On Tue, 07 Jun 2005 15:01:26 GMT, "Henry Kolesnik"
wrote:

Now the diodes resistance can be
easily calculated but I'm not sure how to visualize it. Is this where your
term cathode resistance enters the picture?

Hi Hank,

Getting electrons to "boil" off the cathode (or filament, same thing)
is not a simple task otherwise there would be no filaments needed.
Even with filaments, Edison current is not very considerable unless
you add a monomolecular layer of metal to the surface of either the
filament, or the cathode.

You may note that some tubes are described as having "Thoriated"
filaments. This is that monomolecular addition. Its purpose is to
lower the W, the work function of the interface.

It is far from odd how physics demonstrates at every stage and in
every discipline that interfaces where there is mismatch, there is
difficulty in transfering power. When an electron from the interior
of the metal crystal approaches the surface, it is repelled by that
interface due to the potential of the work function, and is attracted
by the bulk material behind it. The Thoriated surface offers a
matching mechanism between the bulk metal and the free space beyond
the surface. In classic Optics this is known as Index Matching.

To give examples as to how well a monomolecular addition performs:

A Tungsten filament (no treatment) offers a current density of ½A/cM²
This makes for a baseline.

A Thoriated Tungsten filament offers a current density of 4A/cM²

Then we step back to a cylindrical cathode employing Barium.
Such a cathode offers a current density of ½A/cM²

That seems rather regressive to use a cathode, but temperatures are
telling. The simple Tungsten filament is operating at 2500° K and the
cathode needs only to simmer along at 1000° K. This gives
considerably longer life and more efficiency (most of the power for
heating is lost through radiation). Needless to say, cathodes find
more application in low power circuits, or their surfaces are treated
with other low work function metals for greater emission.

Now, when you add a potential gradient, you also lower the work
function of the surface (but it is always an advantage to have it
lowered going into this game). This is called the Schottky effect.
One might be tempted to simply ask, why don't we up the voltage and
discard the filament? This device would be called a Cold Cathode but
the potential gradient then rises to the level where you run the risk
of secondary emission.

When that electron stream strikes the plate and raises the
temperature, it is just short enough power to present this secondary
emission. But if we were to run at 17KV or so, then the electron
stream would be so aggressive as to produce high energy effects such
as X-Ray emission.

So, to return to the resistance of this all, we have physical
impositions of cathode surface area (probably offering the prospect of
being greater than filament surface area), current density, and
potential difference. Discarding all the extraneous surface units
(cM²) and employing the proper division (E/I) we have a resistor that
glows in the dark under extremes of operation. How this fails to be
source resistance is strictly handwaving and the schematic symbolic
mysticism of demanding a carbon composition resistor.

73's
Richard Clark, KB7QHC