METALLIC MATERIALS IN THE TELEPHONE SYSTEM 147 



Die Casting Alloys 



The demand in the Bell Telephone System for the economical pro- 

 duction of large quantities of small complex parts has led to an exten- 

 sive and growing use of die castings. If the past is a guide to the 

 future, further expansion can be expected. Although the zinc base 

 alloys represent the major proportion of all alloys consumed, other 

 materials find application where specific properties are desired. High 

 dimensional accuracy is obtained with tin base alloys; light weight is a 

 notable property of aluminum base alloys. Lead base die castings are 

 used principally in coin collectors where their sound and mechanical 

 damping characteristics are important. To produce the desired 

 properties consistently the metallurgical characteristics of these ma- 

 terials must be known and specific procedures followed. 



Electrical Contact Alloys 



Requirements of a suitable contact are many, and vary with the 

 use to which the contact is subjected. Two requirements that are 

 universal and paramount are that the contact material must provide 

 an electrical path of a low resistance and must not wear away too 

 rapidly. (Some contacts are expected to give satisfactory performance 

 for more than 150 million operations.) In the communication systems 

 both precious and base metal contacts are extensively used. Of the 

 former class, platinum, palladium, silver, platinum-gold-silver, gold- 

 silver, palladium-copper, or platinum-iridium, have given good service 

 performances. Wiping contacts are widely employed in dial central 

 offices. These consist generally of brass and bronze although silver is 

 being used to an increasing extent. 



Some idea of the extent to which our modern communication systems 

 are dependent upon electrical contacts is illustrated by the number of 

 pairs of precious metal contacts that must operate reliably to complete 

 an ordinary dial system call between subscribers in a large city. Such 

 a call brings into operation about three hundred relays involving over 

 one thousand pairs of contacts. In a long distance call between New 

 York and San Francisco about 1500 additional pairs of precious metal 

 contacts must perform dependably for satisfactory transmission. In 

 some years our communication systems have required more than 100 

 million pairs of contacts furnished on different kinds of telephone 

 apparatus. 



It may be readily appreciated, therefore, that knowledge of the 

 factors governing contact performance is of vital importance. 



