The Bell System Technical Journal 



Devoted to the Scientific and Engineering Aspects 

 of Electrical Communication 



EDITORIAL BOARD 



J. J. Carty Bancroft Gherardi F. B. Jewett 



E. B. Craft L. F. Morehouse O. B. Blackwell 



H. P. Charlesworth E. H. Colpitts 



R. W. King- Editor 



Published quarterly by the American Telephone and Telegraph Company, 



through its Information Department, in.behalf of the Western Electric 



Company and the Associated Companies of the Bell System 



Address all correspondence to the Editor 

 Information Department 



AMERICAN TELEPHONE AND TELEGRAPH COMPANY 



195 BROADWAY, NEW YORK, N. Y. 



50c. Per Copy Copyright, 1922. SI. 50 Per Year 



Vol. I NOVEMBER, 1922 No. 2 



Physical Theory of the Electric Wave-Filter 



By GEORGE A. CAMPBELL 



Note : The electric wave-filter, an invention of Dr. Campbell, is one of the 

 most important of present day circuit developments, being indispensable 

 in many branches of electrical communication. It makes possible the 

 separation of a broad band of frequencies into narrow bands in any desired 

 manner, and as will be gathered from the present article, it effects the 

 separation much more sharply than do tuned circuits. As the communica- 

 tion art develops, the need will arise to transmit a growing number of tele- 

 phone and telegraph messages on a given pair of line wires and a grow- 

 ing number of radio messages through the ether, and the filter will prove 

 increasingly useful in coping with this situation. The filter stands beside 

 the vacuum tube as one of the two devices making carrier telegraphy and 

 telephony practicable, being used in standard carrier equipment to separate 

 the various carrier frequencies. It is a part of every telephone repeater 

 set, cutting out and preventing the amplification of extreme line frequencies 

 for which the line is not accurately balanced by its balancing network. 

 It is being applied to certain types of composited lines for the separation 

 of the d.c. Morse channels from the telephone channel. It is finding many 

 applications to radio of which multiplex radio is an illustration. The filter 

 is also being put to numerous uses in the research laboratory. 



The present paper is the first of a series on the electric wave-filter to 

 be contributed to the Technical Journal by various authors. Being an 

 introductory paper the author has chosen to discuss his subject from a 

 physical rather than mathematical point of view, the fundamental char- 

 acteristics of filters being deduced by purely physical reasoning and the 

 derivation of formulas being left to a mathematical appendix. — Editor. 



THE purpose of this paper is to present an elementary, physical 

 explanation of the wave-filter as a device for separating sin- 

 usoidal electrical currents of different frequencies. The discussion 



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