PHYSICS: C. BARUS 
269 
ture of the room. Two such flasks were at hand to be used alternately. The 
coil CC 26 cm. long and 3.7 cm. in external diameter, is wound immediately 
on the tubular water jacket. 
The rod rr fits the tube ww loosely and is centrally detached at the remote 
end by aid of a bushing ^ and a small bolt. The front end is free. The coil 
CC is held in position by a large clutch (not shown) encircling it at the middle 
and attached to the bed plate of the interferometer. It is additionally at- 
tached at the tubulures / and Finally the conical end If of a micrometer 
screw (also rigidly attached to the bed plate) gives the remote end of the rod 
rr any desirable fiducial position. This micrometer M has the further ad- 
vantage of permitting an independent standardization of the contact lever, 
as there is obviously always suflacient elastic yielding in the apparatus to con- 
siderably shift the interference fringes. 
In the experiments made, the breadth of the ray rectangle mm' of the inter- 
ferometer was b = 9.7 cm.; the normal distance between the rod rr and the 
axis a, 7 cm.; the length of the contact lever a to mm', 10.6 cm.; and the length 
of the axle a, 10 cm. 
3. Observations. — The helix C in figure 1 was slender in shape, the length 
being 37 cm. and the diameter within being about 1.5 cm. There were about 
11.2 turns per centimeter per layer and 8 layers of wire so that the field within 
may be estimated at jH" = 110 ^* gauss, i being the current in amperes. The 
current 0.01 to over 8 amperes thus corresponded to field from 1 to over 800 
gauss. 
The first rod selected was of low carbon shop steel 43 cm. long, so that it 
projected a few centimeters beyond either end of the helix. 
The displacement of fringes observed was characteristic, being (in the 
smaller fields) slow and deliberate on closing the circuit (so that their motion 
could almost be followed by the eye), but very rapid on breaking the circuit. 
The experiments were begun with small fringes (about 0.1 mm. in the ocular), 
and the readings were made in terms of an ocular micrometer scale which 
was a centimeter divided into 0.1 mm. This was compared with the datum, 
AiV, of the displacement micrometer normal to one of the mirrors of the 
interferometer. 
If AiV corresponds to the angular displacement, A^, of the contact lever and 
to A/ of elongation of the iron rod r in the helic C (figure 1) we may write as 
above 
2b^e = 2AN cos i (1) 
if i is the angle of incidence (45°) at the mirrors of the interferometer and b 
the breadth of the ray parallelogram. But 
Al = rAd (2) 
if r is the normal distance of the line of thrust of the rod rr from the axis of 
the contact lever. Thus 
Al = (r cos i/b) (AN/Ae) Ae (3) 
If / is the length of the iron rod Al/l will be the datum required. 
