CHEMISTRY IN THE TELEPHONE INDUSTRY 615 



tive of rendering metal brittle has been achieved in general by the prin- 

 ciple of introducing an impurity into the melt, which has a tendency to 

 segregate at the grain boundaries, thus facilitating subsequent fracture. 

 Each alloy requires some special consideration, both as to choice of 

 embrittling agent and heat treatment and working schedules for devel- 

 opment of proper grain size. 



Scarcely any metallic material has given the telephone industry more 

 concern than the lead alloy used for cable sheath. Pure lead is too 

 soft for the purpose and can be too easily damaged mechanically. 

 Years ago about 3 per cent of tin was alloyed with the lead as a harden- 

 ing agent. Tin was later superseded by 1 per cent of antimony, pri- 

 marily for reasons of economy, and it has been reported that twenty 

 million dollars have been saved to the telephone system by this substi- 

 tution alone. 



The use of these hardening agents affords an example of the disper- 

 sion hardening of metals, which has become familiar to the public most 

 conspicuously in the case of duralumin. It is the belief of metallur- 

 gists that the introduction into a molten metal of a constituent, which 

 is precipitated out in very finely divided form upon cooling the metal, 

 diminishes the deformability of the finished material by interposing it- 

 self in the slip planes among the atoms of the metal. 



Dispersion hardening in a metal as soft as lead represents a rather 

 extreme case, for lead at atmospheric temperatures is approximately 

 as deformable as steel at dull red heat. It has been found that the anti- 

 mony used as a hardening agent in lead cable sheath tends, especially 

 under the influence of repeated flexings such as those due to thermal 

 expansion, to redissolve in the metal and redeposit elsewhere. In this 

 fashion large particles of the antimony grow at the expense of small 

 particles, and the hard-worked portion of the metal is eventually de- 

 prived of antimony content and fracture occurs. 



This has been the source of considerable trouble in aerial cables, 

 especially at the bends in the cable which occur at the poles due to 

 expansion. The working out of this problem in fatigue of metals has 

 been a long process, but promises to bear further fruit in the develop- 

 ment of better hardening agents. One of these, a joint development 

 with the Western Electric Company and one which still remains to be 

 tested on a commercial scale, is calcium, which in the minute propor- 

 tion of 0.04 per cent has been found in laboratory experiments to pro- 

 duce a hardening well surpassing that of 1 per cent of antimony. 



Another interesting metallurgical problem is that of solders for use 

 in wiping joints in telephone cables. Somewhat to our surprise we 

 found that some of the supposed prejudices of workmen responsible for 



