Robbie said to my description of using a phone mike insert to measure
a theme suction blip produced by a single snakebite:

>[ Dan's application of the carbon mic as a pressure transducer is
>[ ingenious: the current flowing through the carbon button varies
>[ more-or-less linearly in proportion to the air pressure (or
>[ vacuum).  Dan, did you power the circuit like the traditional
>[ telephone, using a big "No. 6 Dry Cell"?  :-)

It certainly would have been a dry cell, though I can't recall now
just which type !  Yes, through a phone induction coil, in a 1927 Bell
Set No 1 which you could buy surplus in 1952 from the Post Office
stores department for five shillings (then about $0.75) !

The tube from note 88 was taken off an upright's tracker bar and
fed to a sealed box with the insert in it, and part of a roll with
regularly repeated treble-side snakebites played.  The output from
the phone circuit was fed to a cathode-ray oscilloscope (triggered from
the input) where the above blip could be seen repeating on the screen.
Of course, you don't measure actual pressure this way, only the changes
in it.

By the way, my diagram was altered in editing.  Robbie has:

>         .      <-- Theme power
>        : :
>        :  .    This slope steepens if there are a lot of notes open.
>        :   .
> ........    ..............   <--  Accompaniment power
>
>        ^-- snakebite opens "theme" port

This makes my remark:

> Notice that nothing at all happens until most of the snakebite has
> passed the theme port

somewhat mysterious, since the blip above starts simultaneously.  In
practice there is a discrete delay before the secondary valve moves.

My diagram went:

          -- theme power
        .
       : :
       :  . this slope steepens if there are a lot of notes open
       :   .
........    .............. acc power
      === snakebite opens "theme" port

      || <--here's the valve delay, about 0.03s

Notice too that I deliberately showed the theme power some way above
the single-snakebite blip.  A double-snakebite one pretty well reaches
it.  As Julian Dyer remarks, many original Duo-Art rolls used double
snakebites throughout; so these are coded to make them much more
suitable for recutting using large snakebite punches.  But I don't go
along with "doubles are okay for everything" -- singles are critical
for chord shading on some rolls.

Dan Wilson, London

 [ Apologies for my editing errors, Dan; thanks for the corrections.
 [ -- Robbie