Df.SICX CII.IR.ICI riRlSTICS OF F.l.r.CrRO^r.lGXF.TS 229 



Ollu-r ill-signs of relays iisi-tl i-xtonsivoly in the tele|)li<>ne plant 

 are the relays that control the sii[K'rvision of a tclephf)nc connection 

 and the alternatinK current relays which ojHTate on ringinR currents 

 of If) to 20 cycles frwiuency. 



Rela>s which are us^hI for sujierx isor>- purposes and alli-riiatinj; 

 current relays are generally constructetl of silicon steel instead of 

 the customary Norway or niaumtii- iron. The silicon steel is very 



^■C:iKt EXTCN5ID\ 



Fig. 16 



satisfactory for these relays Ixjcause of its comparatively high permea- 

 bility, low coercive force and small hysteresis. The high permeability 

 is advantageous for relays that are required to operate on a very small 

 energy- input and the low coercive force is very efTective for obtaining 

 a quick and positive release of the relay armature, particularly where 

 a leak current exists due to faulty line insulation. A great improve- 

 ment in many of these relays can be obtained by the use of certain 

 nickel-iron alloys which have been recently developed and arc known 

 as "Permalloy." 



A relay for use on ringing currents is shown in Fig. 16. The arma- 

 ture "A" of this relay is attracted to the bifurcated extensions of the 

 core "B." One of these core extensions is completely surrounded 

 by a part of the copper spool head "C." This arrangement is known 

 as pole "shading" or phase splitting and is used to produce a sub- 

 stantially steady pull on the armature when the relay is energized 

 by single phase alternating current. 



Referring to Fig. 17 the theorv' of ofxjration is shown by consider- 

 ing the vector diagram in connection with the schematic drawing 

 of the relay core and armature. When an alternating current is 

 applied to the winding we can assume that an alternating flux 2 <t>m is 

 generated in the core. This flux di\ides into two approximately 

 equal parts in the two bifurcatefl extensions of the core. If these 

 two fluxes can lie flisplacefl in time phase it is evident that the arma- 

 ture will Ix: attracted by one of the bifurcated extensions of the core, 

 while the flux, and consequently the attraction of the other, is passing 



