[GreenKeys] [Bulk] Re: Telephoto machines?

[Jim Haynes]

Western Union developed an electrosensitive recording paper in the 1930s, 
which they named Teledeltos.  There was a black conductive inner layer, 
with a metallic coating on the back and a grayish-white coating on the 
front.  Supposedly it was electrically conductive all the way through, so 
that a voltage applied across it would ablate the front coating to
expose the black interior.  So maybe it wasn't necessary to spark, but
sparking and smoke happened anyway.  As the ingredients were a secret
one could worry whether it emitted toxic fumes.

You can read about it in Western Union Technical Review January 1949 and
some other issues.  Back in the 1930s it was one of their trade secrets.
They had it manufactured in two or more stages by different contractors
so that nobody outside the company knew all the steps in making it.

As for negative images, that's an artifact of the way you have things
hooked up.  When you scan a black-on-white document it is the background
that generates the strongest reflected signal, while the black generates
little or no signal.  A chopper wheel in the light path turns the signal
into AC for transmission over the circuit.  If you applied that signal to
the recorder, where the larger signal makes a black mark, you would get
a negative image.  W.U. had a thing called a regulator-inverter in the
telegraph office end of the circuit.  This applied AGC and also inverted
the signal to that the highest voltage was for black and the lowest was
for white.