208 APPLIED SCIENCE 



failure of the first cable is not likely to be repeated in the case 

 of those now in use : 



The lecturer proceeded to explain that, when tested by the gal- 

 vanometer, there was very little difference in the force of a current 

 sent into 2,500 miles of the Atlantic Cable, whether the circuit was 

 or was not completed. This seemed rather hopeless for telegraphing 

 (he continued), where there was so much leakage, that the difference 

 could not be discovered between want of insulation and the remote 

 end. But if there were 49-50ths lost by defective insulation, it 

 would only make the difference between sending a message in nine 

 minutes instead of in eight. 1 



Sir William Thomson did not on this occasion mean to state 

 that there really was no difference when the farther end was 

 insulated or put to earth, but the instruments employed showed 

 very little difference, and on a subsequent occasion only about 

 one-fourth of the current which started was found to have 

 arrived at the remote end. The difference now is not one three- 

 hundredth part, and the current entering the cable when the 

 remote end is insulated is now, under the most unfavourable 

 circumstances, not one-hundredth part of that passing when 

 the remote end is put to earth, or, in other words, when the 

 circuit is completed. 2 



1 From Professor W. Thomson's lecture before the members of the British 

 Association at Dublin, 1857, as reported in the Glasgow North British Daily 

 Mail of September 4, 1857. 



2 The following data, supplied by Mr. Latimer Clark, Engineer to the 

 Anglo-American Company, will be interesting to those who have made this 

 subject their special study. The total insulation resistance of the whole 1866 

 cable, as it lies at the bottom of the Atlantic, is 1-316 millions of British 

 Association units, or, as Mr. Clark calls them, ohms. This is equal to 2,437 

 millions of ohms per knot after one minute's electrification. The 1865 cable 

 does not sensibly differ from the 1866 cable. Both lose half their charge in fi om 

 60 to 70 minutes The increase of apparent resistance due to electrification is 

 enormous ; thus, after thirty minutes' electrification the insulation resistance 

 is more than 7,000 millions of ohms per knot. Mr. Jenkin, in the Red Sea 

 Cable, did not observe a greater increase than 50 or 60 per cent, due to this 

 cause, and a similar amount has been generally observed on other cables. An 

 increase of 200 per cent, for gutta-percha is perhaps unparalleled, although an 

 even greater increase has been observed with india-rubber prepared by Mr. 

 Hooper. While the cable was on board the ' Great Eastern,' it behaved like all 

 other cables as to electrification, rising, for instance, from 681 to 1,051 per knot 

 during thirty minutes, at 18'3 C., so that the increased effect of electrifica- 



