Re: Fluorescent bulbs
SCHRAMMR@WLSVAX.WVNET.EDU
Fri, 04 Oct 1996 11:32:03 -0400 (EDT)
Terry,
Maybe you have different types of lamps in the UK. "Anode" and
"Cathode" imply positive and negative which requires direct current
not alternating current. The FL lamps we use in the USA run on AC.
The whole idea began with Sir William Crookes who put electrodes in
highly evacuated tubes and applied a source of high potential. He
observed ( via a fluorescent i.e. zinc sulfide coated screen) some
"rays" emitted from the cathode (negative terminal) which he called
"cathode rays." These turned out to be electrons. Later Roentgen, when
repeating these experiments noticed additional "rays" coming from
the "anode" which were a lot different than cathode rays. He called
the "X-rays" i.e. "X" for unknown. One can put many different types
of gasses in glass tube under partial pressure, add electrodes and
a high potential source and you will get ionization of the gas and
a current of positive and negative ions (electrons). The energy
supplied by the electric field excites these gas atoms and what results
is the atomic spectrum of the element e.g. neon, mercury, argon etc.
In the case of mercury, in addition to the visable spectrum, it
produces strong lines in the ultraviolet which is why we can use
flourescent tubes as a strong source of UV light for alt-process
printing. It is also this UV light which causes the coating (phosphur)
to emit visable light. If you examine the light from an FL tube with
a diffraction grating or prism spectroscope, you will see the continuous
background of the light from the phosphor with the strong line of
the mercury spectrum superimposed thus proving that there is mercury
in the tube. A lot of mercury is not required so you might not see
it if it is broken open. BTW, those bluish-green, mercury vapor street
lights contain not only mercury but also helium sometimes. The yellow
ones contain sodium vapor and sometimes helium as well. The helium
is used to start the arc and as the tube heats up the mercury or
sodium vapor takes over. Sorry about the long Physics lecture ;-)
Bob Schramm