270 
PHYSICS: C. BARUS 
It was quite possible to ascertain Ae = 0.1; i.e., elongations of butA/// = 
5 X 10-^ equivalent to A/ = 2.3 X 10"^ cm. The current must exceed 0.02 
am. before any elongation can be detected, after which, however, the elonga- 
tions abruptly begin and increase rapidly to a maximum, which is reached 
before saturation. 
The experiments, figure 2, were made with somewhat greater care and with 
larger fringes. The standardization of the ocular micrometer showed 
^N/^e = 10-5 X 1.82, Al/l = 10"^ X 2.16 A^ 
But for incidental difficulties (tremors, etc,), the results in figure 2 would 
probably be very smooth. 
A number of supplementary experiments (see figure 2) were made to see 
whether the observed A/ = 0 for currents below 0.02 amperes might not be 
equivalent to an initial small minimum. But Al remained persistently zero, 
while currents decreased from 0.02 to 0.001 amperes. At 0.004 am. the 
field was reversed, but no significant Al could be observed. The fringes just 
moved {Ae = 0.1) when i was about 0.035 amperes, indicating a field of 3 or 
4 gauss. 
A rough test made of the equation by pushing the rod rr forward by the 
backstop screw M, figure 1, gave corroborative results. 
If A(p refers to the turns of the screw M 
Al = (r cos i/b) (AN/Ae) {Ae/A<p) Acp 
(AN/Ae) was found to be 10"^ X 3.3 cm. per scale part and (Ae/A(p) scale 
parts per degree of turn. Hence with the above data 
Al = 10-4 X 1.8 
The back screw having 40 turns to the inch, i.e., a pitch of 0.0635 cm. gives 
us 10-4 X 1.76 cm. per degree of turn. 
Another feature may be mentioned here. The expansion of the coil when 
carrying very large currents is a thrust on the back stop ikf, which is quite 
appreciable and appears as an apparent contraction of the rod. 
4. Vibration telescope. — To test the surmise that initial elongations always 
precede the final contraction, the vibration telescope heretofore described^ 
was installed. It was then found that the even band of fringes drawn out by 
the vibrating objective broke up into strongly sinuous lines on making and 
particularly on breaking the circuit through the helix. When the circuit was 
made and broken alternately, the waves broke up further into a succession of 
discontinuous pulses of more than double the amplitudes of the waves. With 
the field properly adjusted by passing 1.8 to 2 amperes through the coil, there 
was no further observable displacement after the strong waves lines, produced 
immediately after closing the circuit, had subsided. 
