1428 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



of relays leads to additional requirements: (e) Substantially equal op- 

 erate and release times, (f) A make and a break contact, and (g) Sub- 

 stantially zero transfer or bridging time when operated or released. 



The first requirement is to avoid correction terms in the test set 

 equations. The second is to make the test set operation conform to the 

 differential equation applying. The third depends upon the fact that 

 meters having dc resistances of the order of 50 ohms are used, and no 

 error deflection due to contact potential effects should be present. The 

 stability requirement results from an objective of studying flux phenom- 

 ena in intervals as smafl as 25 microseconds. The added relay require- 

 ments will be developed during the switching circuit description. All 

 except the last can be met by individual relays. The last one has not 

 been met by individual relays, but by using contacts of two relays actu- 

 ated simultaneously, and adjusting their relative timing by winding 

 shunts, the contact switching interval can be reduced to a few micro- 

 seconds. 



Contact Operation. The switching functions required were shown in 

 Figs. 1 (b), (c) and (d) where the interval ti can be as low as 25 microseconds 

 or almost as long as T. The A switch in unaffected in duty cycle, but the 

 B and C switches vary from to 100 per cent duty cycle. The operate and 

 release times of relays are affected by their duty cycle and for low or high 

 values become very erratic. To avoid this effect all relays are operated 

 with a 50 per cent duty cycle, and the switching is accomplished by com- 

 binations of relay contacts. The epoch of the B relay cycle can be adjusted 

 by manual selection, anywhere between and T/2. This is shown sche- 

 matically in Fig. 8 for the make contacts. The break contacts of course 

 show exactly the reverse behavior. The C make contact is fixed in phase 

 and is set to open just before the A contact closes. 



To produce a B cycle for flux rise shown in Fig. 1 (c) where ^i is less than 

 T/2, a parallel connection of the B and C make contacts is used. To pro- 



Fig. 8 



L -----T J 



The operation of the switching make contacts. 



