66 THE SCIENTIFIC PAPERS OF 



of the best conducting material, in which quality pure copper far 

 surpasses all but some of the precious metals and possibly pure 

 aluminium. If the conductivity of silver is expressed by 100, that 

 of pure precipitated copper may be taken at 90. The conductivity 

 of the copper of commerce varies, however, between extraordinary 

 limits ; and it may be accepted as a rule that all foreign matter 

 contained in it, whether metallic or otherwise, diminishes its con- 

 ductivity. Thus 2 per cent, of alloy is known to reduce the con- 

 ductivity of copper from 90 to 13, and even the best selected 

 copper used for telegraph conductors varies in practice as much 

 as 20 per cent, in conducting power. The foreign substance which 

 it is most difficult to remove from the copper is oxygen ; and a 

 process to effect this would be of considerable value. 



The insulating covering of the conductor is the most delicate 

 and essential part of the telegraph cable. It has to form an 

 effectual barrier against escape of the current throughout the 

 whole length, for a single flaw in this coating causes the failure 

 of an entire cable. Nor does a flaw show itself always in testing- 

 cables, however thoroughly, previous to their submersion ; for 

 experience has proved that flaws are produced gradually by the 

 chemical action of the galvanic current itself in any places where 

 the thickness of insulating coating has been considerably below 

 the average, either owing to an air bubble forced open by the 

 pressure of the water, or owing to an eccentric position of the 

 conductor. The latter defect may be produced either in the cover- 

 ing machine, or afterwards by exposure of the cable to the heat 

 of the sun, or to a strain producing a permanent elongation of the 

 copper ; in consequence of such elongation the gutta-percha en- 

 deavours to return to its original length and causes the copper 

 core by degrees to assume a serpentine position in the covering. 

 Gutta-percha was till lately thought almost a perfect non-con- 

 ductor of electricity ; but in dealing with long lines of submarine 

 electric telegraph its conductivity has become well established, and 

 is often a source of painful anxiety to the electrical engineer, 

 obliging him to search for other insulating materials. Glass and 

 other vitreous substances, which possess the highest insulating 

 properties, are of course inapplicable ; and amongst the resinous 

 insulators there is none that combines insulating quality with 

 tenacity and other desirable mechanical properties in so high a 



