Electrical discharges through air are the first  kind of clean artificial light.   The rarefied gas electric discharge tube was  discovered by Nollet in 1740.   Further work was done on this lighting  technique by Faraday, Geissler  and Edison  before the end of the 19Th. century.   With the help of Sprengel's Mercury siphon  vacuum pump of 1865 the development of the  illuminated city of night began.

Close-up of a neon  sign tube.

An electric spark is held small by the pressure of the  surrounding air. The discharge shown here has about   2500 volts across the electrodes with a current of about  4 miliamps. The current limit is set by series resisters.    As the air expands, the  spark becomes larger and fainter.  The amount of  power is fixed,  so it is is spread over a larger volume.   As the air is given more room by the pump,  the discharge will fill and be stopped by the chamber  walls.

 Note, plans for a simple power supply are underway.  There are plenty of
books  and perhaps some people around schools who have knowledge that can
lead  the building of high  voltage power supplies of the kind needed for this
demonstration.

 The voltages used here are potentially dangerous. Connect clip leads between
the lamp and the supply terminals before plugging in the power supply.
 
 

Cartridge fuses of this type are a good source  of short lengths of small diameter glass tubes.   Get a number of fuses to practice on.   Grasp the fuse with a slightly damp heavy  duty paper towel.  Place the tines of a fork  against the inner edge of the cap on the other  end and push gently outwards while bringing  the fuse end over a lit candle. The glue and the  solder should melt and the cap slide off.   Repeat  for the other end.

 

Clean the remaining glue from the glass  tube ends with acetone-moistened paper  towel.   Make sure to clean all lint and moisture  from all mating surfaces.   Assemble in a dust and lint free area.   Silicone grease could assist assembly  and sealing. A solid 1/4" Dia rod can be  used instead of the soldered tube segment  as shown to the right side of this picture.

 

The tube is ready for connection to the  pump and power supply.

 

Old tube type lasers are now obsolete and many of the tubes have  gone bad.  Now many of the power supplies are unpaired with no  further purpose.  Surplus dealers and some flea market venders sell  them for a buck.   These electrical connections should be made before applying power.    Small power supplies can deliver quite a jolting shock. Hazard comes  not only from the effect of the electricity, but also injury from hitting  one's self against surrounding things and other people nearby.    Do not leave out the 25 K ohm series ballast resister.   This resister reduces the current and prevents it from rushing or  surging out of the power supply in bursts.   These power resisters come a glazed ceramic tube ranging from a  little over an inch 3/8" dia for a 10 watt to a 25 watt one of 4 inches  long by 5/8 th.of an inch dia.

 
 

Rarefied  nitrogen gas gives off  a violet light when a current  passes through it.  This turns to a  pale blue gray when water vapor  given off by the internal surfaces  of the setup, replacing the dry air.    These pictures were taken when  I had a shorter glass tube section.   There is a considerable, but  harmless amount of long wave ultraviolet  light from this setup.  The glass  of the tube blocks the  harmful short wave ultraviolet  rays.   An experiment was tried using a  quartz tube with the rarefied air  discharge. It still did not produce  short wave uv. A mineral  prospector's uv lamp and a  postage stamp was used to verify that tube was quartz.   More on this later.   As the air becomes more  rarefied,  the discharge assumes  a peculiar striated appearance

 
 

This electrical discharge is  affected by magnets.    Here is a current flowing in a  gas which is free to move about.   Just as an unconstrained wire  near  a strong magnet moves  when a current passes trough,  this discharge will show the  magnetic deflection principle.   The larger tube shown here has  a white phosphor on its lower  inside surface for the electron  beam to excite.

 

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