sewing machine speed control

I opened up a 1970s vintage sewing machine treadle the other day expecting to find some triac type chopper controller but instead found a weird plunger type of thing with what looks like carbon rods either side.

what's the mechanism?

I guess I should post a pic but its in the shed and I cant be bothered to at the moment

 

Many old sewing machines it was a simple resistor, Phaff was one of the first with electronic control, that was mid 80's.

 

a simple resistor eh?
what about a complicated resistor?
Guess this must be something like that. a pressure-varying resistor I suppose. don't they get a bit hot?

 

Just a variable resistance. Well worth the effort of converting one to a modern triac control.

 

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