356 THE ELECTRIC TELEGRAPH. 



The highest value, probably, yet found for the conduct- 

 ing power of pure copper is sixty times that of pure mercury. 

 Commercial copper may be considered of good quality when 

 its conducting power is over fifty. 



Dr. Matthiessen's results on this subject are valuable. He 

 has found the conducting powers of annealed wires of the 

 following sorts of commercial coppers, when the conducting 

 power of pure copper at 15'5C. is taken as 100, to be : 



1. Lake Superior, native, not fused 98-8, at 15-5 



2. Ditto, fused, as it comes in commerce ... 92*6, at 15 -0 



3. BurraBurra 88-7, at 14*0 



4. Best selected' 81-3, at 14*2 



5. Bright copper wire 72*2, at 15*7 



6. Tough Copper 71-0, at 17'3 



7. Demidoff 59-3, at 12-7 



8. RioTinto 14-2, at 14-8 



76. Measuring the Insulation Resistance of the Core. The 

 first measurements of the electrical conditions of submarine 

 wires were made with a simple galvanometer, inserted 

 between one end of the cable to be measured and one of the 

 poles of a galvanic battery, the other pole of which was to 

 earth. When the farther end of the cable was put in con- 

 nection with the earth the current of the battery passed 

 through both cable and galvanometer, deflecting the needle 

 of the latter to a greater or less degree as the cable was 

 shorter or longer, or the section of the conductor greater or 

 smaller, than that of some piece of cable taken as a standard. 

 This test served to show at the same time that the conductor 

 of the cable was entire, and gave a remote idea of its length. 

 When the end of the cable was insulated, however, or dry and 

 free in air, the current of the battery passed through the gal- 

 vanometer as before, but, having to traverse the insulating 

 medium surrounding the conductor, its intensity was materi- 

 ally lessened, depending upon the degree of insulation of the 

 cable. If no deflection was observed, it was presumed that 

 the cable was good ; and if the current was strong enough 

 to deflect the needle, the magnitude of its deflection gave an 



