HENRY AND THE TELEGRAPH. 279 



dation, produced but a comparatively small declination of Scbweigger's 

 needle. In fact, no multiplication of galvanic elements was successful 

 in increasing the deflection of a given galvanometer. On the other hand, 

 the same galvanometer was found to have its deflections greatly reduced 

 with every increase in the length of the interi)osed circuit. And here 

 again an increase of surface in the galvanic pair failed to overcome the 

 increased resistance of a lengthened conductor. There was also an 

 early limit found to the number of turns in the galvanometer coil, which 

 could be efficiently employed with any given surface of oxidizable metal 

 in the single galvanic element. 



In 1824, Peter Barlow, the eminent English mathematician and mag- 

 netician, taking up Am lucre's suggestion, endeavored more fully to test 

 its i^racticability. He has thus stated the result : "In a very early 

 stage of electro-magnetic experiments, it had been suggested that an 

 instantaneous telegraph might be established by means of conducting 

 wires and compasses. The details of this contrivance are so obvious, 

 and the principle on which it is founded so well understood, that there 

 was only one question which could render the result doubtful ; and this 

 was, is there any diraunition of effect by lengthening the conducting 

 wire ? It has been said that the electric fluid from a common [tin-foil] 

 electrical battery had been transmitted through a wii'e four miles in 

 length without any sensible diminution of efl'ect, and to every appear- 

 ance instantaneously ; and if this should be found to be the case with 

 the galvanic circuit, then no question could be entertained of the prac- 

 ticability and utility of the suggestion above adverted to. I was there- 

 fore induced to mate the trial ; but I found such a sensible diminution 

 with only 200 feet of wire, as at once to convince me of the impractica- 

 bility of the scheme. It led me however to an inquiry as to the cause 

 of this diminution and the laws by which it is governed." * 



From the rapid reduction of effect observed with increasing lengths of 

 conjunctive wire under the conditions tried, Barlow (from a considerable 

 series of experimental results) was led to believe that the resistance of 

 the conducting wire is api^roximately proportional to the square root of 

 its length, t 



Notwithstanding therefore Ampere's "completely successful" experi- 

 ment " through a very long conducting wire" and Schilling's later work- 

 ing of his telegraph " through a great length of wires," (the precise 

 length of the circuit not being stated in either case,) the problem of the 

 electro -magnetic telegraph could hardly be considered as satisfactorily 

 solved for any practical purposes of communicating to great distances. 

 In the deliberate judgment of one of the most eminent of English phys- 



* "• Ou the laws of electro-magnetic action." Edinlur(jh PhilosojjMcal Journal, Jan. 

 1825, vol. xii, p. 105. 



tPp. 110, 111 of the JoiTrnaljnst cited. Later experiments nnder varied conditions 

 have shown that Ohm's law (announced three years after Barlow's) of a simple ratio 

 of resistance to length is approximately correct. 



