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THE BELL SYSTEM TECHXICAL JOURNAL, JANUARY 1954 



cuit must be taken as that of Fig. 3. In this case it is the armature flux 

 <Pa , rather than the total flux (p, which approaches a saturation value 

 limiting the mechanical output. In the low density region, where the 

 reluctances are constant, the conditions of equivalence given by equa- 

 tions (25) may be used to evaluate a simple parallel equivalent to the 

 armature path reluctance, reducing the circuit to the form of Fig. 21, 

 and this may in turn be reduced to an equivalent circuit of the form of 

 Fig. 23. In the high density region, however, it is the armature rather 

 than the core reluctance which must be expressed in terms of the Froe- 

 lich-Kennelly approximation. 



For reed relays, the structural schematic is as shown in Fig. 24. In 

 some cases, the external shield may be omitted. The magnetic circuit 

 involves an air path through the coil in parallel with a path through the 

 reeds. With a new interpretation of the terms, the circuit of Fig. 3 may 

 be taken as applying, with (Re taken as zero, and (Rlc representing the air 

 leakage field. With some correction for the effect of the shield, this may 

 be estimated by means of the solenoid reluctance expressions of Section 

 5. The controlling path is that through the reeds, represented by (Ra. for 

 the saturating section of maximum density in series with the parallel 

 paths between the reeds: the useful path at the gap and the leakage field. 

 The latter may be estimated from the relations of Fig. 13. 



For polar relays, the network contains both the permanent magnet 

 magnetomotive force 'i^M and the coil magnetomotive force $Jc • A typical 

 case is shown in Fig. 25. The constants applying can be evaluated by the 

 procedures described in Section 5, and the somewhat complex circuit 

 equations resulting can be written by the application of Ivirchoff's laws 

 in a manner wholly analogous to that for the corresponding electrical 

 circuit. 



REEDS'' 



Fig. 24 — Magnetic field of a reed relay. 



