II IS TORY OF ELECTRICAL RESONANCE 



425 



As we consider the use of resonance in electric communication there may 

 occur to some readers a recollection of a very early system of multiplex 

 signaling known as the "harmonic telegraph", representing the attempts of 

 Elisha (iray, Alexander (Iraham Bell, E. Mercadier and others to transmit 

 simultaneously a number of telegraph messages over the same line; experi- 

 ments which, in the case of Bell, led to the invention of the telei)hone. 

 These various schemes, however, were all based on the principle of mechani- 

 cal resonance; electromagneticall}' driven luned reeds at the receiving end 

 were set to vibrating by signaling currents generated by corresponding 

 reeds at the transmitting end. The principle of electrical resonance was not 

 involved in such methods. 



Fig. 2 — Combination of series and shunt resonant elements to lessen interference of 

 power source low frequencies with higher telejihone frequencies; from U. S. patent of 

 Stanlevand Kelh , 18')]. 



Suggestions for the practical application of electrical resonance began to 

 appear in the early 18Ws. By this time, as our history shows, the phe- 

 nomenon was generally understood by the technically trained and the well 

 informed; it was one of the facts of science open to all. Henceforth, 

 |)rogress in the putting to use of it was largely in the hands of inventors and 

 its history is to be found in the study of patents. 



In telephony, one of the earliest proposals is illustrated b}' a United States 

 patent issued to Stanley and Kelly in 1891,'' showing methods for preventing 

 interference with telephone currents by lower frequency currents induced 

 in the line by power sources. ()n2 of the methods described was the inser- 

 tion in series with the receiver of a capacity making a combination resonant 

 to the mean speech frequency, supplemented by a shunt combination of 

 capacity and inductance resonant to the interfering frequency. It need 

 hardly be said that such an arrangement, favoring only a narrow band of the 

 speech frequencies, would greatly promote distortion and woukl find hi tie 

 favor with telephone engineers. 



Another application, for a different purpose, appeared in a I'Vench patent 



