414 



REPORT 1864. 



fore, that gutta percha is the mo.st desirable material that can be employed 

 as an insulator, it then resolves itself into the question, What additional cover- 

 ing, and what additional strength, is necessary to enable the engineer to pay 

 out of a ship a length of 1600 miles into deep water so as to deposit it with- 

 out strain at the bottom of the ocean ? This is one of the questions the 

 Committee was called upon to solve, and for this very important object the 

 following experiments were instituted :— - 



Experiments to determine the Strength of the Central Core, and the 

 Materials of which it is composed. 



Summary of Results. 



(a) In this experiment the core was not broken, but laid open for inspection. 



(b) One wire broke first, and subsequently the others followed. 



It is of considerable importance in marine cables to have all the parts as 

 nearly uniform as possil)le, and in the foregoing experiments on the central 

 core will be observed the dift'crence of elasticity which exists between the 

 copper-wire conductor and the insulator or gutta-percha covering. In the 

 former case we have at the point of fracture an elongation of 6-71 inch 

 and a permanent set of 6-71 inches in a length of 2 feet 6 inches, whereas in 

 the insulating material there is 8-735 inches of extension and only 0-215 

 inches of a permanent set iu the same length. These discrepancies of elasti- 

 city and elongation are of considerable importance, in so far as they show that 

 in cables of this description we have to contend with materials of different 

 properties, the first being to that of the second as 6-71 : 6-215, or as 1-08 : 1 ; 

 in other words, the gutta percha is 8 per cent, more elastic than the copper 

 conductii g wire ■s\hich it covers. These facts account for the extraordinary 

 developme it which presented itself on cutting a slice of the gutta-percha 

 covering from the wires which, on being liberated burst through the opening 

 in the form of loops, as shown in the annexed figure. 



the wire bursting out in this and in a former experincnt, after being 

 forcibly stretched and liberated from its confinement, in the foim shown 

 above at a, a, a. 



Prom these experiments will be noticed the facility with which the copper 



