Richard Vance has made a point that there is a difference between
solenoids and electromagnets; I must point out that the technical
difference is different than he suggests.

An electromagnet is a device which is usually made of ferromagnetic
material (like iron) and a coil of wire which converts electricity
into force and, hopefully, into kinetic energy.  Even a flat coil
or a single loop qualifies; this includes solenoids.

A solenoid is that special case of a coil on a spool with a solid
armature moving through the center.  Naturally, these are made with
a return path for the magnetism, for efficiency.

There are such a variety of implementations of electromagnets (and even
solenoids) that Mr. Vance's generalization of force-vs-distance, overall
efficiency, and gap size in relation to geometry, is too broad.

In selecting a unit to drive a piano action, the force, distance,
placement in the action, efficiency and resulting power are all
important, but cannot simply be solved by selecting one type of
electromagnet.  I would _very_ much like to see a force-vs-displacement
curve and matching efficiency curve for a wide range of candidates.
This would be fascinating and great fodder for discussion and
development.  Surely we have not seen the optimum design on the market
as yet!

Even I have a personal design portfolio of proprietary linear
electromagnetic actuators which have escaped notice of the patent
office since Siemens patented the voice-coil in the 1880s.  The designs
which actually have either been patented or manufactured number in the
thousands and dozens used with some success in "solenoid" pianos.

Karl Petersen
Rushville, Illinois

 [ The primary requirement, I believe, is small size, so that the player
 [ action doesn't create a big bulge on the bottom of the grand piano.
 [ Good engineering design takes a back seat to the demands of the
 [ piano dealers.  ;-)  -- Robbie