326 Transactions . — Miscellaneotts . 



tor ; (2) the covering to prevent the dissipation of the electri- 

 city, known as the insulator ; (3) a padding of some soft sub- 

 stance, as jute ; (4) the iron sheathing. 



1. The Conductor. — This, of course, is the essential part, 

 the other parts being subsidiary to it. In cables it is always 

 made of copper, on account of the high conducting-power of 

 that metal. Copper has been considered by many physicists 

 to have the highest conducting-pow^er for electricity of all 

 known substances, though other physicists have thought 

 silver to be slightly superior. The question is not one of any 

 importance to the telegraph engineer, as the high price of the 

 latter metal precludes its use. It is interesting, however, to 

 observe that very carefully devised experiments in this matter 

 have not been attended with uniform results. These dis- 

 crepancies are easily accounted for by the difficulty of ■ getting 

 the metals absolutely pure. Alloys are usually of much 

 greater resistance, not only than the mean of their con- 

 stituents, but greater than of either constituent. A very small 

 admixture of a second metal with one of high conductive 

 power such as copper produces a very marked effect in the 

 resistance, while a trace of a non-metallic body increases the 

 resistance enormously. It is stated that the copper of com- 

 merce has six or seven times the resistance of the pure metal. 

 Thus telegraphy has created a demand for pure copper. It 

 has also done much to supply that demand, as the residue 

 from batteries in which copper is reduced to the state of 

 metal furnishes a copper very suitable for telegraphic pur- 

 poses. It may be mentioned here that, while silver and 

 copper have the highest conductivity for a given volume, alu- 

 minium has the highest for a given weight. The latter metal, 

 therefore, may prove of service for some sorts of electrical 

 communication, but it is not likely to be used for submarine 

 cables. In the first place, the weight of the copper, varying 

 from 100 lb. to 400 lb. per nautical mile, is only a fraction 

 of the weight of the cable. In the next place, an aluminium 

 conductor of less w^eight but of greater sectional area would 

 require a greater amount of insulating material, and this, 

 again, would requii'e a greater amount of sheathing-wire. 



Copper, then, as having practically the highest con- 

 ductivity, or, in other words, the smallest resistance for its 

 volume, and not being too dear in price, always forms the 

 conductor. In ocean cables there is usually a strand of seven 

 wires twisted together. This arrangement is considered 

 better than one thick wire, in which a flaw involving fracture 

 would render the cable useless till repaired. A fracture of one 

 or more wires of the strand has no effect on the signalling, 

 unless all the wires are broken in the same place. Joints are 

 the weakest places, but there the precaution is always taken 



