DESIGN FACTORS IXFLIENCIXG RKLIAIULITV OF UKLAYS i)!)l 



pprformanco of twin contacts with single contacts, and arrivinp; at a 

 lignro of merit. Data liaxc also boon colloctod from i'(>Iays in soi'Nicc in 

 the tclophoiic plant on \\\v basis of nnmbci's of foniid oixmi tronl)l('s on 

 both l>'pcs of contacts. As might bo o.xpoctcd the results Nai'iod widely, 

 with the twin contact being superior ])y a factor of anywhere from 3 to 

 100 with pci'liaps 10 as a rc^isonablc lignre. 



MAGNETIC STABILITY 



Magnetic niateiials in rela>'s liaxc been found to change^ in tlieii' 

 magnetic characteristics with time and temperatures to which they are 

 subjected in their normal usage. This effect is known as aging. The dii'oc- 

 tion of th(> change is such as to decrease the permeability and increase 

 the coercive force of the material. The dogi'ce of change in certain ap- 

 plications, such as i-elays in mai'ginal and time delay circuits, may be so 

 large as to be of serious concern. 



A high grade of magnetic iron which has been extensively used in the 

 telephone system has been found to age considerably under conditions 

 simulating operation in the plant. Aging of iron is attributed to the pre- 

 cipitation of impurities such as carbon, nitrogen, and oxygen. The solu- 

 bility of these elements decreases with decreasing temperature. When 

 iron is cooled from a high temperature, impurities, such as carbides and 

 nitrides, do not have sufficient time to precipitate completely, so a super- 

 saturated solid solution is produced. C'onsociuently the impurities tend 

 to continue to precipitate slow^ly at low temperatures \vhere the diffusion 

 rate is extremely sIoav, and internal strains are ])rodnced which affect 

 the magnetic properties.'^ 



It has been found that if these parts are annealed in atmospheres of 

 dry hydrogen instead of the ordinary "pot" anneal, this aging effect is 

 greatly reduced. Not only is the aging effect reduced to where it is of 

 no great engineering importance, but the magnetic properties of the 

 material are improved. The maximum permeability is increased and the 

 coercive force is decreased both by a factor of about two. The use of 

 relays in critical applications is thus greatly enhanced. 



The degree by which magnetic materials change by aging may be 

 determined readily by laboratory tests. Long time aging effects can be 

 simulated by baking ring samples of the material or the relays at 100°C 

 for several hundred hours and measuring the magnetic jn-opei'tios of the 

 ring specimens oi' the operating charactei'istics of the relays befoi-(> and 

 after aging. Foi' "pot" annealed magnetic iron the effect of such aging 

 is to decrease the maximum permeability by about 50 per cent and to 

 approximately double the coercive force. When the iron is hyth-ogen 



