1148 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



its upper or lower tolerance limit. On the corresponding resistance test, 

 the sensitrol will balance and not operate either way. Without an anti- 

 stall device the test cycle would then be stalled until the balance failed. 

 Current flowing through the A arm would eventually heat it up and 

 vitiate the temperature compensation feature. The anti-stall circuit in 

 Fig. 7(b) is essentially a slow release device to which external energy is 

 interrupted at the same time as the sensitrol reset is released. Energy 

 stored in the 50-mf capacitor prevents release of the relay for about 3 

 seconds, long after the bridge test is ordinarily finished. If at release the 

 bridge is still balanced, a 50,000-ohm resistor is thrown in parallel with 

 that ratio arm which will make the sensitrol accept the test winding. 

 A prominent and hitherto valuable feature of this test set is its adapta- 

 bility to a large variety of coil assemblies. Some hundreds of distinct 

 designs of product are presently accommodated. In the Kearny relay 

 coil shop there are four sets of the design described here and four sets of 

 earlier designs. It is possible that future development, if justifiable, will 

 be directed toward greater automaticity for some of the simpler and 

 more numerous product codes, with less emphasis on universal applica- 

 tion. 



THE CALIBRATING MACHINE FOR 56-A OSCILLATOR FILM SCALES^ 



Photographic films are used for the frequency scales of some oscillators 

 to afford scale length and enhance readability. There have been several 

 successive designs of film scale calibrators built and put in use at the 

 Bell Telephone Laboratories and at Kearny. Some have been described 

 in the hterature.^^' "■ ^^ One very early design is still in use on production 

 at the Marion Shops in Jersey City. In its use, a calibrating run requires 

 about an hour, and the possibility of frequency drift due to temperature 

 variations makes the use of an air conditioned room essential. All of those 

 used at Western, prior to the one described here, depended for accuracy 

 on the film scale of a standard prototype of the oscillator to be calibrated. 

 Using a frequency controlled servo linkage, the scale of the standard was 

 reproduced photographically on the film of the product. Some of the 

 prior art appears in the design of the new machine. In order to describe 

 the principle clearly, it seems necessary to discuss some features which 

 were previously covered, but which now are used in new ways. 



The 56A is a heterodyne oscillator designed for use in the field testing 

 of L3 installations.*^ It has a usable range of 50 kc to 10 mc. One com- 

 ponent oscillator is fixed at or near 90 mc and the other may be varied 

 between 80 and 90 mc by means of a tunable cavity. The calibrated 

 portion of the 35-mm film scale geared to the cavity tuner is about 17 



