140 BELL SYSTEM TECHNICAL JOURNAL 



The history of cable sheath development illustrates the value of 

 metallurgical research to the telephone system. Unalloyed lead was 

 first used because it was pliable, resistant to corrosion and could easily 

 be manufactured into pipe. Nevertheless, it has serious shortcomings. 

 Brittleness would not be expected in a material so soft and ductile, 

 yet repeated stresses caused by w^ind sway, mechanical vibrations, and 



Fig. 2 — View of piece of old cable sheath made of commercially pure lead, which 

 failed in service from intercrystalline fracture. 



movements due to temperature changes produce fine cracks in the 

 cable sheath through which moisture may enter the cable. An ad- 

 vanced stage of such cracking is shown in Fig. 2. In fact this effect 

 is so serious^^that, unless precautions are taken to minimize vibration, 

 cables sheathed with unalloyed lead cannot be shipped for long dis- 

 tances by rail or boat without serious damage. 



It was early found that the addition of three per cent of tin to lead 

 greatly decreased the susceptibility to this type of failure. This alloy 



