470 



MEMOIRS OF THE AMERICAN ACADEMY. 



compare these reduced lengths. But other things besides the dimensions and quality 

 of the wire influence the velocity of electricity. Ohm's formula, already discussed, 

 contains a letter which represents the electrostatic capacity of the conductor. But he 

 did not live to see its full significance as revealed in subterranean lines of telegraph. 

 In the very year (1854) when Ohm was seized with a mortal illness, Faraday com- 

 municated to the Proceedings of the Royal Institution * of Great Britain, some re- 

 markable experiments, made originally by Mr. Latimer Clark, and then repeated, 

 under his inspection, at the Works of the Electric Telegraph Company. One hundred 

 miles of copper wire, covered with gutta-percha, were immersed in water, and whe 



n 



nt was sent through this circuit, phenomena resulted which 



were never 

 observed when the wire was surrounded by air. The immersed wire was charged like 



a Leyden jar, the gutta-percha serving as the dielectric, and the water as the outer 



coating. 



In th 



particular 



the copp 



was one sixteenth of 



inch 



in 



diameter, and the gutta-percha about one tenth of an inch in thickness 



Leyd 



© 



ement, the 



Regarded as 



inner coating, or surface of the copper, contained 8,300 



square feet, and the outer coating 



surface of 



with the gutt 



percha, contained 33,000 square feet. Experiments were made upon wires covered 

 with gutta-percha, and then enclosed in tubes of lead or iron, or buried in the earth, 

 and with similar results. By connecting the various subterranean wires between Lon- 



don and Manchester, a total length was obtained of 1,500 miles 



were introduced 



intervals of about 400 miles, they 



When galvanometers 



same observer in London 



If 



all under the eye of the 



ively affected ; the last 



current was sent into this wire the galvanometers 



Faraday explained this 



only after the long interval of two seconds 



the velocity of electricity through 

 supply the place of the oute 



transmission through such wires, when compared with 



(where 



no 



metallic surface 



the Leyd 



conductor is present to 



gement), to the 



increased electrostatic capacity of the wire under the inductive action, and to the time 



quired for the battery 



Thomp 



2 



furnish the additional amount of electricity. Sir William 



has proved that the electrostat 



VkS 



pacity of such a wire is equal to 



which ( V) 



th 



potential, (k) the specific inductive capacity of gutta- 



percha, (S) the surface of the wire, (r) the radius of the copper or the inner surface of 

 the gutta-percha, and (r') the radius of the outside surface of the gutta-percha. Every 

 hundred miles of wire, similar to that used in Faraday's first experiments, had an elec- 

 trical capacity at least equal to a Leyden battery with 8,300 square feet of coated sur- 



Royal 



1854. 



Electrostatic 



t 



