24. THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



and to its improvement. This article gives the background of this tech- 

 nolog}^ as it apphes to the dc telephone relay. 



A telephone relay is an electromagnetic switch which actuates metallic 

 contacts in response to signals applied to its coil. Its characteristics as a 

 component of switching circuits are defined primarily by (1) the contact 

 assembly, (2) the coil resistance, (3) the current flow requirements for 

 operation, holding, and release, and (4) the operate and release times. 

 The design of the contact assembly determines the niunber and nature of 

 the contacts, the sequence of their operation, their current carrying 

 capacity, and their reliability and life. 



Structurally, the design of the relay, including both the electromagnet 

 and the contact assembly, is governed not only by the performance 

 requirements but also by the manufacturing considerations which deter- 

 mine the relay's initial cost, and by equipment considerations relating to 

 the way it is mounted, wired, adjusted, and maintained. The ultimate 

 objective is to perform a circuit function at a minimum over-all cost, 

 including the costs of installation, power consumption, maintenance and 

 replacement, as well as initial cost. 



The design of the contact assembly determines a force-displacement 

 characteristic representing the mechanical work which must be done by 

 the electromagnet. The relations between this mechanical output and 

 the electrical input to the coil are determined by the magnetic design of 

 the relay, or specifically of its electromagnet. The electromagnetic design 

 is therefore subject to the performance reciuirements, to the design of 

 the contact assembly, and to the manufacturing and equipment con- 

 siderations applying to the whole relay. 



Magnetic design thus requires the ability to determine the effect on 

 the performance of alternative choices of the configuration, dimensions, 

 and materials of the electromagnet, of alternative load characteristics 

 corresponding to variations in the design of the contact assembly, and of 

 alternatives with respect to the coil dimensions and characteristics. The 

 basic relations required for such prediction of performance are the mag- 

 netization relations, which express the field strength of the electromag- 

 net as a function of the ampere turns, and the armature position. 



The present article describes the evaluation of magnetization rela- 

 tions, and their use in relating the mechanical output with the electrical 

 input to the coil. The time reciuired for operation, which also depends 

 upon the magnetization relations, is discussed in a separate article.^ 

 While much of the material given here has a broader application, the 

 discussion of its use will l)e confined to the case of the simple neutral 

 (non-polar) relay. 



