G3YNH.info Spectrum Tubes

Capillary spectrum tube.

Crookes tube with capillary.
Capillary spectrum tube, ca 1930, labelled C2H2 (acetylene).
This item is in the collection of the Porthcurno Museum of Global Communications. (where it was catalogued, somewhat generically, as a "Crookes Tube" when viewed by the author some time before 2012).

A spectrum tube contains gas or vapour at low pressure. The narrow (capillary) middle section eliminates the need for a slit-collimator when examining the light output with a simple prism or grating spectroscope (it also 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 power supplies designed for the purpose. The 'safety' of the Teaching PSU comes from recessing the tube end-caps during operation, but freedom from accidents is by no means assured. The PSU can typically produce up to 4kV to start the tube, and will then run it at about 1kV and 10mA. Note that this corresponds to about 10W, which means that  tubes can get very hot and should only be used for short periods. Some power supplies have a control enabling the brightness to be reduced after starting, but true constant-current operation is typically not achieved 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 calibration.
     Eisco states that its tubes have pure Nickel electrodes. Other makes are probably the same.


Hg spectrum tube operating at low intensity
A Mercury spectrum tube operated at very low current by using a smal DC fluorescent tube inverter. The brighness is not sufficient for recording the spectrum.


Ne spectrum tube operating at low intensity
A Neon spectrum tube operated at low current. In this case a good high-brightness arc has struck in the gas column. The beading effect is an acoustic resonance that depends on the inverter running frequency. 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.


Tube suppliers:
www.eiscolabs.com/ Eisco scientific online catalogue.
www.rapidonline.com/ [Eisco tubes - type 'spectrum tube' into the search box]
www.betterequipped.co.uk/ [type 'spectrum tube' into the search box].
Philip Harris [type 'spectrum tube into the search box]
Select School Supplies [type 'spectrum tube into the search box]

See also Indian tube and PSU 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, C2H5OH, Ne, N2, I2, Cd, Cl2, CO2, H2, He, Kr, H2O vapour, Na, S and O2.


Acknowledgement:
Thanks to Peter Jameson for supplying the inverter used for the low-current discharge pictures above. Peter makes tubes and sells them on ebay.
Hg spectral tube 280mm.
Hg spectral tube 190mm.
 Ne spectral tube 280mm.
Ne Spectral tube 190mm.
Peter's tubes are superior to the typical Teaching-Lab spectrum tubes from educational suppliers because they have Barium Oxide coated thermionic cathodes as opposed to coiled Ni wire inserts. They can be operated continuously from a 3kV 30mA neon-sign transformer to give a nominal life of 2000 hours.
     Pete has no website, but an article is planned on this site to detail his tube-making activities  and products.


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D. W. Knight,
Page started 2013.
Update 2021-June-5.