In a simple two electrode glow discharge tube, for
any given set of conditions (voltage and pressure), the physical form of the
negative region is a constant entity. If the pressure is lowered, the negative
region will expand and the positive column will shrink. If the electrodes are
moved closer, the positive column will shrink to the point where it may almost
completely disappear. With long electrode separations (as is the case with a
neon sign), the positive column occupies the tube to the extent that the
negative region is almost unnoticeable. On the other hand, if the anode is
moved closer to the cathode, such that it closely approaches then enters the
negative region, the voltage will have to rise steeply in order to maintain a
discharge. This is because there is less space for ionization to occur. This
condition is termed an obstructed discharge. A cute device that
demonstrates this is the so-called detour tube developed by Johann Wilhelm
Hittorf in 1884. Here the direct path between electrodes becomes shorter than
the length of the cathode region as the pressure in the tube is lowered. When
that occurs, the discharge takes the longer route through the side tube where a
normal glow discharge can form. The action of the Hittorf tube illustrates the
Paschen effect, also known as the minimum sparking potential. One
practical use of this effect is for high voltage feedthroughs into vacuum
chambers. Instead of trying to insulate the negative connection (which will
more than likely result in virtual leaks and other problems), if the lead-in is
surrounded by a coaxial anode with a spacing that is smaller than the negative
region, the gap will hold off a substantial voltage. This approach is widely
used in sputtering systems and other plasma apparatus. (The preceding text
was drawn from the First Five Years compilation.)
The video shows the operation of a detour tube as
the pressure is lowered. The gap is approximately 3/8 inch and the power supply
is approximately 4 kV with the cathode at the top. As the pressure declines
filaments initially form. These soon give way to a glow that fills the
electrode gap. With further pressure decline the dark space broadens and the
discharge begins to follow the path of the detour. Eventually the glow in the
direct path disappears and the discharge is only seen in the detour. Finally,
with a further decrease in pressure even the discharge in the sidearm is
extinguished. This is the point at which the glow becomes a constricted
discharge, a consequence of the small diameter of the tube and the low
pressure.
