98 TUB TRANSATLANTIC LONGITUDE. 



circumstances; so that under no ordinarily practicable circumstances could a signal 

 from either station fail to traverse both parts of the circuit at that station before 

 passing on to the other. 



Since the investigation^ in 1850 to which I have alluded, the progress of science 

 has thrown light upon many points which then were subjects of doubt or of indi- 

 vidual opinion. The condition of an open galvanic circuit is now almost universally 

 conceded not to be essentially different from that of an interrupted conductor to an 

 electrical machine. The velocity of a current is also known to be dependent upon 

 its quantity, and therefore generally upon its intensity, as well as upon the resistance 

 of the conductor. But it appears questionable whether the law is so simple as has 

 been supposed by some, who have regarded the velocity as inversely proportional 

 to the capacity of the conductor multiplied by its resistance, and therefore, in a 

 homogeneous conductor, to the square of its length. For the problem, as it now 

 presents itself, does not pertain so much to the time for transmission of a given 

 signal, as to the time for its transmission with -a certain force, depending on the 

 sensitiveness of the receiving apparatus ; since the electrical impulse or disturbance 

 consists of a continuous series of molecular influences which propagate themselves 

 in every possible direction according to the inverse ratio of their several resistances. 

 And the form of the conductor, as well as other conditions, may essentially modify 

 the time requisite for the attainment of the prescribed force at the other extremity 

 of the line. A current may thus be temporarily established in part of an open 

 circuitj continuing until the battery and conductors have attained an electrostatic 

 equilibrium. The time required for attaining this equilibrium depends of course 

 simply on the capacity and form of the conductors, and on the energy of the 

 battery; but the first electrical impulse may reach. the most remote point of the 

 circuit before a portion nearest the battery has received its full charge. Similarly, 

 in a closed circuit, the distant extremity of the line may well be supposed to per- 

 ceive some slight electrical disturbance from a signal, before its full force is mani- 

 fested at intermediate points ; so that a signal might be received with a delicate 

 galvanometer at the farther extremity, before it could be recognized upon an electro- 

 magnet at half the distance. And this, too, apart from any consideration of in- 

 creasing intensity in the electromotor. 



The circuit formed by the two cables might, although broken at Valencia, thus 

 serve to establish what would practically be a momentary current at Newfoundland 

 when the battery at that station was introduced, deflecting the galvanometer there for 

 an instant ; and the change of statical condition in the cables at Valencia would 

 thereupon be manifest to the electroscope. But the closure of circuit at Valencia 

 would be accompanied by instantaneous deflection of the galvanometer, with cor- 

 responding insensibility of the electroscope. Thus a signal given by closing or 

 interrupting an insulated circuit at any point is instantaneously transmitted from 

 that point in both directions, and at fidl speed ; but the interval before it attains its 

 total force at any other point, must depend upon the character of the intervening 

 conductor. 



» Proc. Ainer. Assoc. Adv. Sci., 1850, p. 71 ; Am. Jour. Sci. XI, 6"?, 151. 



