SUBMARINE TELEGRAPHY 241 



the meeting of the British Association this year at Nottingham, 

 and published in the ' Athenaeum ' of September 15. 



The signalling arrangements on board ship and on shore 

 present this peculiarity, that there is no voltaic circuit. The 

 current is received at Valentia in a Leyden jar or condenser, 

 which acts as a sort of elastic reservoir. When receiving, it is 

 alternately charged by the cable, and discharged back into the 

 cable ; while the galvanometer placed between the condenser and 

 the cable indicates the alternate forward and backward tide in 

 the current. Similarly, when Valentia wishes to send signals, it 

 charges the condenser positively or negatively by induction from 

 the battery, and thus causes corresponding movements in the 

 charge of the cable. This arrangement, already alluded to in 

 the testing arrangements, is due, we believe, to Mr. Varley, and 

 prevents to a great extent the action of earth-currents, which 

 would otherwise be found troublesome with so sensitive a re- 

 ceiving instrument as the mirror galvanometer. Much has been 

 said concerning these earth-currents, and some people thought 

 they would render signalling across the Atlantic impossible. 



Different parts of the earth and sea are found to be at dif- 

 ferent electric potentials. One part is electro-positive or electro- 

 negative to another. There is, that is to say, the same difference 

 between two parts of the earth that exists between the two poles 

 of a battery. If, then, these two points are joined by a wire, a 

 current will flow through that wire as if from a battery, and 

 this current is termed an earth-current, to distinguish it from a 

 current produced by an ordinary voltaic battery. This difference 

 of potential between two given spots, such as Newfoundland and 

 Yalentia, is not constant, but continually varies. So does the 

 current it produces. The current, and the variations in the 

 current, interfere with the signalling current, disturbing the 

 distinctness of the signals. When no voltaic circuit exists, no 

 direct current will flow from one end of the cable to the other, 

 except that caused by the discharges into and out of the con- 

 denser ; but a change in the potential of either station will still 

 have some disturbing effect, by changing the charge in the con- 

 denser. When very rapid changes take place in the electric con- 

 dition of the centre, a magnetic storm is said to be taking place, 

 and this, on all lines, will occasionally put a stop to signalling. 

 VOL. n. R 



