On Systems with "Propagated Coupling." 827 



dynamical case illustrating maintenance of vibrations/' the 

 authors give what comes to a theoretical treatment of what 

 happens when an ordinary telephone receiver is connected 

 to a battery. Professor Porter and Mr. Gibbs arrive at the 

 unexpected conclusion that this telephone receiver will 

 produce a continuous sound. 



This obviously cannot be the case, and the erroneous 

 result arrived at is due to an unfortunate oversight of a 

 minus sign. In fact the second equation of p. 435 must be 



_aC + («^+r^.+ K)y = 



instead of 



if the first equation is maintained. 



This error, which is repeated a few times (pp. 437, 438, 

 440), just makes the maintenance of free undamped 

 vibrations, which the examples are intended to illustrate, 

 impossible in all cases treated, none of the differential 

 equations given having purely imaginary roots. 



In order to obtain spontaneous oscillations a variable 

 resistance must be present in the electrodynamical system, 

 as was the case in the authors 5 experiments, where a tele- 

 phone receiver was coupled acoustically to a microphone 

 transmitter, and not to another telephone receiver. 



I am, Gentlemen, 

 Yours truly, 



Baj/th. van der Pol, Jun. 



Physical Laboratory, 

 Teyler's Institute, Haarlem (Holland). 



To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



We must thank Dr. van der Pol for pointing out the unfor- 

 tunate slip in our paper in connexion with the sign of a. 



We do not agree with him, however, in regard to the 

 necessity for assuming a variability in the resistance — though 

 this variability was certainly present either in the micro- 

 phone or in the valve set. It may be pointed out that the 

 experiment works equally well with Brown magnetophones. 

 The effect of the propagation of the mutual action is to 

 create a phase-difference between y and C to an amount 



