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BELL SYSTEM TECHNICAL JOURNAL 



issued to Hutin and Leblanc during the same year.^^ These engineers were 

 pioneers in the attempt at multiplex telephony by means of high-frequency 

 carrier currents, a method now so greatly extended. In May, 1891, several 

 months before their patent papers were filed, they had reviewed in a French 

 journal the theory of resonance in an inductively coupled circuit in the 

 course of a general article on alternating currents, and had briefly suggested 

 therein its appHcation to multiplex signaling.'^ The scheme disclosed in 

 their patent comprised the transmission over the line of a number of super- 

 audible frequencies, now called carrier currents, the modulation of each by a 



Hg. -^ — Multiplex carrier telephone circuit of Hutin and Leblanc. as in their French 

 patent of 1891. At each terminal are shown four branches, each of these branch circuits 

 being tuned to one of the carrier frequencies. 



separate telephone transmitter, and means for separating and detecting 

 the individual messages. At both the transmitting and receiving ends, in 

 their plan, there were branches from the connecting line, each of these 

 branch circuits being tuned by means of a capacity which balanced the 

 inductance in the circuit, so that each responded by resonance to its own 

 carrier frequency to the exclusion of the others.* Here was a substitution 

 of electrical resonance for the mechanical resonance of the older harmonic 

 telegraph. 



At about this same time Professor Pupin in his early research work at 

 Columbia had developed a method of analyzing a complex current by 

 picking out its components in an inductively coupled resonant circuit — a 



* While their patent drawing fails to show tuning condensers in the receiving branches, 

 without which the scheme would be inoperative, and the description on this jioint is vague, 

 in the interference cases that later developed in the U. S. Patent OtVice (reference 24) 

 it was claimed that this omission was an oversight. In a later French patent. No. 2.34,785, 

 granted March 5, 1894, as in their U. S. patent. No. 8.^8, .S45, this defect was corrected. 



