SCIENCE-GOSSIP. 



95 



REPAIRING AN ELECTRIC CABLE. 

 By A. H. Bechervaise. 



"TTORTY years have elapsed since the first 

 submarine cable was successfully laid be- 

 tween Ireland and Newfoundland by the steamship 

 "• Great Eastern."' an event which excited general 

 public interest at the time. Since then, cable 

 laying and repairing has become such an every -day 

 and matter-of-fact occurrence that it is rarely 

 thought of by these not personally connected with 

 this important and interesting scientific industry. 

 To them, however, it has great fascinations, and 

 an account of a recent repair of a submarine cable 

 in these islands may be not without interest to 

 your readers. 



unknown resistance. In the old days it was very 

 difficult to ascertain and allow for this resistance, 

 and it is due principally to Mr. A. E. Kennelly 

 that this difficulty has now been overcome. He, 

 finding that a natural law governed the action of 

 different strengths of current passing through the 

 unknown resistance, formulated a method by which 

 it could be accurately ascertained and eliminated. 

 This method is to-day generally adopted in cable 

 testing so successfully that it is possible sometimes 

 to locate a break to within a few yards. 



The cable between the islands of Teneriffe and 

 La Palma being broken, and tests from shore 



Broken Marine Telegraph Cable. 



The question is frequently asked, " How do you 

 find out where the cable is broken ? " When the 

 break is a clean one, roughly, in theory this appears 

 to be a simple matter, as the resistance offered by 

 the copper core of the cable to a known battery 

 power is most accurately ascertained, both before 

 and after the cable is submerged. A cable which 

 offers, say. a resistance of 10,000 ohms (the 

 ohm is the unit of electrical measurement of 

 resistance, in the same sense as the inch or 

 foot is in the measurement of length) when 

 perfect and 5,000 ohms when cleanly ruptured, 

 may be presumed to be broken exactly in the 

 \t of its length. This, however, is by no 

 means always the case, as immediately the cable 

 is broken and the copper conductor exposed to the 

 sea. apart from the natural magnetic— or, as they 

 are termed, ullurtc — currents which circulate 

 through the cable, chloride salts form on the 

 exposed copper and offer a very varying and 



Tiwsrt.iV I'. ' 



having satisfactorily located the position of break, 

 the cable steamer was about to start for the 

 repairing ground, and I was ordered to proceed to 

 the Teneriffe end of the broken cable at the other 

 side of the island. I left Santa Cruz in a large, 

 roomy carriage, which, however, was so packed 

 with the necessary apparatus, batteries, etc., as to 

 allow little spare room for myself After toiling 

 up the steep road to a height of nearly 2,000 feet, 

 we reached, and passed through at a gallop, the 

 fine old city of Laguna. Leaving this behind, the 

 luties of the island began to present them 

 The road was exceedingly pretty, — a 

 continuous avenue of eucalyptus, oleandei 

 acacias, and cork trees, between which 1 gol 

 glimpses of the pretty landscape, dotted with 

 orange trees laden with golden fruit, date-palms, 

 and fields green with maize Wnh ,u< li plea .run 

 surroundings the lime quickly passed, and I 

 toppl place, La Matan/.i, 01 



