66 



DISPLACEMENT INTERFEROMETRY BY 



In the absence of current the fringe bands were nearly horizontal parallel 

 lines. The secondary was closed with 5,000 ohms and the inductance of the 

 three telephones. From the position b (ellipse i, fig. 73, quiescent) the inductor 

 telephone was quickly displaced to position c. The enormously eccentric, 

 finally linear ellipse 2 follows, which then rotates and contracts counter- 

 clockwise through the figure 3 and 4 into the sharp bands (usually but not 

 always) No. 5. These duplications then separate on further rotation into the 

 final quiescent form, 7. The arrows indicate the drift of one of the four points 

 of tangency. On returning from c, by quickly sliding the telephone inductor 

 into the position b, the figures roll clockwise from 7' to i'. Number 7' passes 

 at once through the highly eccentric ellipse 6', though in other slower adjust- 

 ments intermediate sharp duplicates like 5 may be detected between 6' and 7'. 



a 



72 



4' 5' 



The stretched ellipses, which follow immediately after the change of aspect 

 of the telephone bobbin to the magnetic lines, are noteworthy. They indicate 

 the inductive effect of the reversal of the magnetic field, impressed on the 

 vibrating system and observed in spite of it. Ellipses cross over, or change 

 sign of rotation, at 2 and 6', but not near 3' or 5, the latter being oscillations. 



At a large resistance, jR = 25,ooo ohms, the range of the ellipses was incre- 

 mented by about 4 scale-parts; at 5,000 ohms by 12 scale-parts. Figure 73 

 as interpreted by 72 would suggest a passage from/ to b of the latter, the case 

 a corresponding to No. 2 of the former, so that 4 to 7 are oscillatory. The 

 interpretation is difficult, because the four points of contact of the ellipses 

 lie in a vertically expanding, contracting, and eventually oscillating rectangle. 



The corresponding cases for capacity (fig. 74) are similar on the whole, 

 though less pronounced. From telephone positions c to b the ellipses 5' pass 

 through strong, duplicated fringes 4', when i microfarad is in circuit. With 

 0.5 microfarad in circuit, the ellipse 5' remains banded. The telephone 

 continued to be appreciably audible, even when the capacity was reduced to 

 io~ 4 microfarad; but the vibrator was not influenced by less than o.i micro- 

 farad. Moreover, the forms i and 5 are not in opposed phases. 



An example of the change of ellipses for a passage from self-induction to 

 capacity (i m.f ., resistance 5,000 ohms) need not be given, as it merely involves 

 a smaller degree of rolling with variation of size. Thus, for instance, in figure 

 74, the case 3 for capacity became the case i for inductance; or 3 passed to 5. 



