Anyone know how these work?

The foot control of my wife's 1980's Singer sewing machine started
emitting smoke a couple of days ago and, at one stage, the motor ran
without having her foot on the pedal.

Mr Google suggests that it contains a capacitor which, if it develops a
short circuit, can exhibit these symptoms. I opened it up to have a look
and there was was a nasty smell from the vicinity of the capacitor, but
nothing obviously burnt out. I've ordered a new capacitor, anyway, and
will fit it when it arrives. Meanwhile, I've blown all the dust out of
the foot control with an airline, and tested it - and it seems to be
working ok.

But speed control isn't very progressive - it tends to be virtually all
or nothing, like a car accelerator which only has two positions. [This
isn't new - it was like this before it started smoking.]

So I'm trying to understand how the speed control is supposed to work.

https://app.box.com/s/inzt3elo35uq13srz5fqi7qg15ei7z8x shows a photo of
the foot control with the hinged top open. The brown and blue wires are
connected together via a ceramic cylinder which has a piece of carbon
rod protruding from each end. The one at the left hand end is spring-loaded.

When the lid is closed and pressed down, a post on its underside presses
down on the centre of the long copper? strip so that it makes contact
with another strip below (hidden in the photo) which is connected to the
brown wire - swiching on the motor. As the lid is pressed down more, the
upturned part at the left hand end of the copper strip moves to the
right and presses on the spring-loaded carbon rod. I assume that it is
somehow meant to increase the motor speed, but I don't understand what's
going on inside the ceramic cylinder.

Can anyone throw any light on this?
--
Cheers,
Roger