1144 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



the operator must make disposition. The sequence of events within the 

 set is controlled by a switching circuit containing thirty telephone relays, 

 a sensitrol relay and two electron tubes. The sensitrol is used in succession 

 to detect the existence and sense of unbalance of six dc bridge circuits 

 (high and low limit for each of three windings) . The operation sequence 

 for primary windings is shown in Table III. 



Fig. 6(a), shows schematically a typical bridge arrangement for testing 

 a winding at one tolerance limit. A and B correspond to the ratio arms 

 of an ordinary Wheatstone bridge, and are nominally 1,000 ohms each. 

 The temperature compensation referred to above is obtained by including 

 the same resistance percentage (within 2.5 per cent) of copper in the A 

 arm of the bridge as there is known to be in the winding. Inspection of 

 the bridge balance equation in Fig. 6(a) will show that an error in X could 

 be compensated by a proportional error in either A or C. A is chosen as 

 the compensating arm because of its simplicity. It has available twenty 

 resistors of copper and twenty of low temperature coefficient resistance 

 wire. Each resistor is 50 ohms, measured at 68°F. The selector switch is 

 arranged so that the arm always has tw^enty resistors, the indicated per- 

 centage being resistance wire. 



For proper compensation it is necessary that the A arm be as near am- 

 bient temperature and the temperature of the coils as possible. The di- 



Table III — Sequence of Events in Test of Primary Winding 

 FOR High Limit Resistance 



