1140 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



transients from this and other circuits. During the short interval of actual 

 test no other switching takes place in the machine. 



A fixture that has no network because of rejection at an earlier test 

 position or because of operator failure to load it, would cause open circuit 

 in one bridge arm on capacitance test. Without intervention this would 

 cause a violent unbalancing of the bridge, overloading of the detector 

 system and possible damage to the sensitrol. Ordinary methods of limit- 

 ing the overload signal would be only partially effective and would de- 

 tract from the sensitivity. To forestall this trouble from empty fixtures, 

 each capacitance test position is equipped with a microswitch which is 

 operated by a dog at the bottom end of the ejection arm of any empy fix- 

 ture (Fig. 3). When the microswitch operates it causes the bridge to be 

 disconnected from the test leads and connected to a capacitor that is just 

 out of limits, several tenths of a second before the removal of the short 

 circuit from the sensitrol. Then w'hen the test is made it results in a re- 

 jection. 



There is also an interlock circuit which will stop the machine if a failure 

 of the bridge and detector system causes an empty fixture not to show 

 rejection. This serves as a random occasional check on the functioning 

 of the circuit. 



The conditioning of the three capacitors for the leakage current tests 

 l)egins at position 16. Because of charging and absorption currents ob- 

 scuring the effect of pure leakage, the test for leakage is made to an arbi- 

 trary current limit specified at one minute of charge. To insure that good 

 units pass the test, it is desirable to use the whole minute. But if the leak- 

 age current reading is taken after more than a minute of charge, quality 

 is jeopardized. Accordingly it is necessary to make sure that the charge 

 is for a minute and no longer on each capacitor. Therefore, at position 16 

 the first unit is put on charge, at 17 the second, and at 18 the third. Then 

 at position 32 the first unit is tested while the other two remain on charge. 

 At 33 the first unit is discharged, the second tested, and so on. 



The leakage test itself is made by measuring the voltage across a large 

 resistor in series with the test capacitor and a dc voltage source. The 

 energy in this signal is small and must be amplified before there is enough 

 to operate a sensitrol. A dc amplifier with high input impedance is used 

 for this purpose. In addition the mechanical bias of the sensitrol is kept 

 small to increase sensitivity, and a carefully controlled dc biasing source 

 is used to insure accuracy and stability. 



At position 37 three capacitors and a coil ^\•inding are given a final 

 check for contirmity. The test of the winding is made by connecting it in 

 series with a relay coil (say No. 1) and battery. If current passes, relay 



