300 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



6.2 Terminating and Adjusting to Value 



Transformation of a carbon coated ceramic rod into a completed resistor 

 requires the application of low resistance contacts to the carbon film. Such 

 electrodes may be applied directly to the carbon film by use of a water sus- 

 pension of colloidal graphite (Aquadag) or of suitable metallic paints, or 

 by other means. Except for resistors of low resistances, colloidal graphite, 

 burnished and baked, provides an excellent termination. It is, however, 

 susceptible to moisture; to provide greater stabiUty and to facilitate subse- 

 quent manufacturing operations, metallic paints are generally preferred. 



To obtain resistors within given tolerances, it is necessary to adjust the 

 resistances of the terminated units, since the statistical variation which 

 the resistances would otherwise exhibit generally exceeds the allowable toler- 

 ance. There are two reasons for this variation: First, there is, despite the 

 most precise control of coating conditions in either the batch or the con- 

 tinuous process, a statistical variation in film thickness; and, second, there 

 are variations in the core surfaces and in the dimensions of the cores. 



Primary control of resistance tolerance is accomplished through control 

 of the conditions of pyrolysis. The close control which is necessary in view 

 of the great sensitivity of film thickness to the conditions of pyrolysis, as 

 shown in Fig. 3, Fig. 4 and Fig. 5, requires careful attention to furnace 

 design. The furnaces illustrated in Fig. 1 and Fig. 2 have proved satisfactory 

 for resistor production: Either of them will furnish carbon films reproducible 

 in film resistance to within about 7 per cent. 



The adjustment by means of which resistors with resistance tolerances as 

 small as ±0.5 per cent or less are produced from coated blanks may be 

 accomplished in two stages: if helixing is employed, by choice of the helix 

 pitch and width; and by removal of a small amount of the carbon film by 

 abrasion. 



The helix is ground through the carbon film by use of a water-cooled 

 metal-bonded diamond cutting wheel. It is essential that the helical groove 

 be smoothly ground in order to prevent the occurrence of cracks or fractures 

 extending into the carbon ribbon, which lead to high noise levels and in- 

 stability. The machines employed are so constructed that the carbon-coated 

 blank "floats" against the abrasive wheel, thus providing a groove of es- 

 sentially uniform depth and width regardless of any slight ellipticity in the 

 cross-section of the core. Provision is made in the machines for continuously 

 varying the helix pitch; and, over the range of pitch normally employed, the 

 resistance of the coated blank can be increased by any desired factor from 

 about 10 to about 8000. With uniformly coated blanks the helixing operation 

 does not increase the spread of resistance values nor the value of the temp- 



