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Re: Antique Rotary Sparkgap
All,
I went to an antique auto show today which had, in addition to many
lovingly restored cars, a large number of "flea market" tables set up
where people could sell various automobile-related gadgets, parts, and
junque. I found a nice 10A variac ($3) and some heavy-duty 60A 240V
4-wire power connectors ($7). However, one thing stopped me in my tracks
- in amongst a whole lot of miscellaneous old auto parts one guy was
also selling a ROTARY SPARK GAP! It turns out it belonged to his dad,
who was an early ham radio operator back in the pre-tube, spark-gap
transmitter days. It was fairly grungy looking, but looked like it had
all the parts, so I bought it for what I think was a steal! (My wife,
however, was not too sure just who did the stealing on this deal...)
(:^)
Once I lugged it home, I proceeded to clean it up and do some repair
work. All major assemblies were mounted on a solid polished 11 1/2" x
13" x 1" marble base via countersunk holes drilled through the marble.
After removing all the hardware, the base cleaned up nicely, revealing
beautiful veins of dark gray/black and golden tans against a white
background. The top surface was slightly polished (sort of like a satin
finish instead of high gloss).
The unit came with an old 5 1/2" diameter universal motor with bronze
bushings and brass oil containers below the bushings. Inside these
containers are fiber wicks which permit oil to be pulled up to the
bearings as needed. The cast aluminum rotor is about 6.5" in diameter,
and has 14 paddle-shaped copper or brass plates, each .062" thick. These
plates extend radially from the rotor about 3/8" and are about 3/4"
wide. These are positioned into slots cut into the rim of the rotor, and
are clamped in place by metal rings on either side of the rotor, which
are attached via short screws to the rotor. This results in the rotor
electrodes being held in place very firmly.
The aluminum rotor assembly is about 1/8" thick at the rim, and about
5/8" thick at the center hole, which is 2" in diameter. Inside the
center hole is an insulating hub, made of some dark brown material
(phenolic??), which is threaded onto a 5/16" diameter brass threaded
shaft, and secured with a single nut. The threaded shaft is also screwed
into a brass shaft coupling. The shaft of the motor is slotted, and a
single setscrew secures the shaft coupling to the 5/16" diameter motor
shaft. Overall space between the hot rotor and the center bolt is only
about 3/4", which would limit its voltage handling capability to perhaps
10 KV or less (versus ground). The motor case does not show any evidence
of ever being tied to anything (like a safety ground...).
Two 3/8" diameter stationary electrodes are at 180 degree positions vs
the rotor, and are adjustable via large thumbscrews electrode holders
which are in turn bolted onto 1" wide x 0.105" thick brass strap
material. The straps are then bent at 90 degress to permit mounting to
the marble base. The stationary electrodes are silvery-gray in color,
are heavier than aluminum, and are non-magnetic. They do not appear to
be stainless steel, they show no sign of rust or corrosion, but they are
definately not tungsten. When I did a closer inspection of the
thumbscrew electrode holders, they appeared to be made to hold larger
(7/16") diameter electrodes.
After cleaning, and replacement of some of the more corroded nuts and
bolts, I reassembled the unit. The motor had been re-wired to a
relatively modern 120 volt cord and plug, which I ran to a 10A 0-140V
variac. The rotor began to turn at about the 40% setting, and rotated
quite rapidly (2000+ rpm) at the 80% setting. The motor shaft has about
1/16" of end-play which causes some unwanted vibration at higher speeds.
Also, the unit is not perfectly balanced, and the insulating hub seems
to be a little out of square relative to the motor shaft, causing some
"wobble" of the rotor. All in all, however, the unit looks like it could
function quite well in low power use (say 1-2 KVA). I wouldn't want to
try frying it with a pig, however.
Anyways... after going to many a hamfest, I've never run into any rotary
sparkgaps - The LAST thing I expected was to stumble upon one at an
antique car show!!
Safe coilin' to ya!
-- Bert --