> No, my guess is that they read the resistivity of the ink track and
> then it activated the electric solenoid action of the special playback
> piano. What is concerning me is this: The ink track is very small and
> any large current that was passed through the ink would turn it into
> instant ashes. I wonder if the system used an electrical bridge, with
> the ink track as one leg of the bridge?
That sounds right. Telegraph people had been using bridges for many
years by that time. The difficulty with any of this, though, is that
without electronic amplification you'd have needed an awful lot of
machinery: a bridge circuit with an energetic and responsive voice-coil
circuit that would change the resistance of a potentiometer that
supplied the current to a magnetic amplifier that ran an AC solenoid
(you've gotta use AC with a magnetic amplifier). Keeping just one of
these reading devices running for any length of time in 1905 or
whenever it was would have been quite a job, and Mr. Welte would have
needed 88 of them, or however many of them that there were.
I think that it's more likely that there was some division of labor
here: the magnetic ink could easily have conducted enough current to
switch a sensitive telegraph relay. Such a relay in 1900 would have
been responsive enough to read the fastest of notes. A bank of 88 of
these telegraph relays (not really a huge number for the telegraph
offices of the time) could switch the solenoid current to push the keys
of the piano in question. This would have given him an electric player
piano: lots of persnickety contacts to keep clean, but it would have
been a fairly straightforward job for anyone with experience at the
local telegraph office.
The dynamics, though, might well have been interpreted by the people
who made the punched rolls that were produced later. They'd look at
the width of the line and, using the eyeball and musical skills that
I've read about in many posts in MMD, code in the dynamics and expres-
sion on the final roll. Thus the immediate playback roll would have
lacked dynamics, but still would have seemed pretty slick at the time.
It's not obvious what the big advantage of the mercury switch
arrangement would have been, though: they'd have been better off just
connecting those knife-edge rolls right to the piano keys.