1886.J On the Self-induction of an Electric Current. 465 



magnetic have an almost invariable quantity dependent entirely upon 

 the coefficient of the electromagnetic action of the iron tube; and 

 independent of the resistance or nature of the wire. 



The experiments with an insulated iron sheath may be considered 

 as a forced condition which does not occur or need not be taken into 

 account, but the reaction of the iron tube is evidently electro- 

 magnetic, and as our atmosphere is also magnetic, we may assume 

 that its reaction would be similar though in a far less degree to that 

 which I have shown. 



Submarine Cables. 



The influence of a magnetic sheath upon an external insulated 

 wire is one of vast practical importance in relation to our electrical 

 submarine cables, as they are all constructed with an insulated wire 

 surrounded by a spiral sheath of exterior iron wires. In order to 

 study this question I made several cables of short lengths (1 metre) 

 and found (most fortunately for our practical applications) that when 

 the exterior iron sheath is divided as in a cable, or when the sheath 

 consists of several large wires in close contact, that its effects are 

 reduced to a mere fraction of what it would be if the external sheath 

 was a continuous tube ; this is entirely due (as I have shown in the 

 case of stranded wires as compared with solid) to the imperfect 

 formation of circular magnetism. I will cite a few examples illus- 

 trating this : a cable was formed of a similar copper wire and 

 insulation to that shown in Table III, the iron tube being replaced by 

 eight iron wires (of 2 mm. diameter each) wound with a slight spiral 

 of five turns per metre, this showed but 100 per cent, increase in the 

 force of the extra currents against 400 per cent, increase when the 

 same wire was in the iron tube ; the increased resistance in the 

 variable period was but 50 per cent, against 600 per cent, for the iron 

 tube, or only one-twelfth of the additional resistance caused by the 

 tube ; these wires were replaced by galvanised iron wire which 

 prevented magnetic contact, and the variable resistance became much 

 less, and when these were replaced by numerous fine iron wires the 

 effect was reduced to the minimum found, or only 20 per cent, 

 increased resistance in the variable period, or 30 times less than that 

 of a solid iron tube of much less iron. 



It is therefore evident that the circular magnetism plays an 

 important role, and that most fortunately our cable manufacturers 

 (without knowing the immense reaction which would be produced bv 

 a continuous sheath of iron) have constructed our cables with a 

 protection of iron divided into several iron wires instead of a 

 continuous iron sheath ; there are, however, many telegraph lines 

 which in passing through tunnels use a continuous iron tube as the 



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