Here are a few more thoughts to add to the speculation regarding the
Welte T100 ["instant"] recording-playback thread.

In the following write-up I have combined what seems to be the agreed
elements of the Welte T100 recording system (carbon rods with variable
resistance, mercury trough) with speculated elements (springs, printing
with conductive ink) and added a couple of elements of my own (relays
that pull in with different currents, a second piano-forte action to
strike the carbon rods, use of the string damper to detect the end of
a note) to create a system to record and playback the expression
(intensity) of the artist's playing.

The values chosen for voltage and resistance are arbitrary.  If they
are changed the corresponding current values will change but the
concept remains the same.  Table 1 lists the current for individual
keys played and the total current in the mercury trough for different
combinations of number of keys played and pp-ff intensity of each key
played.

MAKING A RECORD

Carbon rod:  Initially the rod floats vertically (perpendicular) in the
mercury.  The portion of the rod immersed in the mercury when the rod
is floating before being struck is insulated so that no current is
drawn when no note is played.  The resistance of the carbon rod varies
from a large resistance when first immersed in the mercury by a soft
(pp) note to a small resistance when fully immersed in the mercury by
a loud (ff) note.

Spring:  Carbon rods do not like to float perpendicular to the mercury.
A conical spring is fastened to the top of each rod to maintain the rod
in a vertical alignment with respect to the mercury.  The spring
provides less chance of the rod binding than if the rod were to be
supported by a tube.  The spring also provides electrical contact to
the top of the rod.

Piano-forte:  The rod must be struck so that it is immersed in the
mercury to a depth proportional to the intensity with which the key is
struck.  A piano-forte action similar to that used to strike the string
does this.  The rod piano-forte is separate from the string action but
is operated by the same key.  The "rod hammer" strikes the rod at the
same time the "string hammer" strikes the string.  A piano-forte action
for the rod independent of the piano-forte for the string enables the
rod striking action to be optimized.

Damper:  The duration of each note begins when the string hammer
strikes the string and ends when the damper touches the string.  When
the damper is off the string the pp relay is enabled.  The pp relay
pulls in when the conductive portion of the rod first enters the
mercury.  The pp relay drops out when the damper touches the string.
In recording the note begins when the pp relay pulls in and ends when
the pp relay drops out.

Relays:  The use of relays isolates the printing device from the
vagaries of the varying rod currents.  Five relays corresponding to pp
(very soft), p, mf, f, ff (very loud) with their coils wired in series
are used for each note.  Each relay requires a different minimum
electrical current to pull in.  The relay requiring the least amount of
current to pull in corresponds to pp and the relay requiring the most
amount of current to pull in corresponds to ff.  As the carbon rod goes
deeper into the mercury its resistance decreases from and the current
drawn by it increases proportionally.  The relays are successively
pulled in until they are all pulled in for an ff note.  The basic
circuit consists of a voltage source (V) feeding the five relays
(series resistance of the five relay coils = Rk) and a carbon rod (
varying resistance = Rc) in series (total series resistance = Rk+Rc)
for each playing note.

Mercury trough:  As the note is played the carbon rod makes ground
contact through the mercury trough.  There are two troughs, one for
bass the other for treble.  The current through one rod-relay circuit
corresponds to the intensity of the corresponding note played.  The
current through the common trough corresponds to the combined intensity
of all of the notes played.  Table 1 lists the current for individual
keys played and the total current in the mercury trough for different
combinations of number of keys played and pp-ff intensity of each key
played.

Conductive ink:  Note & Note Intensity:  Each note has from 1 to 5
marks printed on the master record.  The first mark starts when the pp
relay pulls in and ends when the damper returns to the string.  The
second through fifth marks start if and when the corresponding relay p
through ff pulls in and end when these relays drop out.

Bass-Treble Intensity Analog: Bass-Treble intensity corresponds to the
total current through the respective troughs.  This is printed in an
analog manner by a stylus whose position is determined by the trough
current.  The result in this case resembles the "seismograph recorder"
plot as shown in http://mmd.foxtail.com/Pictures/Welte/seismic.html.

Bass-Treble Intensity Discrete: The Bass-Treble intensity is printed in
a discrete manner in addition to or in place of the analog technique
above.  It is possible to have five relays wired is series and operated
by the total current through each trough.  The most sensitive relay
corresponding to pp pulls in with the minimum current and the least
sensitive relay pulls in with a large current corresponding to ff.

PLAYBACK

Assumption: The equipment to playback the master roll need not be
identical to that used to playback an edited roll.  It is possible to
playback the master roll as soon as the conductive ink is dry.  Five
levels of playback intensity when applied to individual notes or to the
common bass-treble provides a reasonable fidelity.

Reading Master Roll

The master roll is read by contacts to sense the conductive ink.  The
note on-off is determined by reading the pp marking of the Note-Note
Intensity print.  The note intensity is determined by reading the p
through ff marking of the four Note-Note Intensity print tracks.  The
bass-treble intensity is determined by reading either the Bass-Treble
Analog or Bass-Treble Discrete print tracks.

Controlling Expression

Individual Note Expression:  Individual note expression is controlled
by five relays corresponding to pp through ff for each note.  Each
relay controls the intensity of the vacuum applied to the corresponding
note playing bellows.  This is done by setting up an expression
governor for each note similar to that used to control the Bass-Treble
expression in the T100 piano.   The five relays operate five solenoids
which in turn determine the how much the knife valve of the expression
governor is opened.  The more the knife valve is opened the louder the
note is played.

Bass-Treble Note Expression:  The bass-treble expression is based upon
reading the analog or discrete printed record.  At this point I can
think of no way to read back the analog expression data without
breaking it up into discrete levels.  This is done using five levels of
intensity.  Data, printed in discrete form initially, is played back by
reading the discrete marks directly.  Expression for Bass-Treble notes
is controlled by a common vacuum intensity control unit (expression
governor) similar to that used to control the Bass-Treble expression in
the T100 piano.  The expression pneumatic of the T100 piano is replaced
by five relays operating five solenoids which in turn determine the how
much the knife valve of the expression governor is opened.  The more
the knife valve is opened the louder the note is played.

         Table 1

INTEN All pp
 #keys              Rz      Ic  Ic_tot
 1 pp            11000  0.0091  0.0091
 2 pp             5500  0.0091  0.0182
 3 pp             3667  0.0091  0.0273
 4 pp             2750  0.0091  0.0364
 5 pp             2200  0.0091  0.0455
 6 pp             1833  0.0091  0.0545
 7 pp             1571  0.0091  0.0636
 8 pp             1375  0.0091  0.0727
 9 pp             1222  0.0091  0.0818
10 pp             1100  0.0091  0.0909

INTEN All ff
 #keys              Rz      Ic  Ic_tot
 1 ff             1100  0.0909  0.0909
 2 ff              550  0.0909  0.1818
 3 ff              367  0.0909  0.2727
 4 ff              275  0.0909  0.3636
 5 ff              220  0.0909  0.4545
 6 ff              183  0.0909  0.5455
 7 ff              157  0.0909  0.6364
 8 ff              138  0.0909  0.7273
 9 ff              122  0.0909  0.8182
10 ff              110  0.0909  0.9091

INTEN Mixed ff/pp
 #keys              Rz      Ic  Ic_tot
 1 ff             1100          0.0909
 2 ff/pp          1000          0.1000
 3 ff/pp/pp        917          0.1091
 3 ff/ff/pp        524          0.1909
 4 ff/ff/pp/pp     500          0.2000
 4 ff/ff/ff/pp     355          0.2818
 5 ff/pp/pp/pp/pp  786          0.1273
 5 ff/ff/pp/pp/pp  478          0.2091
 5 ff/ff/ff/pp/pp  344          0.2909
 5 ff/ff/ff/ff/pp  268          0.3727

Notes:

1. Rc = resistance of carbon rod.  Rc varies from 10000 ohms when the
rod initially makes contact with the mercury to 100 ohms when it is
fully immersed.

2. Rk = resistance (ohms) of the five series relays.

3. Rz:  For one note played Rz = the series resistance (Ohms) (Rc+Rk).
For two notes played Rz is the series resistance of the first note in
parallel with the series resistance of the second note.  For multiple
notes played Rz is the combined parallel resistance of each individual
note resistance.

4. Ic = current (amperes) in one carbon rod.

5. Ic total = current (amperes) in mercury trough.

6. V = input voltage

7. Intensity = pp for soft and ff for loud.

8. #Keys = number of keys struck in bass or treble portion.  Generally
the maximum number of keys is five in bass or treble.

Richard Cardwell