Prof. Faraday on Subterraneous Electro-teleyraph Wires. 397 



send you ; the effects are produced with wii-es covered with gutta 

 percha and laid in the earth. 



" A very remarkable phsenomenon is constantly observed on 

 long, well-insulated telegraphic lines. Suppose one extremity, 

 B, of the wire be insulated, and the other, A, be connected 

 with one pole of a battery of which the other touches the earth ; 

 at the instant of communication a brief current is observed in 

 the near parts of the wire in the same direction as the instanta- 

 neous current which would exist if the extremity B were con- 

 nected with the earth ; on lines of perfect insulation no trace of 

 this current remains. Suddenly replacing, through the action 

 of a commutator, the battery by an earth conductor, a second 

 instantaneous current is obtained of an intensity nearly equal to 

 the lirst, but in the inverse direction. Finally, breaking the 

 communication of A with the battery and also the earth, so as 

 to insulate this extremitj^, and uniting the end B at the same 

 instant with the ground, an instantaneous current is observed 

 nearly equal in intensity to the former, and this time in the 

 same direction as the first, i. e. as the continuous current of the 

 battery. This last experiment can only be made on a double 

 subterranean line ha\ing the two extremities A and B at the 

 same station. One might at first sight suppose these phseno- 

 mena to be due to secondary polarities developed on the wire, 

 but many facts oppose such a conclusion. 1 . The phsenomena 

 are more striking as the wire is better insulated. 2. The cur- 

 rents are much more brief than those due to secondary polarities. 

 3. Their intensity is proportional to the force of the battery, and 

 independent of the intensity of any derived current that may 

 occur in consequence of imperfect insulation ; it follows that the 

 intensity of the instantaneous currents can greatly surpass the 

 maximum intensity which secondary cm-rents in the same cir- 

 cuit could acquire. 4. Finally, the intensity of the instanta- 

 neous currents is proportional to the length of the wire, whilst 

 an inverse relation ought to occur if the currents were due to 

 secondary polarities. 



" The ])ha;nomena are easily comprehended if we recall the 

 beautiful experiment by which Volta furnished the most striking 

 proof of the identity of galvanism and electricity. He showed 

 that on communicating one of the cuds of his pile with the earth, 

 and the other with the interior of a non-insulated Leyden bat- 

 tery, the battery was charged in an instant of time to a degree 

 proportional to the force of the pile. At the same time an instan- 

 taneous current was observed in the conductor between the pile 

 and the battery, which, according to Hitter, had all the proper- 

 ties of an ordinary current. Now it is evident tliat the subter- 

 raneous wire with its insulating covering uuiy be assimilated 



