330 BELL SYSTEM TECHNICAL JOURNAL 



mechanism, merely opens and closes a local circuit and a strong local 

 current performs whatever functions may be necessary in the receiving 

 mechanism. 



These constitute the fundamentals of a complete magnetic telegraph 

 system and left only its perfection in detail, the development of an 

 alphabetical code in dots and dashes, and its commercial exploitation. 

 In 1844 Morse opened his first telegraph line between Baltimore and 

 Washington and in this line utilized the features referred to above, 

 contributed by Henry. From this start the telegraph rapidly grew 

 to a nation-wide and world-wide communication system. 



The next important commercial application of electricity came 

 about 30 years later when Bell invented the telephone. As in the 

 case of the telegraph, important use was made of Henry's contributions 

 to the art. 



Bell's telephone makes use of Henry's work on magnets, as does 

 every telephone receiver today. The telephone bell, with which 

 each telephone is equipped to give an audible signal so as to attract 

 the attention of the called party, consists essentially of a polarized 

 ringer directly suggested by the receiving device employed by Henry 

 in his first telegraph demonstration. But a telephone by itself, even 

 equipped with a call bell, is merely an interesting scientific toy. The 

 telephone is useful only as there are a number of them at the ends of 

 telephone lines, these lines centering on telephone switchboards for 

 the purposes of interconnection. In all telephone switchboards, 

 whether of the manual or of the automatic type, there are multitudes 

 of relays for the purpose of controlling circuits and signaling apparatus. 

 There are perhaps seventy million telephone relays in the United 

 States today and the prototype of all of these is Henry's electromag- 

 netic relay, dating back to 1835. 



Telephony owes still another debt to Henry and to his work. He 

 discovered the characteristic of electrical circuits known as self- 

 induction. This is a property of all electrical circuits unless especially 

 designed to avoid it and self-induction must be taken into account 

 in many phases of electrical design. In long telephone lines it was 

 found to exercise a favorable effect upon telephonic transmission and 

 about 1900 Dr. Michael I. Pupin, a distinguished member of the 

 National Academy of Sciences, showed how self-induction could be 

 added to long telephone lines so as to improve their talking efficiency. 

 Today his invention is very generally used, not only in the lines of this 

 country but throughout the world, and has been an important factor 

 in the extension of long distance telephony and in making possible 

 long telephone cables. 



