AUTOMATIC TESTING IN TELEPHONE MANUFACTURE 1143 



three windings (called primary, secondary and tertiary). The primary 

 and secondary are wired to corresponding pairs of terminals on the ter- 

 minal assembly, while the tertiary leads at this stage are not on terminals 

 and must be connected to the test contact fixture by hand. 



Direction of winding is important in the multiwinding coils because of 

 external fields and the fact that the relays are required to respond to 

 currents in more than one winding and the proper direction of flow in 

 each, relative to the other, must be known. In some relays one or two 

 of the windings may be noninductively wound, to serve merely as resis- 

 tors. Also, many windings are wound part copper and part resistance 

 wire to obtain the desired resistance without unnecessary increase in 

 copper, inductance and response time. In such cases the percentages of 

 copper and resistance wire are known. This is important because of the 

 effect of temperature on the resistivity of copper. Resistance tolerances 

 on the test windings are specified at 68°F, but shop testing is done at any 

 value of room temperature. The effect of the difference on copper is seri- 

 ous enough to cause errors larger than some of the tolerances, and the 

 effect on resistance wire may be neglected. Therefore, it is necessary to 

 have the test set compensated for temperature in such a way as to allow 

 for the proportions of copper and reistance wire. 



The coil test set (Fig. 5) tests all windings for resistance and direction 

 of winding and for breakdown to each other and the core. The maximum 

 total test time for three-winding coils is less than 3 seconds under normal 

 conditions. A borderline winding resistance will cause some delay. There 

 are lamps to indicate the type of failure on a rejection. Other lamps indi- 

 cate satisfaction of the requirements. At the completion of test on a good 

 coil an "OK" lamp lights on the test fixture, so that the operator need 

 look at the set itself only when there is a rejection. 



Requirements data are stored in the set before a given code of coil is 

 tested. The codes come to the set in batches, so that one setup will serve 

 for a large number of coils. Three six-decade resistance standards are 

 set to the nominal values for the respective windings. If there are fewer 

 than three windings, a key is operated to disable bridges and furnish 

 substitute continuity paths. The percentage tolerances for the windings 

 are set on selector switches: ±1, 2, 5, 10 and 15 per cent tolerances are 

 available. Also, the known percentages of resistance wire in the windings 

 are set on selector switches in steps of 5 per cent from to 100. Keys are 

 operated to warn the set of noninductive windings and bypass the direc- 

 tion of winding circuits as needed. 



Once a coil is placed and connected in the test fixture and the fixture 

 closed by operation of a pedal, the test is automatic up to the point where 



