100 DISPLACEMENT INTERFEROMETRY. 



As a tuned system responding to definite periods only, the vibration inter- 

 ferometer is quite sensitive, provided the average currents are of the order 

 of several microamperes. Between the types of compound vibration-curves 

 corresponding to frequency ratios of 4/3, 3/2, i/i, 2/3, 3/4, there is 

 usually an unbroken band of fringes. If the ratio of periods remains fixed, 

 the vibration curve of course remains fixed, which is the usual sharp acoustic 

 criterion. When the ellipses (out of tune) change continuously between 

 lines of different inclination, the passage in one direction is often gradual, 

 whereas in the reverse or return direction it is almost sudden. Linear forms 

 flop into linear forms, as it were. No doubt this is related to the vibration 

 of a bifilar system like the above, where the two ends are liable to vibrate 

 alternately. When the average currents approach the order of io~ 4 cm. the 

 bands become sinuous for all periods not excessively high or low, as already 

 stated. 





