Sir yohn Snell 251 



station at Deptford on the Thames, to supply current at 

 ten thousand volts pressure to transformers in the London 

 area. For this station he designed great fifteen-hundred 

 horse-power alternators worked by Corliss engines, and 

 since ordinary copper wire was not suitable for carrying 

 so heavy a current he invented a form of main consisting 

 of two copper tubes, one inside the other, and insulated 

 one from the other by Manila paper soaked in certain 

 resins and oils. The writer of this chapter was shown a 

 small piece of Ferranti's original main, which is preserved 

 in the office at the Bedford Power Station. 



These first mains were seven miles long, and four cables 

 were laid. They carried their load well enough, but very 

 strange things began to happen. A large current was 

 found to flow into the Deptford main when no current 

 was taken out at the London end, and it was discovered 

 that these long mains actually became condensers — you 

 might say Leyden jars, each with a very considerable 

 electric capacity. But the worst trouble was this. When 

 these long concentric mains were switched on suddenly or 

 disconnected there was often a failure of insulation 

 between the outer copper tube and the protecting steel 

 tube. This was due to the ' mass ' of the electric current. 

 The problem was solved by Partridge, the Electric Supply 

 Company's engineer, who devised a means for switching 

 on the current gradually instead of suddenly. 



Another valuable invention of those early days was 

 the ' cut-out,' the function of which is to interrupt the 

 current in a main or branch should it exceed a safe 

 strength, as may happen in the case of two wires short- 

 circuiting. A cut-out is a safety fuse inserted in the cir- 

 cuit ; usually it is made of a short piece of lead wire, which 

 will melt if the current becomes too strong. 



In spite of many experiments, it was a long time before 

 anything better than the carbon filament for lamps was 

 discovered. True, the lamps were so much improved that 



