120 



DISPLACEMENT INTERFEROMETRY BY 



11' 



10 10 W 40 SO 60 10 80 QO iOO 110 



100 no 



130 m i50 m iro 180 100 zoo 210 o 



&a 



m 



w 



the motion of the needle is practically aperiodic. If we call the double ampli- 

 tude x' , the torsion coefficient 6, the gravitational force F, we may write 



F/b 0.115 (double amplitude) nearly, from the static experiments of 

 the next section. Hence (using the last values) 



* a = 0.15 = o.i i5 



so that the damping coefficient would be about ^ = 7.6. The observed coeffi- 

 cient would, however, seem to be larger than this. 



92. Static elongation. To complete the evidence as to the character of 

 the behavior of the needle, measurements of the elongations were made at 

 long intervals apart, with the attracting mass M alternately on one side and 

 the other of the needle. The results are given in table 7. The apparatus was 

 kept in the dark; nevertheless the individual results are curiously irregular, 

 showing the interference of a foreign effect. It is difficult to suggest a cause, 

 but laboratory tremor affecting the suspension and the warping of the straw 



