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BELL SYSTEM TECHNICAL JOURNAL 



through zero. This may be explained by the vector diagram in which 

 Ri represents the induced voltage in the short circuited copper ring 

 due to the alternating flux <>„. The current in the copper ring I2 

 lags behind the voltage £» as shown and the flux due to this current 



Fig. 17 



is 4),. This flux ^^ has a magnetic path through the bifurcated pole 

 pieces and armature as shown by the arrows. F"ollowing out the 

 arrows it will be seen that this flux adds to the flux <i>„ in the upper 

 part and subtracts in the lower part of the two core extensions. 



The vector addition and subtraction of these two fluxes results in 

 two vectors <j>m-\-<i>r and (t>m — <t>r, each of which represents a flux that 

 crosses an air-gap to attract the armature. These two fluxes difi'cr 

 in time phase as represented by the angle "B" so that a substantially 

 constant attraction results on the armature. The operation of the 

 relay under these conditions is very much the same as that of a direct 

 current relay as no vibration or chatter of the armature or contacts 

 occurs. The minimum effective alternating current ampere-turns 

 required for operation are 70 to 100 ampere-turns. 



Such a relay, of course, operates on direct current as well as on 

 alternating current and in fact the direct current sui>er\isory relays 

 are quite similar to these relays in mechanical design. 



