By EARLE E . S C H U M A C H E R a n d W . C. E L L IS
T N th e d ev elo p m en t of electrical co m m u n icatio n , m etals an d alloys h av e p lay ed a n o te w o rth y p a rt. T o em phasize specifically th e u tilizatio n of m etallic m a te ria ls th e telep h o n e h a n d s e t serves as an a d m ira b le exam ple. T h e assem b ly of in tric a te p a rts in th is sm all piece of a p p a ra tu s , show n sectionalized in Fig. 1, c o n ta in s sev en teen m etallic elem ents, e ith e r alone or in co m b in a tio n as alloys.
T h e Bell S y stem has there fo re c o n d u c ted ex ten siv e m e tallu rg ical researches, an d th e discoveries an d d ev elo p m en ts h av e been nu m ero u s.
S pace p e rm its a discussion of o n ly a few of th e d e v e lo p m e n ts re la tin g to th e m ore ex ten siv ely used m a teria ls. T h ese com prise th e alloys of lead, copper, zinc a n d alu m in u m , an d th e precio u s m etals, a n d m a g n etic m aterials.
Le a d a n d Al l o y s o f Le a d
L ead alloys are used p rin cip a lly as s h e a th in g for cable, an d as solders for jo in in g cable sh e a th a n d m a k in g electrical co n n ectio n s in a p p a ra tu s .
C ables re p re se n t one of th e la rg e st single item s of in v e s tm e n t;
a p p ro x im a te ly nin ety -fiv e p er c e n t of th e Bell S y s te m ’s to ta l wire m ileage is co n ta in e d in lead or lead alloy sh e a th a n d th is sh e a th req u ires a n enorm ous a m o u n t of lead a n n u a lly in its p ro d u c tio n . T h e la rg e st size cable m ade b y th e S y stem co n ta in s 4242 co p p er w ires. T h e sam e n u m b e r of open w ires on telep h o n e poles w ould ta k e 70 row s of poles each ca rry in g 60 w ires. U n d e r one s tre e t to d a y in N ew Y o rk C ity th e re are 282 cables c o n ta in in g a b o u t 560,000 w ires.
Since th e w ires in th e cable a re in su la te d from one a n o th e r only by th e p a p e r or tex tile w rap p in g s o r sh e a th s a n d b y th e d r y a ir c o n tain ed in th e cable, th e presence of even a slig h t a m o u n t of m o istu re will in te rfe re w ith tran sm issio n b y d ra stic a lly red u cin g th e in su latio n resistan ce. A p o sitiv e p re ssu re of d ry n itro g en is m a in ta in e d in som e cables as ad d itio n a l p ro te c tio n a g a in st m o istu re e n tra n c e a n d to d is close sh e a th b reak s. C o n tin u e d efforts a re m ade, th e refo re, to im p ro v e cable sh e a th so as to keep sh e a th failures to a m inim um .
* Based upon a paper published in Metal Progress, N ovem ber 1939.
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Fig. 1—Schematic cross-section of handset showing utilization of metallic materials.
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T h e h isto ry of cable sh e a th d e v e lo p m e n t illu s tra te s th e v a lu e of m etallu rg ical research to th e telep h o n e sy stem . U n allo y ed lead w as first used because i t w as pliable, re s is ta n t to corrosion a n d could easily be m a n u fa c tu re d in to pipe. N ev erth ele ss, it h as serious sh ortcom ings.
B rittle n e ss w ould n o t be ex p ected in a m a te ria l so so ft a n d d u ctile, y e t re p e a te d stresses cau sed b y w ind sw ay, m echanical v ib ra tio n s, an d
1&Pr
Fig. 2—View of piece of old cable sheath made of commercially pure lead, which failed in service from intercrystalline fracture.
m o v em en ts d u e to te m p e ra tu re chan g es p ro d u c e fine crack s in th e cable sh e a th th ro u g h w hich m o istu re m a y e n te r th e cable. A n a d v a n c e d stag e of such c rack in g is show n in F ig. 2. In fa c t th is effect is so serious th a t, unless p re c a u tio n s a re ta k e n to m inim ize v ib ra tio n , cables sh e a th e d w ith u n allo y ed lead c a n n o t be sh ip p ed for long d is
ta n c e s b y rail or b o a t w ith o u t serious d am age.
I t w as e a rly found t h a t th e ad d itio n of th re e p er c e n t of tin to lead g re a tly decreased th e su sc e p tib ility to th is ty p e of failure. T h is alloy
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sta n ces reveals a clue to th e causes p ro d u cin g e a rly failure a n d th u s suggests m eth o d s b y w hich th e failures m a y be elim in ated .
In d eveloping new alloys such as h av e been described a n d in stu d y in g th e causes of failure of these alloys in service, ex ten siv e la b o ra to ry facilities are req u ired . F o r exam ple, th e Bell T elephone L a b o ra to ries possess an ex tru sio n press, show n in Fig. 3, for ex p erim en tal stu d ies
Fig. 3—L aboratory extrusion press for th e study of the extrusion process and for the production of experim ental cable sheathing alloys.
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Fig. 4 Metallographicequipment usedin the studyofmetal problem!
M E T A L L I C M A T E R I A L S I N T H E T E L E P H O N E S Y S T E M 145 lead-calcium sh ea th ed cable w ere in stalled on poles alongside of sim ilar len g th s of cable w ith s ta n d a rd le ad -an tim o n y sh eath . V arious sh e a th thicknesses ran g in g from .075 to th e s ta n d a rd .125 inch were in stalled for com parison a n d to exp ed ite e a rly failure. In ad d itio n to th e co m parison betw een alloys th is te s t will also give in fo rm atio n reg ard in g th e m in im u m thickness of sh e a th w hich m a y be em ployed w ith b o th th e s ta n d a rd an d th e ex p erim en tal alloys.
Fig. 5— X -ra y d iffraction a p p a ra tu s show ing cam eras m ounted for identification of s tru c tu re a n d precision m easu rem en t of la ttic e co n stan t.
Besides th e ir ap p lic a tio n as cable sh e ath in g m aterials, lead alloys a re also exten siv ely used by th e Bell S ystem as solders, storage b a tte ry plates, fuses an d as corrosion p ro tectio n coatings.
Co p p e r a n d Co p p e r Al l o y s
U nalloyed cop p er finds a p p licatio n as wire in th e lead-sheathed cables a lre a d y discussed, in open wire c ircu its an d in cen tral office e q u ip m en t. In th e telephone p la n t th ere are eig h ty m illion m iles of it— enough to span th e d istan ce from th e e a rth to th e m oon th ree h u n d re d th irty -fiv e tim es. T o o b tain th e low est transm ission losses, cable co n d u cto rs consist of high co n d u c tiv ity annealed copper wire.
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148 B E L L S Y S T E M T E C H N I C A L J O U R N A L low m agnetic loss. Likewise, perm anent m agnet m aterials are frequently referred to as “ h a rd .”
M E T A L L I C M A T E R I A L S I X T H E T E L E P H O S E S Y S T E M 149 section w hen h o t, b u t w hen cold it is exceedingly b rittle an d can be pulverized to fine pow der. T h e m anganese c o n te n t of th e allo y m u st also be controlled since it h as a n effect opp o site to t h a t of su lp h u r.
T h e iro n -co b alt sy stem yields a useful m ag n etic m a te ria l, th e one
Fig. 6— T w enty roll cold-reduction mill for producing th in sheet m aterials for experimental studies.
co n tain in g ap p ro x im ate ly equal p ercen tag es of iron a n d co b alt. T h is alloy, called perm endur, is ch ara cterized b y high perm eability- a t high flux densities a n d b y a high reversible p e rm e a b ility w hen su b jected to superposed d ire c t c u rre n t m agnetizing forces. T h e b in a ry alloy c a n
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M E T A L L I C M A T E R I A L S I N T H E T E L E P H O N E S Y S T E M 151 T h e tre n d s in th e use of new m etallic m a terials in th e telephone service are difficult to p red ict. A large class of a p p lic atio n s includes th e in corp o ratio n of im proved m a terials in existing a p p a ra tu s w ith som e m odification in design re su ltin g in a cost saving or in im proved service.
Such m aterials orig in ate from d ev elo p m en ts b y th e m etallu rg ical in d u s try an d from in v estig atio n s by th e S y ste m ’s engineers. E xam ples of this ty p e have a lread y been m en tio n ed ; for exam ple, im p ro v ed cable sh eath in g m aterials, electrical co n d u cto rs, an d m ag n etic alloys. T h is evolution in ap p licatio n of m a terials will u n d o u b te d ly co n tin u e an d c o n stitu te a large p a r t of th e telephone m e ta llu rg ists’ activ ities.
T here is a n o th e r field of ap p lica tio n for m etallic m ate rials, a p p lic a tions in new ly designed a p p a ra tu s or system s of com m unication. H ere th e p ro p erties of existing m a terials are fre q u e n tly in a d e q u a te to p e r
form th e required d u tie s a n d new m ate rials m u st be developed w ith th e necessary properties. One exam ple a lread y cited is th e p re p a ra tio n of m agnetic pow der for in d u ctan ce coil cores. A new system of tra n s mission, a m illion-cycle system , requires new ly developed m aterials in th e coaxial cable a n d th e associated eq u ip m en t. Special p ro p erties are usually involved w hich are of in te re s t only in connection w ith com m unications, an d hence th e dev elo p m en t of such m ateria ls is d ep en d en t alm o st w holly on th e a c tiv itie s of th e S y ste m ’s research groups.