1146 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



vision into twenty resistors helps in this by maintaining high effectiveness 

 of dissipation. In addition, the automatic switching circuits are arranged 

 to keep the duty cycle of current in the bridge arms low. 



The B arm of the bridge is selected by the setting of the percentage 

 tolerance switch. Each resistor is used alone and consists of low tempera- 

 ture coefficient resistance wire as in standard bridge practise. The value 

 of each resistor in ohms is 1 ,000 divided by one plus or minus the corre- 

 sponding tolerance fraction. Thus, for ±1 per cent tolerances the re- 

 sistors are 1,000/1.01 ( = 990.0) and 1,000/0.99 ( = 1010.1), respectively. 

 One setting of the switch indicates zero tolerance and is equipped with 

 1,000-ohm resistors to permit easy checking of the C arm precision. 



The six-decade standard resistor in the C arm, which is set to the nom- 

 inal value of the test winding, is of a high quality commercial type with 

 a range of to 40,000 ohms in steps of 0.1 ohm. Because the C and X arms 

 may contain values as low as 2 ohms, no relay contacts are used in them. 

 Relay switching is done in the A and B arms where the resistances are 

 always of the order of 1,000 ohms and small variations in contact resist- 

 ance are negligible. The more stable wiping contacts of selector switches 

 do appear in the X arm. These switches permit any contact in the test 

 fixture to be connected to any bridge terminal, to enhance flexibility. 



A continuity test on all windings, before resistance test, is desirable 

 for two reasons. The effect on the sensitrol of the severe bridge unbalance 

 caused by an open winding would be life-shortening and is to be avoided 

 if possible. Also, the result of the resistance test would only show high 

 resistance, and separate analysis would be needed to reveal that a wind- 

 ing was open. The continuity test circuit in Fig. 6(b) was devised to prove 

 continuity for windings having resistance values as high as 35,000 ohms. 

 A relay (UA-104) was chosen which is sensitive enough to close a pair 

 of "preliminary make" contacts (m) on 0.005 ampere, and which pro- 

 vides the number of other contacts needed to satisfy circuit requirements. 

 When the test winding is connected at X, the currents through it and 

 the 100,000 ohms combine to equal 0.005 ampere or more. This closes m, 

 connecting the 20,000-ohm resistor in parallel with the 100,000 ohms, 

 thus locking the relay and assuring that all the other contacts operate. 

 In the act of proving continuity, the relay disconnects itself from the 

 test winding and remains locked. The make contacts shown at the right 

 end of the relay symbol are in series with similar contacts on the con- 

 tinuity relays for the other two test windings, and when all are closed 

 they pass operating current to a relay which initiates the first resistance 

 test (for primary high limit). 



In the direction of winding circuit. Fig. 7(a), it is necessary to have a 

 negative pulse from the pickup coil, in the test fixture, cause the 313CA 



