. A: 'Standard' 26cm tube (I2
B: Russian 'Nixie gas' tube Melz TNUV 0482 (He-Ar mix). C: Russian
). D: Russian 19cm tube (H2
E: Peter Jameson
19cm tube (Ne). F: Peter Jameson 26cm tube (Hg-Ar). (E
and F have rare-earth oxide cathodes).
A spectrum tube has a narrow (capillary) mid section
and contains gas or vapour at low
The capillary eliminates the need for a slit-collimator when examining
light output with a simple prism or grating spectroscope, and it
ensures high brightness when a slit is used. This type of tube, with a
wide choice of fills, is used for Teaching Laboratory
demonstrations and is still readily available (cost ca. £10 -
£60 UK inc. VAT from educational suppliers). The 'standard'
length of 26cm allows the tubes to be used in a variety
of nominally-safe high-voltage
designed for teaching purposes. The 'safety' of the
PSU comes from recessing the tube end-caps during operation, but
freedom from electric shocks and burns is by no means assured. The PSU
up to 4kV to start the tube, and will then run it at about 1kV
(depending on the fill type and pressure). Note that, for a current of
10mA, this corresponds to about 10W, which means that
tubes can get very hot and should only be used for short
Some power supplies have a control enabling the brightness to be
reduced after starting, but true constant-current operation is
typically not achieved in the cheaper units, and continuous use is
still not advised.
Note that Hg tubes generally also
contain some Ar to get them started. The Hg
spectrum lines at 436 and 546nm
are typically used for spectrometer
Eisco states that its tubes have pure
Nickel electrodes. Most tubes with coiled wire electrodes are probably
the same. Cup electrodes with rare-earth oxide thermionic coating
(sign-tube cathodes) are can radically increase tube life, but are not
operated at very low current by
using a smal DC
fluorescent tube inverter. The brighness is not sufficient for
recording the spectrum.
operated at low current. In this
good high-brightness arc has struck in the gas column. The beading
effect depends on the inverter running
frequency and gas pressure and is possibly an acoustic resonance. It
often becomes invisible if the frequency is changed by
increasing the current, but then the tube will get hot and
must be switched off after a short period. Beading is discussed in more
detail in Peter
, ca 1930, labelled C2
This item is in the collection of the Porthcurno
Museum of Global Communications
. (where it was catalogued,
generically, as a "Crookes Tube" when viewed by the author some time
Eisco scientific online catalogue.
[Eisco tubes - type 'spectrum tube' into the search box]. Also supplies
Eisco PH1197B tube holder at a good price.
'spectrum tube' into the search box].
[type 'spectrum tube
into the search box]. Also supplies a 26cm tube holder, but more
expensive that the Eisco..
Select School Supplies
into the search box]
See also Indian suppliers on ebay, but beware: if buying a
tube bundle, make sure that the gases to be supplied are listed.
An 18-tube bundle might contain: Hg, Ar, Air, NH3
OH, Ne, N2
, Cd, Cl2
, He, Kr, H2
Na, S and O2
Neon tube transparent supports for 6 - 20mm
for Gas Discharge Tube. Patent WO 1991/06197
Mentions beading on p2.
Thanks to Peter Jameson
for supplying the inverter used for the
low-current discharge pictures above. Peter makes tubes and
scintillation screens and sells them
Peter Jameson's ebay shop
Peter's tubes are superior to the typical Teaching-Lab spectrum tubes
educational suppliers because they have Barium Oxide coated
thermionic cathodes as opposed to coiled Ni wire inserts or plain
metal. They can be
operated continuously from a 3 kV 30 mA neon-sign transformer to give a
nominal life of 2000 hours. The ZnS:Ag scintillator screen can be used
to detect α-particles and UVC from silica-walled Hg tubes.
Thanks also to Pete for reminding me of
the neon signs and Fire Safety switches that were everywhere when I was
Pete has no website, but there is now
an article on
this site: The Art of Discharge Tube Making
in which he describes his various experiments and activities:
D. W. Knight, 2013 - 2021.
Last update: 2021 July 23rd