

CHAPTER VI. 







THE VIBRATION OF THE TELEPHONE PLATE. 



49. Phenomena. Intense white light (sun, electric arc) from a collimator, 

 figure 80, passes the half -silvers hh' and the opaque mirrors mm' and is observed 

 in the vibration telescope oe (vibrating objective o) . The slit must be very 

 fine and the interference fringes brilliantly beaded . The very small light mirror 

 m is at the center of the plate p of an ordinary telephone pt. This is excited by 

 a little induction coil, with high resistance in circuit. It is also convenient 

 to pass the current from hand to hand through the body and vary the contact 

 at the fingers. A second telephone should be in circuit to test the loudness of 

 response by the ear. 



What one observes is a very beautiful phenomenon of apparently beating 

 wave-trains ; and this in spite of the fact that the telephone at the ear does not 

 suggest the slightest departure from a uniform rapping; or otherwise a steady, 

 low-pitched note. 



I have endeavored to analyze the phenomenon observed, though almost 

 infinitely varied and perplexing, in figures 81 and 82. When the telephone 

 current is sufficiently strong, it consists essentially of detached equidistant 

 groups of fringe-waves w, w', etc. and vertical equidistant lines of white light 

 s, which are temporary slit-images. There may be many more waves and lines 

 than drawn and differences in amplitude; but this is immaterial here. For 

 simplicity, one fringe only is treated. In one extreme case waves and lines may 

 alternate, figure 81; in the other, figure 82, coincide; and there are all inter- 

 mediate cases. If the telephone current is weak, the lines 5 vanish, but the 

 beating-fringe wave-trains are left. In other cases the lines may be replaced by 

 fringe-free gaps (Cf. figure 87) in the wave-field. Both the lines 5 and the 

 grouped waves w move with the objective; and since the objective vibrates, 

 they also vibrate, as suggested by the arrows in the two parts of each figure. 

 The waves at any part of the field have the appearance of being on an axis 

 alternately stretched longitudinally with loss of amplitude, and compressed 



*h&P 



i/.. 



/"m> ' / 

 80 



A 



AAA 



1 /m! 



8J 



VW 



<U]* ' 1H 



A 



n 



82 



/w 



*U) 



or crumpled by end-thrust with increased amplitude. In such cases the wave- 

 length may decrease continuously almost to zero and the waves become sharp 

 cusps. When the amplitude is a fringe, the displacement at the center of the 



plate would be but y = \/(a cos 6) =4Xio~ 5 cm. 



The telephone then shouts 

 55 



