I just wanted to add a thought to the discussion of the Welte recording
process.  The carbon rods would touch the bottom of the key, but would
not be attached to it.  The harder the blow on the key, the more
momentum given, and the deeper the rod would go into the mercury.
There would be no need to attach springs to the carbon rods since they
would rapidly rise in the dense liquid mercury on their own accord.

This may cause a feeling or sensation in the key when the rod rises to
hit the bottom of the key, and it would be comparable to the feeling
you get when the damper up-stop rail on a grand piano is adjusted too
high.  What happens is the damper lever leaves the back of the key,
flies up and bounces off the up-stop rail and then strikes the back of
the key, giving a certain feeling to the key that should not be there.

Padding the bottoms of the keys where they touch the carbon rods will
help to a certain extent.  The less mass there is to the carbon rod
will reduce the feeling that will be transmitted back to the key.

Randolph Herr