I was fascinated by the two articles on this.  This scheme is very
similar to the Choralcelo that was discussed a while ago, with one
major difference.

In the Choralcelo, an alternating current is generated separately
from the piano string "harp" assembly.  When applied to the string of
the same fundamental pitch, by an electromagnet essentially identical
to the one used in the "microphone" system, the string vibrates and is
heard through its sounding board.  No microphone or interrupter
contacts are needed.

The "bug" in the Choralcelo, which I suspect was very hard to overcome,
is that musical strings have a very high "Q" or narrow bandwidth,
meaning that to make them vibrate with an alternating current, the
current's pitch has to match that of the string almost exactly.  A
Choralcelo's generating plant, spinning away in the basement, would
have to be maintained at a very, very constant speed, a fraction of
a percent.  Meanwhile, the strings would have to remain in tune.

If weather caused the piano harp to go flat, the technician could
adjust the generators a little slower for the day, to match, but as
individual strings went their separate ways, the effect would not be
one of out-of-tuneness, but rather a weak sound from any detuned
strings, since they would not quite match their generator's frequency.

The "microphone" version overcomes this problem, by taking oscillation
feedback directly from the string.  It is guaranteed to "sing" at the
string's pitch.  So tuning problems are no worse (or better) than in
any piano or organ.

But that carbon button or contact pair per string would be a real bear
for reliability.  It was hard to keep a straight face while reading the
quoted article's comments on the complex and unreliable piano action!

This instrument's biggest problem would be a slow attack on each note,
but some may find that pleasing, like a bowed string.  This could be
reduced by adding a capacitor in shunt across the carbon button, to
give a good initial "kick" to the string.  After-sound could be stopped
by using a second electromagnet to lift a damper per string, like a
piano.  A damper pedal would energize all these magnets at once,
amusing the audience by dimming the room lights in time to your
pedaling :-)

Wiring would not be complex or messy.  One wire from each key to its
note assembly.  Each assembly has a short wire from its coil to its
button mike, which is then returned to ground.

Tone quality would be determined by where along the string the magnet
and button mike were placed, just like a hammer strike point, only at
"loops" rather than "nodes."  Alternately, the mike buttons could be
spread around the sound board or bridges.

I've heard of modern gadgets that combine the drive electromagnet and
the feedback pickup coil (in place of mike button) that can simply be
held over a steel string to make it vibrate.  I think they're used to
tune certain instruments.  Probably much more reliable (and expensive)
than the mike button scheme, but the slow attack problem remains.

On a related topic, I've seen pictures of a German "Geigenwerk", or
keyboard violin, looking like a piano, but each key completed a wooden
linkage between its string and a revolving cylinder covered with rosin.
No idea how well it worked, but it had to be less finicky than the
electrical schemes.

Mike Knudsen