70 



DISPLACEMENT INTERFEROMETRY BY 



30,000 ohms inserted. The ordinates 5 (scale-parts) show the range of ellipses 

 in the direction of the fringe vibration. The fringes being about a scale-part 

 in width, the sensitiveness at high resistances was about io~ 7 ampere per 

 fringe. In case of figure 76 the secondary consisted of but one turn (e= io~ 3 

 volts) and relatively low resistances were admissible. The sensitiveness here 

 is about io- 8 ampere per fringe. In case of the large fringes used in series 

 7 and 8 below, I found about o.5Xio~ 8 ampere to the scale-part, which is 

 probably the limit of the present apparatus. At R= 20,000 ohms the tele- 

 phone was practically silent to the ear, but the vibrator gave distinct evidence 

 of current even above 30,000 ohms. The curves (figs. 75 and 76) are so nearly 

 hyperbolic that the inductance must be negligible compared with the high 

 resistances. 



In the first series Rs is practically constant, a result which might be used for 

 standardizing s in case of these particular fringes. In series (3) r is computed 



from (R+r)s = const. For R below 500 ohms, the results for the effective 

 resistance of the circuit come out between 400 and 500 ohms. To compute 

 L 2 a) 2 =A(s(R-{-r))*/As z would require a more accurate specification of r and 

 better individual values of 5 than the method provides in the secondary. 

 Thus, if r 2$o ohms, Leo = 640 to 660; but this is clearly over five times too 

 large. Probably 5 and i do not pass through zero together. In fact, if r = 280 

 ohms and if i/(R-\-r) and s be constructed as in figure 76, the relation is so 

 nearly linear that L will be merged in the errors of observation. But the line 

 suggests an initial s=i scale-part. 



In a later adjustment I reduced the induction till the ellipses remained in 

 the field in the absence of extra resistance R. The resistance of the circuit 

 itself with its 3 telephones was r = 28o ohms. Results were obtained, for 

 example, as follows: 



R s La> 



O 



100 



15 



I2O 



After removing the auxiliary telephone (r = 187) : 



o 

 too 



24 



73 



