ELECTRICAL AND MECHANICAL ANALOGIES 411 



to wire lines, with the line taking the place of the ether as the wave trans- 

 mitting medium. Previously, radio systems had been developed which 

 would transmit messages in detinite frequency ranges. These transmitting 

 ranges were selected from other frequency ranges by means of electrical 

 tuned circuits, which were themselves a borrowing from the acoustic resona- 

 tors of Helmholtz devised many years earlier. Tuned circuits are not 

 advantageous for selecting out channels in a carrier system, because with 

 them it is not possible to regulate the band width received or to get the neces- 

 sary discrimination between the pass band and the attenuated region. It 

 was found, however, that filters could meet these requirements and conse- 

 quently they were applied in separating the channels of the first carrier 

 systems. 



This use stimulated the further development of electrical network 

 theory. Filters with sharper discriminating properties, composite filters 

 containing sections of like image impedances but different attenuation char- 

 acteristics, transforming filters, impedance corrected filters for reducing 

 reflections, filters using mutual inductances, attenuation and phase correct- 

 ing networks are among the later developments. These investigations were 

 carried out by a large number of individuals among whom may be mentioned 

 Bartlett, Bode, Carson, Cauer, Foster, Fry, (luillemin, Johnson, Norton, 

 Shea, ..Wagner, and Zobel. Electrical network theory has progressed to 

 such an extent that it is now possible to select substantially any desired 

 frequency range, with very little of the frequency range wasted in obtaining 

 the desired selectivity, and to control the amplitude and phase of the 

 currents received over long distance lines so that a high degree of fidelity 

 of the received signal can be maintained. 



Borrowings of Mech.amcal Theory from Electrical Network 



Theory 



While this development of electrical theory was progressing, very little 

 development of a parallel nature was being carried out for mechanical theory, 

 due probably to the lack of a corresponding stimulus. With the advent of 

 the vacuum tube, public address system, and radio broadcasting, however, 

 .1 demand developed for loud speakers and related equipment. It was 

 shortly realized that the parallel developments of electrical network theory 

 provided a base for the design of such equipment. One of the first to recog- 

 nize this possibility was Professor A. G. Webster,* who pointed out the use- 

 fulness of the concept of impedance in mechanical systems. He applied 

 the concept to the phonograph and developed the first theory of the action 

 of acoustic horns. After this occurred the widespread application of the 



* A. G. Webster, "Acoustic Impedance, and The Theory of Horns and of the Phono- 

 graph," Nat'L Acad, of Science, Vol. 5, p. 275, 1919. 



