1430 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



instrument and damping resistor switching. The latter is to eUminate 

 errors due to momentary circuit configurations not in accordance with 

 the basic equation (3). 



The final circuit developed is shown in Fig. 10, which uses two B 

 relays, called Bl and B2, a single C relay, and compatible timing require- 

 ments for the two B relays whose windings are connected in series. In all 

 cases Bl shall be faster than B2. The former controls the damping re- 

 sistor while the latter switches the instrument. A composite circuit is 

 drawn, with the cross marks indicating a closed contact for the function 

 being measured, open otherwise. The change from rise to decay in this 

 circuit requires four switch contacts compared to two for the minimum 

 contact circuit. As before, the relay contacts are all shown in the un- 

 operated position. A preliminary sorting of the Bl and B2 relays is 

 required and is based on operate and release time measurements. The 

 faster relay is assigned as Bl, the other as B2. B2 is further slowed down 

 selectively to match the A relay as to operate and release time by the 

 shunts and diode shown in Fig. 10. 



B2 can be lined up with the A contact both for simultaneous closure 

 of the make contacts when ^i = and for simultaneous closure of the 



ADJUST 



RISE AND 



DECAY 



+ VA- 



© 



© 



1; % 



ADJUST 

 RISE 



r 



•^ 



(b) 



Fig. 10 — Final schematic of basic contact circuit. 



