Metal Working Exercises 5 1 



connection with electrical work, as the insulation would be 

 destroyed. Resin is therefore used as flux. 



Powdered resin being sprinkled well over the joint, the flame 

 from a blowpipe, or blowlamp, is allowed to play upon the joint 

 until it is well warmed up, when the joint is touched with a strip of 

 soft solder, also dipped in powdered resin. If a suitable tempera- 

 ture has been attained, the solder will run into the joint without 

 further trouble. A soldering iron is of little use in the jointing of 

 cables, as the cable conducts the heat away from the joint as 

 quickly as the iron supplies it, but when a joint has been " sweated " 

 as above, it may well be trimmed with a soldering iron. Files 

 should not be used for trimming ; all soldered joints should be 

 sufficiently clean from the iron, and it should be remembered that 

 one test of excellence is how little solder can be used to make a 

 perfect joint. 



Resin is not so easy a flux to work with as zinc chloride, the 

 tendency being to apply it too soon and ignite it, when it blackens 

 the joint, which consequently needs freshly cleaning. 



A great deal of trouble occurs in soldering through not using a 

 sufficiently high temperature. It should be remembered that the 

 temperature of the solder is of secondary importance ; it is the 

 work that must be hot, and as the soldering iron usually supplies 

 the heat by conductivity of the solder, the solder is generally well 

 melted when the work is still too cold for the solder to flow. When 

 the work is hot, however, the solder runs in quite easily, being kept 

 up to temperature by the work itself. 



A little practice with the iron will soon convince a student that 

 soldering is an easily acquired art, and he will be rewarded by the 

 production of neat and smooth-surfaced "jobs," instead of the 

 heavy mainly solder productions of the early stages. 



(B) Insulated. Cables such as those treated of above are 

 usually insulated, and this insulation must be replaced after solder- 

 ing, except of course, in the" case of "flexible" (e.g. 55 strands, 

 No. 32 wire), which need not be so treated as a rule. 



Insulation generally consists of four layers 



1. Pure unvulcanized rubber laid on the wires. 



2. Vulcanized rubber, one or two layers (if more than one, a 

 difference in colour is made to indicate the number). 



3. Spiral tape, soaked in rubber solution. 



4. Woven jute wrapping, impregnated with a bitumenous 

 composition. 



The insulation must be removed in successive layers, exposing 

 about an inch of each, and leaving sufficient wire or cable exposed 



