparatus as a whole, M M' 
N N' being the mirrors of the interferometer (M' on a normal microme- 
ter screw), m, m' the auxiliary mirrors on the vibrator cc' \ T' , T, are the 
telephones (one provided with a switch r), V the vibration telescope, 
I the mercury interruptor. T" is an auxiliary telephone for the ear. 
The primary consists of the linear coil P described in the preceding 
paper, the storage cells E (usually four), and the two small electro- 
magnets e, e' , for controlling the interruptor and the objective of the 
