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http://www.google.com/patents/US2517180?printsec=drawing#v=onepage&q&f=false

Art Davis worked for Cinema Engineering, which was marketing a form of this kind of attenuator in 
1953 and probably earlier.

See:
http://www.preservationsound.com/?p=4611
(I provided the scans, from old magazines)

As we know, slide-faders became the preferred interface for analog mixing boards, eventually 
replacing rotary faders in almost all applications (although small-format mixers sold by companies 
like Behringer, Mackie, Alesis and others still use rotary faders). There have been many varients on 
slide-faders over the years. In the 60's, Fairchild sold a fader that controlled the intensity of a 
light source, which then interacted with a LDR to control gain, branded "Autoten." In the 70's, 
Audio Designs & Manufacturing (ADM) patented a mechanical system that drove a pot that controlled a 
VCA to control gain. The advantage of the ADM "Slidex" attenuator was that liquid (perhaps coffee at 
a TV station, beer at a radio station or something more exotic at a recording studio) could spill 
across the slider section of the console and not short out anything, within reason.

Here is the Slidex patent:
http://www.google.com/patents/US3736801?printsec=drawing#v=onepage&q&f=false

-- Tom Fine