The pedal on Mary’s most recent Kenmore 158 lost its low-speed control, which meant I must add a few more graphite / carbon disks to the stacks:
The contacts needed a bit of attention, too:
Contrary to what I found in the previous rheostats, these stacks end with a double-thick graphite disk backed up by a disk of brass shimstock, all of which needed cleaning, too. No broken disks, none severely eroded, no debris, just a general shortening of the stacks; I think the disks gradually turn into carbon dioxide.
Each stack has 42 graphite disks that average 0.79 mm thick, the double-thick disks measure 1.5 mm, and the brass shims are 0.30 mm = 12 mil. The punched contacts on those brass plates stand 0.95 mm proud of the surface.
With the big graphite plugs in place, the ceramic housing had 37 mm deep holes for the disk stacks. Subtracting the 0.95 mm contact leaves about 36 mm and, seeing as how the stacks add up to just under 36 mm overall, there’s barely room for one additional disk. I added one to each stack, buttoned the pedal up, and it works perfectly again.
Good thing I have a bag of those disks from the crash test dummy machine!
The Smell of Molten Projects in the Morning 
I think you’re right about the discs gradually turning into carbon dioxide. I suspect the stacks act as fairly ineffective (but probably pretty in the dark) arc lamps.
Surely, the Bakelite case has opaque black filler specifically to block any light leaking around the edges of that ceramic block! [grin]
Speaking of arcs, I’m still amazed that:
I’m impressed that any of us survived our childhood. I suspect that we’ve been unwittingly selecting for dumb luck, if that’s inheritable.
High Truth: all adult human males can tell at least one story using the words “Except for that inch / second, I’d be dead…”