998 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



was still unsatisfactory and attempts were made to obtain cathodes more 

 nearly conforming to the design requirements. In the meantime a sizing 

 operation was introduced in the shop to adjust the existing cathodes to 

 size. By the additional sizing operation, the nominal value was reduced 

 to 0.03102" almost exactly the desired value. However, the standard 

 deviation of 0.00014" and 3-sigma spread of =b0.00042" were still twice 

 the design intent. 



A second run of cathodes was obtained in which the average measured 

 0.03099" and the 3-sigma limits were ±0.00021". This substantially 

 fulfilled the design requirements but made it imperative that the average 

 be held precisely at nominal. To avoid differences between measuring 

 instruments an agreement was reached to use a common type of elec- 

 tronic micrometer at both the suppliers' plant and the Western Electric 

 Company. Control charts are kept by the supplier using a modification 

 of the Continuous Production method outlined in the distribution speci- 

 fication. The application of this procedure has resulted in maintaining 

 close control of the averages. Correlation studies on tubes manufactured 

 with these cathodes, have shown that the limits for individual cathodes 

 could be expanded to ±0.00033". 



3.632 Coated Cathodes 



Three characteristics of oxide coated cathodes are particularly im- 

 portant in producing adequate emissive qualities, satisfactory life and 

 the close spacing required in the L3 carrier tubes. These characteristics 

 are weight of coating, density and coated dimensions particularly length 

 and minor axis. In order to eliminate cathode blank variations from the 

 measurements, dummy cathodes of known weight and precise diameter 

 are included with each spray rack load of 41 cathodes. Measurements of 

 gain in weight and of the increase in diameter due to coating are made 

 on these dummy cathodes. The density can be calculated from these 

 measurements. In addition to the measurements made on the dummy 

 cathodes, control charts are kept on coated length and coated diameter 

 on sample cathodes from each coating lot. 



3.633 Control Grid 



Close control of the minor axis for the frame type of control grid used 

 in all three tube types is obtained by precision manufacture of the molyb- 

 denum side rcxis used in making the grid frame. The design specifications 

 require a tolerance of ±0.0001" on the diameter of these rods. Adjust- 

 ment of this diameter is obtained by tumbling the parts, which rounds 



