of a circuit — in one code either all of the current 

 flowed or none of it flowed; in another code the 

 current could flow either in one direction or the other. 



As a result of certain experiments, it was at first 

 argued that a galvanic current could not be trans- 

 mitted to a point at any great distance. Thus, in 

 1825 Peter Barlow^ attacked Ampere's proposal for 

 a telegraph on the basis of the slight effects produced 

 in what Barlow considered very long circuits. Bar- 

 low found that he could detect little Oersted effect 

 at the maximum distance of his circuits, which was 

 200 feet. Barlow's conclusion was that the effect 

 diminished approximately as the square root of the 

 distance along the wire from the battery. From 

 this hypotheses Barlow decided that an electric tele- 

 graph based on the Oersted effect was not only im- 

 practical but was theoretically impossible. 



In spite of Barlow's animadversions, some inven- 

 tors tried to devise apparatus for a needle telegraph. 

 In 1830 William Ritchie^ described an astatic needle 

 galvanometer that could be used as a receiver in such 

 a system. Ritchie agreed with the conclusions of 

 Barlow as to how the current varied along the line 

 but argued that this variation could be overcome 

 by modifying the battery: the longer the tele- 

 graph line, the more pairs of plates were neces- 

 sary to signal over the line. With a needle galvanom- 

 eter and a larger battery than Barlow's, Ritchie 

 found that he could signal over a distance of several 

 hundred feet. Ohm too stated in 1832 that one 

 needed only to increase the number of plates in the 

 battery and the thickness of the wire in order to 

 produce an effect over a distance.'" 



A more thorough solution to the problem of trans- 

 mitting electromagnetic signals to a distant point 

 was announced by Joseph Henry." In 1830 

 Henry demonstrated that an electromagnet could be 

 operated through a thousand feet of wire if an intensity 

 battery, of many pairs of plates, were connected to 

 one end of the line and an intensity electromagnet, of 

 many turns of wire, to the other end. In 1831 and 



8 Peter Barlow, "On the Laws of Electro-Magnetic Action 

 . . . ," Edinburgh Philosophical Journal, 1825, vol. 12, pp. 105-114. 



9 Philosophical Magazine, 1830, vol. 7, p. 212. 



1" G. T. Fechner, ed., Repertorium der experimental Physik, 1832, 

 vol. l,pp. 402-403. 



"Joseph Henry, "On the Application of the Principle of the 

 Galvanic Multiplier to Electro-Magnetic Apparatus, and also 

 to the Development of Great Magnetic Power in Soft Iron, 

 with a Small Galvanic Element," American Journal of Science, 

 1831, vol. 19, pp. 400-408; "Proceedings of the Board of 



ABCDEFGHIJKLMNOPQRSTUVWXYZ: ; .' 



Figure 8. — Alexander's telegraph in which a 

 moving magnetic needle would uncover a 

 letter. From La Lumiere electrique, 1883, vol. 8, 

 P- 333- 



1832 he showed his classes at the Albany Academy 

 that by having the electromagnet operate a clapper 

 and strike a bell he could transmit signals through a 

 mile of wire (fig. 9). Sometime in 1836 or 1837 

 Henry added a relay to a similar long line that had 

 been set up for his classes at Princeton in 1835. An 

 intensity electromagnet actuated by a distant intensity 

 battery closed the local circuit of a powerful quantity 

 electromagnet and a quantity battery. Although 

 Henry realized at an early date that he had all the 

 components of a complete electromagnetic signaling 

 system, he did not attempt to make an invention of 

 them. 



Regents of the Smithsonian Institution," Annual Report of 

 the . . . Smithsonian Institution . . .for the Tear 1857, 1858, pp. 85— 

 117; William B. Taylor, "Henry and the Telegraph," Annual 

 Report of the . . . Smithsonian Institution . . .for the Tear 1878, 1879, 

 pp. 262-360; Thomas Coulson, Joseph Henry, His Life and Work, 

 Princeton, 1950. 



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