274 BELL SYSTEM TECHNICAL JOURNAL 



Since the application of vitreous enamel to the resistances involved 

 plunging the porcelain cores into a hot furnace and their removal into 

 cold air without cracking, and since in use they were also subject to 

 considerable heat shock, it was necessary to develop a body with 

 thermal shock resistance characteristics that would still have the 

 necessary fired strength. It was also required that this body be 

 suitable for the extrusion of lound cores and for the molding of more 

 complicated shapes from a granular body. A somewhat porous fired 

 structure was also desirable to facilitate the application of the enamel. 



These desired characteristics indicated that a clay-talc combination 

 would be suitable. Mixtures containing a greater percentage of clay 

 than the eutectic mixture of the two minerals were investigated to 

 avoid vitrified parts at the temperatures then used in the tunnel kiln 

 for other products. Tests were made of extrusion and molding proper- 

 ties, fired, breaking and impact strengths, and resistance to thermal 

 shocks. On the basis of these tests a talc-clay body composition was 

 selected. Dies were then constructed based on the fired shrinkage of 

 this body of about fifteen per cent in extruded and twelve per cent in 

 molded forms. 



Using these cores, suitable sizes -of wire were selected considering 

 the dimensions of the core, the resistance value desired, heat to be dis- 

 sipated at certain wattages, and the maximum operating temperature 

 permissible, and satisfactory winding methods were established. 

 These methods were based on the use of motor driven machines in 

 which the wire was spaced on the revolving cores by the transverse 

 movement of a guide controlled by lead screws of various pitches. To 

 facilitate any necessary minor resistance adjustments, methods were 

 provided for checking the resistance of the units before connecting the 

 wire to the second terminal. Since the heating received by the wire 

 during the enamel firing increased its resistance value, tests were made 

 to establish resistance value factors for winding use. 



Difficulty was experienced with the resistance becoming open in the 

 firing operation and it was shown that this condition was caused by 

 the formation of a film of glass between the resistance wire and the 

 terminal during the firing operation. Various methods of attaching 

 the resistance wire to terminals were tried and it was found that the 

 use of lead as solder would prevent the formation of a film of glass 

 between the wire and the terminal even though the fusion temperature 

 of the lead was considerably below the enameling temperature. 

 Ordinary soft solder could not be used due to the tin content embrit- 

 tling the standard copper lead wires during the enamel firing. While 

 silver solder could be used it was costly both for material and in 

 application. 



