Just a bit of nit-picking about nomenclature. The electrically
actuated pianos made in the old days, Telektra as well as Mills, used
electromagnets, not solenoids.
The distinction being that a solenoid is a non-magnetic, hollow spool
containing the windings, with a moving, cylindrical armature that can
move through the spool. When current is applied, the magnetic circuit
around the armature exerts force in an attempt to get 'shorter',
pulling the iron armature towards the center of the coil.
An electromagnet consists of an almost continuous 'loop' of iron, with
a small gap in that loop. The windings surround one or more of the
'legs' of that loop. When current is applied, again the magnetic
circuit exerts force, attempting to get shorter. If the gap in the
iron loop is small, that force is concentrated at the gap, and tries
to close the gap.
The Telektra magnet is particularly clever. Here the ends of the fixed
legs of the loop have between them a short piece of magnetic material,
positioned so as to be able to rotate. The rounded ends of this piece
closely fit rounded spaces machined in the ends of the two 'pole' legs.
The rotating piece starts out at an angle to the plane of the poles; as
current is applied, the gaps, already very small, can not get smaller.
But the magnetic circuit can get shorter, by making the rotating piece
move towards a position parallel to the plane of the poles. When the
armature turns, it pulls down on a link wire screwed to the underside
of the key, thereby playing the piano.
With permanently small air gaps in the magnetic circuit, the force
across them is very strong, even near the beginning of the stroke.
Compare this to the action of a solenoid, where the force is quite
small, until the armature approaches the center of the coil. Even
then, the very long magnetic path in air, around the outside of the
coil, is inefficient compared with a magnetic circuit consisting mostly
It is significant that telephone relays, organ direct-action electric
valves, multi-pole organ relays and switches, dot-matrix printer pin
drives, and other industrial applications, still use small-gap
electromagnets, rather than solenoids, where power-to-weight ratio
It is a shame that modern electric pianos use solenoids instead of
electromagnets. A solenoid's force is maximum at the end of its
stroke, just the opposite of the force from a pneumatic. Electromagnets
would be more expensive to make, but they would be very powerful by
comparison. And, if carefully made, they would be much more amenable
to any type of solid-state force or motion control, and easier to
regulate mechanically to boot.
When I saw Mr. Billing's beautiful Telektra in Boston, I was amazed by
the sophisticated design and power of the magnets. Even though they
pull down the keys at a mechanically 'inefficient' point near the
center pins, they exerted plenty of force to play the piano at any
loudness one would want.