RF.LIVS IX THE BF.I.I. SYSTliM 35 



rimiil iiiul is furtluT insured liy the fact ill, it llic ()|H-iiini; of tlu'\\iM(l- 

 in^ of " C" (Hxnirs at a hack contact of relay " B" while the locking; 

 of "C"* occurs only after the relay has pulled up to close its front 

 contact. 



The secjuence of operation and release resulting from this sorii-s 

 of relay operations atTorils protection against false signals since relay 

 ".1 " must operate continuously until " B" has released and " C" has 

 o[H>rato<i before the lanip circuit is closed. Relay "B," in addition 

 to being slow in operation, is al.so slow to release, so that the time 

 interval thus introducei! tends to briflge over any transient impulses 

 that may tend to operate the signal. 



The slow operation of relays is securetl by means of a copper sleeve 

 o\er the relay core. Slow operation results from the transient condi- 

 tion existing during the time between the application of voltage to 

 the relay winding and the building up of the magnetic field to a 

 steady state. Slow release results from the transient condition 

 existing during the time between the removal of the voltage from the 

 relay winding and the decay of the magnetic field until the magneto- 

 motive force falls below the armature restoring force. These condi- 

 tions are more easily seen when the relay winding is considered as the 

 primary of a transformer and the copper sleeve as a short-circuited 

 secondary' winding consisting of a single turn having a very low 

 resistance. The operating current, before it reaches its steady value, 

 may be considered as an alternating current of one-quarter of a cycle, 

 starting from zero and building up to a maximum value. Slow oper- 

 ation of the armature results from opposing the building up of the 

 tlux in the core. Slow release is due to retarding the decay of the 

 tlu.x in the core. The speed at which the armature operates or re- 

 leases is not changed but in the first case the application of the magneto- 

 motive force required to move the armature is delayed, and in the 

 second case the removal of the magneto-motive force holding the 

 armature in the operated position is also delayed. When a voltage 

 is first applied to the terminals of the winding, the current tends to 

 build up and establish the magnetic flux at its maximum value in the 

 relay core. The instant the Hu.x threads the copper sleeve, a \oltage 

 is induced in the latter, causing a current to How' in it. This current 

 in the copper sleeve sets up a flux in the same magnetic path which 

 opposes the flux building up from the current in the relay winding. 

 Due to leakage, the winding flux is greater than the opposing flux 

 set up by the sleeve and the resultant flu.x continues to build up until 

 it reaches a maximum value. This opposition of the winding flux 

 and the flux prmlured by the induced current in the copper sleeve 



