204 APPLIED SCIENCE 



many ways. It was found that if large enough batteries were 

 used, and care taken to obtain very sensitive instruments, some 

 current might always be made to pass between the copper and 

 the outside of the insulator ; in other words, no insulator offers 

 an infinite resistance to the passage of a current. It was not 

 difficult to judge roughly whether the amount of leakage, as it 

 might be termed, was serious enough to damage a cable ; but 

 unfortunately, small faults are apt with time to become large 

 faults, and the rough method was quite useless as a means to 

 detect small faults in long cables. As the cable increased in 

 length, the leakage even through a good insulator became so 

 considerable, that two or three bad places would make no very 

 sensible difference in the deflection observed ; and the galvano- 

 meters used became less and less sensitive as their deflections 

 increased, so that the addition caused by a moderate fault be- 

 came imperceptible. Then the galvanometers were not constant 

 in their indications, so that the deflection of to-day was a very 

 imperfect guide as to the deflection to be expected to-morrow. 

 The galvanometers used by different observers were seldom or 

 never compared. Moreover, the batteries used varied, and their 

 properties were not examined ; little attention was paid to the 

 temperature of the cable, although this has an immense effect on 

 the leakage to be observed ; finally, and worst of all, the cables 

 were not immersed in water, and fifty faults might in that case 

 exist in a cable without producing any sensible effect, either on 

 this old rough test or on any other. Under these circumstances, 

 is it surprising that cables were laid which contained many 

 serious faults, and that, after a short and uncertain period, 

 depending on many circumstances, they ceased to transmit 

 messages ? Is it unreasonable to expect that, under a system 

 by which the existence of any sensible inequality in the insula- 

 tion of a cable is rendered impossible, the cables recently laid 

 may continue in perfect working order for an indefinite period ? 

 All experience has shown that sound gutta-percha retains all its 

 valuable properties in deep or shallow water, completely unin- 

 jured by use or time. The only decay ever observed has been 

 at bad joints, air-bubbles, or impurities. 



It is, again, to Sir W. Thomson that we owe the first sug- 

 gestion of an accurate method of testing the insulation of a cable. 



