382 A Fundamental Experiment in Electricity. 



electrical machine was placed below the middle of the tubes 

 between these two tables. 



The experiment consisted in watching the interference- 

 bands as the potential- difference rose and then suddenly fell 

 on the passage of the spark, and looking for a shift of the 

 bands. To aid in the detection of the shift, the edge of a fine 

 silk fibre, fixed in the middle of the field of the cylindrical 

 eyepiece, served as a fiducial line. When care was taken 

 not to touch either table, no shift whatever could be detected 

 either when the spark occurred or while the potential- 

 difference was accumulating. 



With the slits D D about 4 mm. apart the bands as viewed 

 were just about 1 degree of arc apart, and a shift of ^ - of this 

 distance could have been easily detected. Sometimes the 

 slits D D were placed wider apart and the beams brought 

 together by means of a parallel plate Jamin separator shown 

 at H in fig. 1. With this arrangement the separation of the 

 middle of the bands was about | of a degree, but the definition 

 was not so good. Sometimes the slits D D were traversed 

 by the light after it had passed through the tubes, instead of 

 before; but the result was always the same. 



The difference of potential when the spark passed was 

 ascertained to be about 60,000 volts. The wave-length of the 

 light to which the predominating part of the illumination 

 was due may be taken as 57 X 10~ 6 cm., and the experiment 

 therefore shows that a difference of potential of 60,000 volts 

 does not, in a length of 152 cm. of air, produce a relative 



retardation of as much as ^ of 57xl0~ 6 cm.; so that, if 



there is any difference in the velocities through the tubes ? ^ 



is less than 150 x on x j^ °f the whole, i. e. less than ™ f 



one-millionth of the whole. 



Observatio?is on the Experiment. 



It is satisfactory to notice that the observations were not 

 appreciably disturbed by any secondary effects due to dis- 

 placement of the optical parts through the action of electric 

 forces. Even if such effects had, however, been observed, 

 they could have been distinguished from the effect sought, 

 through their remaining unaltered when the charges of the 

 tubes were reversed in sign. 



The sensitiveness of the experiment could probably be 

 considerably increased by extending the length of the tubes, 

 for this should not increase any secondary disturbance due to 

 electrification. There might be some difficulty in procuring 



