108 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



deflecting forces arising in the difference of potential of the quadrants. In 

 the same way the negative effect, as regards sensitiveness, of an increase of 

 the potential of the needle above a certain value is accounted for. Table 13 

 contains data bearing on this inference. 



TABLE 13. Needle II; quadrants II; steel frame. 



Thus the sensitiveness rapidly reaches a maximum when the potential of 

 the needle is about 150 volts, after which it more gradually diminishes (see 

 fig. 66, curve 6). 



Furthermore, in Series III, where the needle is at the highest potential 

 applied, the sensitiveness seems to increase with the voltage measured. This, 

 however, is merely the result of the fact that there is apparently a small fixed 

 voltaic potential difference between the quadrants, even if they are nominally 

 identical or in the connections. Thus in fig. 66, curve a, AV and A7V are pro- 



volts 



VOliS 



portional within the inevitable errors; but the deflections begin with a differ- 

 ence of potential of about 0.012 volt. In measuring such small voltages 

 electrostatically these voltaic differences become of serious moment. 



The maximum sensitiveness obtained is not as large as above, being but 

 40X10-* volt per vanishing ring. Finally, the water damper was removed, 

 so that the needle was subject to air damping only. After a long trial it was 

 necessary to abandon the work, as, in consequence of the excessive drift, 

 measurement was out of the question. Most of this drift is probably intro- 

 duced by the steel frame. 



