986 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



longitudinal direction, but the washer can move with the coil, although 

 eliminating lateral movement. Use of this washer has practically elimi- 

 nated fatigue lead breakage. 



CONTACT RELIABILITY 



Since the opening and closing of contacts are the prime objectives of 

 a relay, it is extremely important that the contacts themselves are made 

 reliable. To realize these objectives, several factors should be taken into 

 consideration. First is the contact material. While much could be said 

 regarding the behavior of contact metals, space does not permit more 

 than a brief treatment. The contact should maintain reasonably low re- 

 sistance and under the environment in which it is used, be able to with- 

 stand the erosion. Electrical resistivity of most metals is low enough to 

 be satisfactory from the resistance standpoint, but unfortunately^ most 

 of them develop tarnish or corrosion films when exposed to the atmos- 

 phere, thus increasing the contact resistance and rendering them unsuit- 

 able for a contact. These metals are sometimes referred to as "base" 

 metals, and include aluminum, brass, bronze, copper, chromium, nickel 

 and stainless steel. There is a much smaller group of metals kno\Mi as 

 "noble" or precious metals, such as platinum, palladium, gold and irid- 

 ium. These are relatively free from the tendency to tarnish and will 

 maintain low contact resistance. Alloys of these metals and certain alloys 

 in which silver is included are widely used in the telephone plant. Pure 

 silver is also used and is attractive because of its low cost; however, it 

 has a tendency to form high resistance tarnish films and therefore has 

 limitations in its use. It is employed in signaling circuits Avhere the con- 

 tact makes or breaks current. Its contact resistance remains low because 

 the films that form on the silver are broken down or destroyed by the 

 arc. It is not emploj'ed in circuits carrying voice currents on account of 

 its tendency to introduce noise. 



Enough metal must be proA'ided to give satisfactory^ life. Each time a 

 contact makes and breaks an electrical circuit, a small part of the metal 

 may be lost, so that life may be considered roughly proportional to the 

 volume of metal available for erosion. The pair of contacts must have 

 sufficient height to proA'ide enough contact spring clearance to allow for 

 spring adjustment and to insure that the springs will not touch during 

 the normal operation of the relay. At least one contact of a pair must 

 be large enough, that is, present a sufficiently large target area, to insure 

 full registration of the contacts with normal manufacturing \'ai'iations 

 of the position of the contacts on the springs and with the variations in 

 alignment of the springs during assembl3^^ 



