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Carbon Film Resistors: Methane Cracked on Ceramic Rods, Helixed and Varnished (Bell Labs 1951)
Ceramic rods tumble in a hot, gas-tight rotating furnace while methane cracks on them and leaves a thin, hard film of carbon. The coated rods get conducting ends, a helix ground through the film to bring them near their value, a light rub to the exact value, and a coat of baking varnish.
Result: carbon film resistors adjusted to as close as ±0.5 %. The helix alone can raise a rod's resistance by any factor from about 10 to about 8000.
Source: Grisdale, Pfister and van Roosbroeck, Pyrolytic Film Resistors, Bell System Technical Journal 30 (1951). Unknown: the ceramic of the rods, the end caps and leads, and which baking varnish; the paper does not give them. Picture: Wikimedia Commons.
Advanced
2 days
Instructions
1
1
Load the furnace
Load the furnace
Load cylindrical ceramic rods into the gas-tight refractory core of a rotating (or rocking) batch furnace, with silica sand to support the rods and keep them from damage as they tumble. Unknown: which ceramic.
Materials for this step:
Insulating Resistor Core1 batchTools needed:
Rotating Batch Furnace
Quartz Sand2
2
Heat under nitrogen
Heat under nitrogen
Turn the core and bring it to temperature, usually about 975 to 1300 °C, flushing it with nitrogen before any hydrocarbon goes in.
Tools needed:
Rotating Batch Furnace
Nitrogen3
3
Crack the methane
Crack the methane
When the temperature is steady, let in the coating gas and keep it flowing for the time needed. The film grows at a steady rate: at 1000 °C and 30 % methane the paper runs from 0.2 to 10 hours. Longer gives a thicker film of lower resistance; temperature sets the rate more than anything else, and more methane deposits faster.
Materials for this step:
Methane1 flowTools needed:
Rotating Batch Furnace4
4
Cool under nitrogen
Cool under nitrogen
Shut off the methane and let the core cool while oxygen-free nitrogen keeps flowing. The films come out clean, even and firmly stuck.
Tools needed:
Rotating Batch FurnaceNitrogen5
5
Terminate the ends
Terminate the ends
Paint the ends of each coated rod with a water suspension of colloidal graphite (Aquadag), then burnish and bake it; this suits all but low-value resistors. Fit the end caps with their leads. Unknown: the caps and leads, which the paper does not describe. Ours: an agate burnisher and an oven.
Materials for this step:
Colloidal Graphite1 as required
Resistor End Caps with Leads2 per resistorTools needed:
Agate Burnisher
Oven6
6
Grind the helix
Grind the helix
Grind a helical groove through the film with a water-cooled, metal-bonded diamond wheel, the blank floating against the wheel so the groove stays even. Pick the pitch to land slightly below the wanted value (330 Ω, 10 kΩ or any other). The groove must be smooth: cracks into the carbon ribbon bring noise and drift.
Tools needed:
Resistor Helix-Cutting Machine7
7
Rub to value and varnish
Rub to value and varnish
Turn the resistor while measuring it continuously, and rub its whole surface lightly with a cotton pad wet with an organic solvent until it reaches its value. Leave it just below tolerance, then give it one or more coats of baking varnish, which raises the resistance by generally less than 1 %. Unknown: which varnish and its baking. Ours: acetone as the solvent, a Wheatstone bridge to measure.
Materials for this step:
Baking Varnish1 as requiredTools needed:
Wheatstone Bridge
Cotton Wool
AcetoneOvenMaterials
5- 1 batchPlaceholder
- 1 flowPlaceholder
- 1 as requiredPlaceholder
- 2 per resistorPlaceholder
- 1 as requiredPlaceholder
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