EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 109 



73. Observations, continued. The framework of the needles above was of 

 steel. It was supposed that even if not originally magnetic, such a needle 

 might be subject to variations of the earth's field, through which it becomes 

 temporarily magnetic by induction. Accordingly a needle of the same dimen- 

 sions as the preceding was constructed on a frame of thin copper wire and 

 tested, with the results of table 14. 



TABLE 14. Needle II, copper frame. Quadrants II. 



The sensitiveness is on the average 70 microvolts per ring, a smaller value 

 than in the last experiments, owing to a somewhat greater weight of the needle. 

 The voltage was now increased further and the following experiments made: 



TABLE 14. Continued. 



As before, the sensitiveness passes through a maximum when the voltage 

 of the needle is about 180, and is about as great for the voltage of 140 volts 

 as for 250 volts (see curve /, fig. 66). The maximum sensitiveness is 53 

 microvolts per ring. Though the drift was not quite removed, the stability 

 of the needle under any given circumstances proved in fact to be greater than 

 before, indicating a marked improvement as the result of replacing the steel 

 frame by one of copper. The curves c, d (raised 0.05 cm.), e (raised o.i cm.), 

 show that AF and AA7 are proportional within the limits of error. The latter, 

 e, seems to begin with an initial potential which would mean that the sensi- 

 tiveness is even lower at 250 volts than at 140 volts. 



It therefore seemed necessary to replace the needle in some other of the 

 above experiments by structures not containing steel. Thus the needle and 

 quadrants used in 7 1 with this improvement gave the results shown in table 1 5 . 



In view of the low voltage of the needle, the sensitiveness attained is, as 

 above, exceptionally high. The displacement AJV is proportional to AV (see 

 fig. 66, g), but begins with a permanent potential of 0.008 volt. Something 

 similar to this occurs in some of the above results on a smaller scale, so that 



