SCIENCE AND PRACTICE. 369 



warms the percha, for a distance of about three inches from 

 each of the ends, with a spirit-flame, and, when sufficiently 

 soft, pushes it back until it forms an enlargement as at A and 

 A t , Fig. 170. The wires of the copper strand are then cut 

 off at different distances between the bulbs and the ends. 



There are two ways in which joints are made in the con- 

 ductors of cables. The first is called the " scarf-joint." It 



Fig. 170. 



is made by filing off the two ends of the strand at a corre- 

 sponding angle ; fitting and soldering the slanting faces. 

 When this is done, the whole is wrapped round with two 

 coats of fine copper wire, the first wrappering being soldered 

 all the way, the second only on each side of the joint ; by 

 which, should the conductor part at this point, the outer 

 wrapper-wire will preserve electric continuity. Such an 

 accident might easily occur ; and, in that case, the outer wire 

 would only be extended like a spiral spring. The other 

 method, which is sometimes used, is less convenient for 

 strands of seven than for those of three wires. The wires of 

 the opposite ends of the strand are opened out, and each are 

 joined separately. Each separate joint is made with slanting 

 faces, soldered like the scarf-joint, but not wrappered, and 

 a distance of at least half an inch left between each sepa- 

 rate soldering. When the soldering is finished, the work- 

 man endeavours to bring the same spiral twist into the 

 soldered wires as in the rest of the conductor. This is 

 best done by having previously coiled a length upon the 

 floor, and put a couple of reverse twists in it, which are 

 afterwards concentrated in the soldered place, to give it the 

 right spiral. 



The soldering completed, the operator washes his soldered 



B B 



