544 





[October 8, 1891 



Fig. 5, has brown paper soaked in ozokerit rolled round 

 it to a thickness of about five-eighths of an inch. Outside 

 this is slipped a larger copper tube, B, Fig. 5, and the 

 whole is drawn through a taper die under great pressure, 

 which has the effect of forcibly compressing the paper 

 and consolidating the mass. Next, more brown paper 



Wrought 

 ircn tube 



Inner 



copper tube 



Waxed 



paper 



insulation 



Fig. 6. — Cross-section of the Ferranti main ; full size. 



soaked in melted ozokerit is rolled on, to a thickness of 

 one-eighth of an inch, and the whole slipped loosely into 

 an iron tube, D, Fig. 5, which protects the cable sub- 

 sequently from mechanical injury. To fill up any air 

 spaces that may have been left between the iron and the 

 outer copper tubes, the 20-feet section is placed over a 



The object of using concentric tubes is twofold— first, 

 as the outer copper tube is kept practically at the poten- 

 tial of the earth, it is impossible to get a severe shock 

 unless the inner tube is touched, and this, of course, 

 can only be done by first cutting through the outer ; 

 second, the effective increase of the resistance and of the 

 self-induction which occurs with rapidly alternating cur- 

 rents in consequence of the mutual action of Jthe currents 

 in different parts of the conductor on one another is 

 much less for a given cross- section of copper with con- 

 centric tubes than with two insulated rods placed side by 

 side. For example, Sir William Thomson has calculated 

 that if copper be employed in the form of a solid rod, 

 I "2 inch in diameter, the resistance for an alternating 

 current of a frequency of 80 per second will be 31 per 

 cent, greater than for a steady current. 

 \ It is very questionable, however, whether these ad- 

 vantages of using concentric tubes are not more than 

 compensated for by the large electrostatic capacity that 

 such a cable possesses. For, as is now fully recognized, 

 the combination of capacity and self-induction can by a 

 species of resonance cause the difference of potential in 

 the circuit to be far greater than the E.M.F. of the 

 dynamo itself, and in certain cases^ very dangerously 

 greater. 



As soon as the Deptford main was constructed to stand 

 10,000 volts, it was found that one of the dynamos seen 

 in Fig. 4 broke down at this pressure, and therefore for 

 many months the current was sent from Deptford at only 

 5000 volts ; next, the transformer room at the Grosvenor 

 Gallery was burnt down through carelessness, some ^8000 

 worth of transformers destroyed, and a portion of Lon- 

 don left in darkness for two or three weeks. New trans- 

 formers were hastily, too hastily, constructed, and the 

 current was turned on again at the commencement of 

 last December ; but after a few days the transformers 



Fig. 7. — Ends of two pieces of main, tapered ready for jointing, a, copper rod to malje electric connection between inner copper tubes ; e, waxed paper 



coned like a pencil. 



fire, and melted wax pumped in between the two through 

 a tube inserted in a hole drilled in the middle of the iron 

 tube. 



Fig. 6 shows a cross-section of the finished main full 

 size, and as the sectional area of the metal in each of the 

 copper tubes is about a quarter of a square inch, the main 

 can transmit about 2000 horse-power at io,cco volts. 



were, one after another, short-circuited by the electric 

 current sparking from the primary coil to the iron core 

 of the transformers, and all the houses on the London 

 Electric Supply Corporation's system again left in dark- 

 ness during the nearly perpetual night of a densely foggy 

 winter. The Metropolitan Electric Supply Company— 

 which also distributes an alternating current by means of 



Fig. 8. — Ferranti main, jointed, f, copper sleeve slipped over two ends of outer copper tubes, and then corrugated with special tool ; 

 over two ends of iron tubes, and corrugated with special tool ; h, screw-hole to run in melted wax, i. 



sleeve slipped 



The main being constructed in lengths of only 20 feet, 

 some 1500 joints have had to be made in 6 miles of main, 

 or 6500 joints altogether in the five mains which have 

 been laid from London to Deptford. These joints have 

 been made without solder, in the way shown in Figs. 7 

 and 8, pressure alone between the copper tubes having 

 been relied on to maintain good contact. 



NO. 



I 145, VOL. 44] 



transformers, but from several central stations in the heart 

 of London itself, and therefore requiring to use only 1000 

 volts and a single transformation — came to the rescue in 

 certain districts, but in others the householders had to be 

 left to their fate, as it would have been far too expensive 

 to run special mains from the Metropolitan Company's 

 stations merely as a temporary expedient. 



