THE AID OF THE ACHROMATIC FRINGES. 29 



was found to be 10-^X3.3 cm - P er scale-part and (A<?/A<p) = n scale- 

 parts per degree of turn. Hence, with the above data, 



Xn = io- 4 Xi.S cm. 



The back-screw having 40 turns to the inch, i.e., a pitch of 0.0635 cm -> gives 

 us i o- 4 XL 7 6 cm. per degree of turn. This is as close as the observations 

 warranted. The rod must of course be free at both ends, except for the stop- 

 screw and the contact lever. Even in such a case the intermediary rod rr, 

 whose end-face is not rigorously true, is not favorable to sharp results. 



Another feature may be mentioned here. The expansion of the coil when 

 carrying very large currents is a thrust on the back-stop M, which is quite 

 appreciable and appears as an apparent contraction of the rod. The effect 

 is eliminated in the triplets. It was not quite eliminated by the water-jacket, 

 which protects the rod only or chiefly. 



16. Vibration telescope. To test the surmise just stated, 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 separated 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 2.0 amperes through the coil, there was no further observable 

 displacement after the strong wave-lines produced immediately after closing 

 the circuit had subsided. 



The question is therefore pertinent whether in continually stronger fields 

 the contraction of length (which follows the expansion) continually increases 

 long after the rod is saturated. I therefore made a large number of measure- 

 ments in stronger fields with the coil suitably water-cooled. The flow of water 

 produced some slight sharp sinuosities in the otherwise even fringe-band of 

 the vibration telescope, but this was not seriously objectionable. The main 

 results obtained were 



I . 4.1 ampere, A#= 10 scale-parts 

 (AAYAe) = io- 5 < 4 -7 y = 7 . 4 ampere, 18 scale-parts 



The other data for the rod correspond to 



A//7= io~"X5.64A and A/= 10-^X2. ^2Ae cm. 



Thus the contraction continually increases long after practical saturation, 

 even in fields approaching 800 gauss. It is therefore desirable to reduce the 

 mean data of figure 31 and the present data to the same scale, and to plot 

 the whole curve. 



The results as given in the curve figure 3 2 are not as smooth as was expected ; 

 but observation is difficult, because the heat produced by strong electric 

 currents strikes through the water-jacket when the water-current is slow, and 

 a strong water circulation is apt to shake the fringes abnormally, and therefore 



