36 DISPLACEMENT INTERFEROMETRY APPLIED TO 



respects the symmetric and non-symmetric inductions are equivalent in effec- 

 tiveness. But the former (as would be supposed) do not admit of commu- 

 tation, the maximum remaining a maximum on reversal of the telephone 

 current. Adequate installation for the treatment of symmetric induction is 

 yet to be made. 



31. Closed region with pipe. The final experiments were made with the- 

 telephone and brass pipe (13 cm. long, i cm. in diameter) when the latter was- 

 closed with a perforated rubber cork, through which the probe could be in- 

 serted into the pipe, the fit at the cork being very snug. In other words, in. 

 figure 43, the pipe t was closed at the outer end; but the probe was also pro- 

 vided with a lateral branch at which a pin-hole o' could be inserted. An in- 

 terposed stop-cock C allowed the pin-hole to be removed from action when, 

 not wanted. 



The pipe / was first tried with the outer ends open as usual, but with the: 

 pin-hole leak o' respectively in action and closed off. The graphs so obtained 

 were the same as to the distribution of their minima (near /', \>a' ,&', e", a"} 

 and maxima (near g', a', d" , bg"; d" very pronounced) . They differed in the 

 amount of displacement at those pitches. The additional pin-hole does not,, 

 therefore, interfere with the action of the probe; but its efficiency is weakened 

 nearly one-half. Moreover, since the pin-hole o' here acts like a valve, it would 

 seem to follow that the reservoir R' vibrates. 



The pipe t was now closed at the outer end by inserting the perforated cork 

 and a survey was made between g' and a" with the auxiliary pin-hole o' in 

 action. The graph naturally differed from the preceding, having maxima near 

 g' b', g", the latter very pi enounced, and minima near b f , c", and a". The main 

 pitch had thus risen from d" to g". 



The pin-hole o' was then shut off, but the cork in /' loosened. This left, 

 the maxima (particularly the d" maximum) nearly intact ; but the c" minimum 

 was shifted to e". 



Finally the cork at the end cf t was closed tight, so that the region was 

 completely closed. The resulting behavior was peculiar, inasmuch as large 

 fringe displacements were attainable, but only after the lapse of considerable 

 time (minutes). The same conditions have been instanced above and are 

 due to the fact that the very small leaks in the region are taken advantage of 

 for on breaking the current the fringes return to zero with the same slow motion. 



Allowing the necessary time to elapse, however, the maxima for the corked 

 pipe t (with pin-hole 0') were reproducible ; but the former minima respectively 

 at c" and e" had now shifted to d". There was one other noteworthy difference, 

 as the a" minimum, immediately following the very high g" maximum,, 

 now appeared as a deep negative fringe displacement or dilatation. 



All the pipes showed a maximum, more or less developed, at g' and at g",. 

 the unstoppered pipes being far weaker. Between these limits of pitch, the 

 unstoppered pipe is in its regular behavior almost an inversion of the 

 stoppered pipe. 



