158 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



impedance, not including a capacitance, will always increase the operate 

 time. This will be made more evident when the operate times of relays 

 with their windings in series are considered. 



The curves can be used in either of two ways (1) given the relay 

 circuit for which the applied power and coil constant can be computed, 

 the average operate time can be found from the chart, or (2) given a 

 required average operate time, the necessary circuit power and coil 

 constant can be found from the chart. 



These curves can be plotted in this form to exhibit the best winding 

 directly because of the independence to contact spring load. For the 

 load controlled case, the contact spring load was an essential parameter. 

 For the mass controlled case, the armature travel becomes the outstand- 

 ing parameter to be considered, but there are only two or three of these. 

 All the other factors except contact load also enter and need to be evalu- 

 ated for two reasons, (1) to provide an estimate of the range in operate 

 time to be expected and (2) to adjust experimental measured time data 

 to average. For any experimental setup, it is seldom possible to provide a 

 structure which is average in every respect. For any one structure, how- 

 ever, all the factors known to affect its performance can be measured. 

 Comparison of these to the manufacturing specifications locates the 

 experimental setup in the universe of all relays as regards each of the 

 factors. It is thus necessary to develop representations relating each of 

 these factors to the operate time, which will suffice for the two uses 

 named above. The development of these relationships will be the subject 

 of the following sections. 



Waiting Tijne 



The waiting time, whether an electromagnet is mass or load controlled, 

 is given by the same form of equation used for the total operate time of a 

 load controlled relay: 



U = U{Gc + G'^) In —^ . (13) 



1 - 91 



where now qi is determined ]:)y the armature back tension Fi ; Li applies 

 to the open gap; and no sleeve conductance is present. For the present 

 purpose, it is desirable to rewrite this equation in terms of the funda- 

 mental parameters of the relay. For the open gap case the magnetic 

 material is operated in its linear region and the open pole face gap pro- 

 vides additional linearity. For these reasons the expression is quite ac- 

 curate. The sketches of Fig. 7 show the factors to be used. The value of 



