272 BELL SYSTEM TECHNICAL JOURNAL 



surface obtained in dusting a base plate having a groove around the 

 edge prevented the entrapment of air between the plate and the 

 printing pad during the printing operation, thereby resulting in a 

 simplification of that processJ 



As a result of our development of enameling over the black oxide 

 finish it would have been possible to replace the previously described 

 copper-base number plate by one employing a sifted coat of enamel 

 over such finish on a steel blank. However, the application of the 

 black oxide finish on enameling iron was so costly that manufacture of 

 number plates by this process was not competitive. We therefore 

 continued our developments and found that it was possible to enamel 

 directly over an electroplated copper-nickel finish consisting of a 

 minimum of 25 m.s.i. each of copper and nickel on a mild steel blank 

 and get a smooth enamel coat having very good adherence.^ As this 

 finish had the necessary rust resistance and the blank was relatively 

 flat, the enamel could be applied in a single sifted coat on the face only 

 to produce a satisfactory number plate. Also with the steel base it 

 was not necessary to have a thick coating of enamel for strength as was 

 the case with the copper base number plate. In fact, due to the good 

 adherence of the enamel coat, if the thickness of enamel after firing 

 was less than 0.010 inch the plate could be flexed considerably without 

 chipping the finish. On the other hand, it was necessary to have a 

 minimum of 0.007 inch of enamel to hide sufficiently the gray color of 

 the nickel surface. Additional refinements of the enameling process 

 were eff^ected by improvements in the uniformity of enamel distribution 

 and in the printing of characters; and the process was generally autom- 

 atized. These developments produced a number plate of superior 

 quality and appearance at a reduced cost. As all final details for 

 commercial manufacture have not been completed further details of 

 this process will not be given here. 



Vitreous Enameled Resistances 

 With the increased use of panel-type machine switching, the de- 

 mand for vitreous enameled resistances for controlling the current for 

 operating relays and switches increased materially and manufacture 

 of these parts was undertaken. These resistances were required to 

 dissipate a considerable amount of heat in service and to reach a high 

 operating temperature without being damaged. The units therefore 

 consisted of a suitable resistance wire wound on a ceramic core and 

 covered with a vitreous enamel. Some of the types now being manu- 

 factured at Hawthorne are shown in Fig. 7. 



^ W. J. Scott Patent 2,020,476, "Ceramic Articles," November 12, 1935. 

 «S. R. Mason and W. J. Scott Patent 2,020,477, "Ceramic Article," November 

 12, 1935. 



