146 BELL SYSTEM TECHNICAL JOURNAL 



For line wire in open wire circuits, hard drawn copper wire is used in 

 order to take advantage of the conductivity of copper and the in- 

 herently greater strength resulting from strain-hardening. Line wire 

 is subject to ice and wind loads, vibration fatigue, and in some localities, 

 severe corrosion. Where loading conditions are severe the copper- 

 cadmium and other high-conductivity, high-strength materials have 

 attractive possibilities but require further evaluation before their 

 introduction for general use. 



For drop wire — the conductor running from the telephone poles to 

 subscribers' buildings — a material with somewhat different properties 

 is required. Here lower conductivities can be tolerated but higher 

 strengths are necessary since the wire is smaller in size and long spans 

 are sometimes necessary. Several materials have been utilized. The 

 alloy most generally in service in the Bell System is composed of 98.25 

 per cent copper and 1.75 per cent tin. This is being replaced now as 

 a result of research development with a higher strength copper alloy 

 containing 3 per cent tin. This substitution makes possible a reduction 

 in gauge size of conductor from 17 to 18 without sacrifice in the strength 

 characteristics of the conductor. 



For most purposes ordinary electrolytic copper containing a fraction 

 of a per cent of oxygen is satisfactory. There are some limited appli- 

 cations, however, where the copper is subjected to high temperatures 

 in the presence of reducing atmospheres at some stage in the manufac- 

 turing process. Under these conditions, the presence of oxygen in the 

 ordinary copper produces a well-known embrittling effect. For these 

 applications a copper free from oxygen is used. 



A small but important application of copper in telephone circuits is 

 in the production of copper-oxide rectifiers. For this purpose a copper 

 imported from Chile is ordinarily used; for some obscure reason domes- 

 tic brands of copper have not generally proved so satisfactory. 



Copper in the alloyed form also is used extensively in the telephone 

 plant. One application, that for drop wire, has already been men- 

 tioned. Other extensive applications are for springs and contacting 

 members in electrical circuits and for structural parts where corrosion 

 resistance or other desired physical properties justify their use. 

 Nickel silver and to a lesser extent phosphor bronze find application 

 for springs. Brass is used primarily for wiper contacts since it lacks 

 the desirable spring properties of nickel silver and bronze. Included 

 in satisfactory spring requirements is long service life which depends 

 upon good fatigue characteristics and freedom, in many instances, 

 from the tendency to season crack. 



