370 THE ELECTRIC TELEGRAPH. 



wire with naphtha, warms it with his spirit- flame, and smears 

 it with some of the compound of resin, tar, and gutta-percha, 

 invented by "Willoughby Smith. The joint is then screwed 

 into a holder (Fig. 170), where one of the knobs of gutta- 

 percha, A AI, is gently warmed until the gum is soft. It is 

 then drawn carefully up to the other knob, leaving, on its 

 way, a perfect tube of gutta-percha upon the wire. The 

 superfluous percha is. removed, the other knob warmed and 

 drawn in the same way over the tube already formed, which 

 is at the same time heated sufficiently to make the two 

 adhere. A thin coating of Chatterton's compound is put 

 over this ; and, finally, after kneading the plastic tubes well 

 together between the finger and thumb, an oblong piece of 

 sheet gutta-percha is warmed in the lamp, and the whole 

 joint clothed with it. When he has kneaded the whole into 

 a homogeneous mass, warming it gently from time to time 

 as it gets consolidated, the operator works the joint as cylin- 

 drical as possible with his hands, and finishes it off on the 

 outside by burnishing or ironing with a small polished steel 

 tool, heated to a degree sufficiently high to smooth the 

 percha. 



The Gutta-percha Company do not make two tubes of the 

 knobs AAj, but simply draw them together over the joint so 

 as to form a single tube, which they cover up in a piece of 

 sheet gutta-percha. The methods are, perhaps, equally good. 

 Sometimes also joints are made with alternate coverings of 

 gutta-percha and compound, so as to resemble exactly the 

 insulator. 



83. Testing the Joints. Joints are the weak points of 

 submarine cables. It is therefore essential to test those 

 which occur between the lengths of core with the utmost 

 precision. The old method of testing joints was to get a 

 steady deflection by the insulation current after the battery 

 had been on the core or cable some time, during which the 

 joint was held in the air, dry ; it was then suddenly plunged 

 into water in electrical connection with the earth ; and any 

 movement of the galvanometer needle, however slight, was 

 taken as an indication of an inferior joint. 



