Re: async rotory gap?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Bart, David, All!

>As you know, with a sync gap we have the ability to fire the gap at
>concentric points along the AC cycle. The async of course cannot
>sync and therefore will fire at good and not so good points along
>the AC cycle. There are a couple basic async gaps. One is an AC
>motor. This will run at something like 1750 or 3450 rpm's. The other
>would be a DC motor. The DC motor has the ability to run at
>variable speeds as a function of input voltage. With DC, you get to
>play around with the break rates and can find break rates which
>please spark growth. With the AC motor, you would need to build a
>circuit to perform this function.
>
>I've run all three types.
<snip>

There is one further variety, which AFAIK has not seen service since
Marconi, but maybe there's someone out there who has done it and can
comment, and I'm posting this in the hope they will step forth and do
so!  It would be a particularly neat project for 2001 as it was the
device used in the first successful transatlantic transmission a
century ago.

The main problem which the rotary was designed to cure is quenching at
high power, i.e. the prevention of continuous arc formation.  Whilst
the rotaries used by most (all?) these days use fixed and flying
electrodes, which limit the discharges to specific points on the
timing cycle (or pseudo-random if asynch) Marconi's original idea,
used in the Poldhu transmitter 100 years ago (the infamous
transatlantic "S" signal) was to use two (or more) rapidly rotating
disks spaced a short distance apart, their axes at right angles, with
the spark taking place between the rims of the disks.

In this way, from one moment to the next, the spark was taking place
from a cool metal surface, all the rest of the disk periphery where
there was no spark was of course cooling quite nicely due to its rapid
rotation through the air and also its considerable heatsinking ability
to the main mass of metal.  There was also considerable amounts of
cold air being propelled through the sparking zone by friction of the
air with the disk, rather like a centrifugal fan (or even Tesla
turbine!)  The spark length was adjusted exactly as for a standard
static gap by the spacing of the disk rims, and the timing of the
discharges would be just the same as for a static gap.

I wonder if anyone has tried this with a TC?  In "wireless" service it
was good for a few hundred kVA with steel disks around a metre across
and half an inch thick, so a one foot diameter disk should be good for
a fair few kVA.  The rotary-with-electrodes was only introduced
apparently because the radio waves generated by the smooth disks were
insufficiently distinguishable from natural static to give optimum
reception and the "characteristic rasping tone" from the
rotary-with-electrodes (especially the synch rotary and a 400 or
800c/s power supply) which all TC builders know and love was much more
identifiable to the wireless op's ears.

More trivia at:
http://home.freeuk-dot-net/dunckx/wireless/sparksnarcs/sparksnarcs.html

Dunckx
Geek#1113 (G-1)