Re: Thorated vs Pure Tungsten

To Terry, All,

Thoriated tungsten (from thorium, which is radio active,) has its 
proper place in Tesla coil designs.  Namely, as  electrodes
(with hemi-spherical ends) for both rotary and stationary
rods, where the input power is moderate to high.  It is true
that the thorium helps to liberate free electrons, that aid in the
ionizing process, that leads to a saprk breakdown between the
gaps.  This can be either a good or bad thing, depending on the
overall gap design.

Typically, in electrode applications, the thorium "mix" is
limited to about 2 % by volume of the material.  This is not
all that much, but it does make a significant difference, as Terry
has observed in his safety gap application.

A much better choice for a stationary safety gap would be
copper or brass balls, highly polished and of the appropriate
diameter and spacing, for the intended "hold-off" voltage.  Many
text book references exist, to assist the builder in selecting the
appropriate values for this type of safety gap.  And, this type
of gap can be found to be extremely reliable, with no sporatic
behavior in the tank circuit.  Typically, you want to "snub"
the voltage spikes that come along, for whatever reason, without
a power-robbing follow-on high current arc across the gap.
Careful experimentation will nearly always yeild satisfactory

Best regards,

Bill Wysock. 

> Date:          Sun, 13 Jun 1999 20:13:59 -0600
> To:            tesla-at-pupman-dot-com
> Subject:       Thorated vs Pure Tungsten
> From:          Tesla List <tesla-at-pupman-dot-com>

> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
> Hi All,
> 	I use 1/8 inch diameter thorated tungsten rods in my new neon protection
> circuit as safety gaps.  However, they tend to sporadically fire at low
> voltages.  The ends are cut pretty square instead of being rounded.  I can
> replace the rods with the pure tungsten ones and grind them round on the
> ends.  However, I was just wondering if the thorium was mostly to blame for
> the sporadic firing or the square ends?  This happens with nothing else
> connected except the neon running at full normal voltage.
> 	I really think, now, that the pure tungsten is by far the best for Tesla
> coil applications.  The slight radioactivity of the thorated versions is
> unsettling and this sporadic firing thing is a pest.  The tungsten rods
> seem to be far far better than anything else I have used and are definitely
> the way to go for gap electrodes, as so many have found...
> Thanks,
> 	Terry
Tesla Technology Research