[ 432 ] 



XXXVII. On Systems with Propagated Coupling. By 

 Alfred W. Porter, D.Se., F.R.S., F.Inst. P., and 

 Reginald E. Gibbs, B.Sc, A.Inst. P.* 



IN the majority of electrically coupled circuits, the two 

 circuits are so close together that the time of propa- 

 gation of the mutual action can be neglected. For great 

 distances of separation or for very rapid frequencies this 

 would no longer be true. When the coupling is of a 

 mechanical nature (instead of being electrical), and the 

 actions are propagated with the velocity of sound, the entry 

 of time would have to be considered for even quite small 

 frequencies at moderate distances. For example, with a 

 frequency of 1000 per second, through air, even 30 cm. 

 separation would change the phase of action through a 

 complete period. 



A fairly simple case in which acoustic coupling comes into 

 play as mentioned above, is that of a microphone transmitter 

 and a telephone receiver coupled electrically through a valve 

 set, and simultaneously acoustically through the intervening 

 air. It is well known that a transmitter and receiver can 

 maintain each other in vibration as in the case of the singing- 

 telephone ; the introduction of the valve set is merely to 

 amplify the action and allow of its study at greater distances 

 of separation. 



To maintain continuous oscillations in any system, definite 

 relations must exist between the phases of the forced and 

 forcing systems, as is often stated by saying that they must 

 be " in step " with one another. In the case under consider- 

 ation the electric coupling in the valve is constant,, whilst the 

 acoustic coupling between the transmitter and receiver is a 

 variable quantity dependent on their separation. Thus at 

 some distances this ki phase condition" will be realized, at 

 others it will not, so that a periodic variation of intensity with 

 separation is to be expected, while experimentally it is found 

 that if the receiver be moved gradually away from the 

 transmitter in any direction, a series of maxima and minima 

 of mutual action can be obtained. 



The whole of the room in which the experiments are con- 

 ducted can be mapped out in this way into surface-loci of 

 maxima and minima alternating with one another every 

 quarter wave-length. The first impression, that these surfaces 

 represent nodal and ventral surfaces of a system of stationary 

 waves, cannot be true, for the conditions were not suited 



* Communicated by the Authors. 



