52 Laboratory Arts 



to permit the making of the joint as already described. Great 

 care must be used in cutting the last layers of insulation, as a nick 

 in the wire will generally cause fracture there. 



After jointing, sweating, and trimming, the whole is cooled, the 

 joint well rubbed with rubber solution, and wound spirally with 



Jurecf^ SPIRALLY vtojcttaEo PURE WIRE on 



lissSsI 

 CABLE 



1. ,. ^ 3rff ' 



COVE* 



FIG. 46. Showing layers of insulation and cable ready for jointing. The extent 

 covered by successive wrappings of fresh insulating material, after jointing, 

 is also indicated. 



rubber tape, with considerable tension upon it. This will replace 

 the pure rubber insulation, the most effective of all. It should 

 overlap the next layer about " to J". The upper surface of that 

 already wound on is next smeared with rubber solution, and the 

 winding continued with the same material, but with much less 

 tension, again overlapping the next layer by ". The end of the 

 rubber strip being held, a quantity of " sticky tape " ordinary tape 

 soaked in bitumenous rubber compound is spirally wrapped along 

 the joint until the whole is of the original diameter, and the ends 

 of the hemp covering are enclosed in the final wrapping. This 

 tape adheres without additional treatment, and the free end may be 

 left simply pressed against the underneath layer. 



The rubber and " sticky " tapes are most serviceable when " 

 wide. They are sold by weight. Chatterton's compound may be 

 found useful in finishing these joints, especially when the joints are 

 exposed in damp situations. Chatterton's compound is applied hot, 

 like sealing wax, and may be moulded into any desired form by 

 the fingers, to which it adheres excellently unless they are frequently 

 dipped in cold water, or smeared with powdered chalk. 



EXERCISE 2. To make a box to hold i cubic inch. This 

 may be constructed in sheet brass. 



On a sheet of thin brass, the development of a cube is to be 

 drawn by means of a sharp instrument, e.g. a steel knitting needle 

 well ground to a very sharp point. The plan may be either of those 

 shown in Figs. 47, 48. Allowance must be made for the thickness 

 of the metal, and for flanges which will be subsequently soldered. 



