Counting tube bulk gas illumination
The bulk gas of a discharge tube can be made to ionise by subjecting
the tube to a strong electric field such as occurs in the vicinity
of a normal-mode helical antenna or a self-resonating coil. In
lamps that are intended to be used as cathode-glow devices, the
bulk discharge is usually supressed by using a short inter-electrode
distance (so that the Crookes dark-space fills the tube). Thus
RF excitation in this manner can give additional information
about the fill gas.
In the photograph above, a coil
is excited at its self-resonance frequency by means of an induction
loop driven using a radio transmitter (see coil
). A weak glow in a nearby tube can
be obtained without electrical connection, but earthing an internal
electrode to the cable feeding the loop increases the field gradient
and causes the whole tube to light brightly.
The tube shown above is a GR10G
. The anode pin is earthed, since this (the outer mesh
cage) creates the greatest capacitance between the tube and the
nearby coil.. The frequency is 24.7 MHz. A close-up of the tube
is given below, with the normally-operating tube shown on the
right for comparison.
Not all manufacturers use the same gas mixture. The GR10G appears
to be filled with a neon-argon penning mixture, but other tubes,
as shown below, seem to contain mercury vapour and neon.
Bulk glow of the red-coated Mullard
tube (anode wire is grounded via metal forceps).
The discharge appears green through the red filter, but the cyan
colour (blue-green) can be seen through the un-coated base. The
red dip-coating is evidently effective at removing the blue component
of the Hg discharge, but does not greatly attenuate the spectral
components in the green-yellow region.
Bulk glow of clear glass Hivac XN12 (anode
wire grounded). The cyan colour is characteristic of mercury
vapour. The pinkish colour corresponds to the region closest
to the induction coil and is due to the glass.
Although the bulk gas glow appears cyan,
the numeral display with 200V DC on the anode and a 33 kΩ
series resistor is orange. The running voltage is ca. 140 V,
and so the current in this case is 1.8 mA.
The reason for the extreme difference between the two discharges
is that the RF-induced glow is essentially a positive column
discharge (having the spectrum of an arc discharge even if the
current density is low). In cathode-glow lamps, the positive
discharge is not usually noticeable, due to the short inter-electrode
distance, but it appears in the high-frequency bulk gas discharge
because the electrodes are not involved and there is little overall
ionic migration. With DC bias across the electrodes however,
we see the cathode glow, and the positive column glow is practically
eliminated. In this case, the positive column has the cyan colour
characteristic of Hg vapour, whereas the cathode glow is characteristic
of neon. Thus the fill gas is Ne, with a trace of Hg.
No cute furry animals were harmed
. . . . .
The photographic animation sequence of the working XN12 on the
right (above) was taken after the abuse depicted in the photograph
on the left. Evidently the tube is unfazed by its ordeal (but
it did get quite warm).
© D. W. Knight, 2013