96 



SUBMABINE CABLE LAYING AND EEPAIEING, 



same number of holes, to guide the wires immediately before 

 they pass on to the cable and keep them at equal distances 

 apart. A short space farther on, the cable passes through a 

 circular die in halves (the pressure being regulated by a hand 

 screw), which smooths down the wires as laid. The bobbins 

 are provided with rope or steel friction brake bands (seen in 

 Fig. 45), by which the tension on the wires can be adjusted. 



The machine is stopped at intervals to weld fresh wire on 

 and replace the empty bobbins by full ones. The welds in 

 different wires should always be at least 12 ft. apart. The 

 Thomson process of electric welding is now largely used for 

 welding the iron or steel armouring wires, and a short descrip- 

 tion of this process may be of interest. The two ends of the 

 wires to be joined are gripped in a special machine, and a heavy 

 current of low voltage is passed between them. In consequence 

 of the resistance offered to the passage of the current at the 

 joint, the ends of the wires are rapidly raised to a welding heat, 

 and when suflBciently soft, are forced into intimate contact by 

 means of mechanical pressure and firmly welded together. 

 Owing to the comparatively small resistance of the material to 

 be welded, it is necessary to use current of great volume to 

 obtain the high temperature required for welding. To 

 generate such currents directly and convey them to the 

 work would involve the use of extremely heavy conductors, and 

 to avoid this alternating currents are used, generated at a 

 convenient voltage, from 150 to 350 volts, and converted, 

 by means of a transformer, to a very low voltage, usually 

 about 1 or 2 volts, with a corresponding increase in current 

 strength. 



The following table shows the power expended and the time 

 required for electrically welding wires in common use in sub- 

 marine cables and in grapnel and buoy ropes : — 



