Function of a three-terminal spark gap

[ChristofferB]

Hey guys,

I picked this oddity up today at my local electronics store. It seems to be a spark gap, with a fixed and a sliding terminal (sliding terminal has a milimeter scale etched in it) - on a bakelite base. I think it looks 30's-early 50's ish.

The odd part to me is the third, angled terminal below the two other ones. it doesn't have any movable parts, and doesn't really have a screw terminal as the other two, just a 2mm cross hole. it's also very close to one of the terminals.

The guy at the store said he thought it had been used for ignition coil testing, but it doesnt strike me as a mechanic's tool.

Can anyone venture a guess as to its function or origin?

I'm at a loss. Looks good on a shelf of oddities though.

--Chris

[mikerj]

Could be a trigger electrode I suppose.

[floobydust]

More about them here:
https://www.eevblog.com/forum/projects/high-voltage-vintage-equipment-question-for-the-physicists/msg1286145/#msg1286145

edit: changed link to post about how the "teaser electrode" works

[Tom45]

In other words, this is a triggered spark gap.

Thanks. Triggered spark gap is something I've never encountered or heard of in my long life until now.

[ChristofferB]

that's exactly what I was going to say.

Thanks for the quick responses. good to know.

--Chris

[MagicSmoker]

There is even an official name for this contraption: trigatron.

[Alex Eisenhut]

https://en.wikipedia.org/wiki/Krytron

[NiHaoMike]

With the scale, it looks like a primitive device for measuring high voltage, with the pilot electrode to improve repeatability.

[coppercone2]

i made one of these with a screw for simple experiments (triggered by bbq starter)

[tautech]

I picked this oddity up today at my local electronics store. It seems to be a spark gap, with a fixed and a sliding terminal (sliding terminal has a milimeter scale etched in it) - on a bakelite base. I think it looks 30's-early 50's ish.

The odd part to me is the third, angled terminal below the two other ones. it doesn't have any movable parts, and doesn't really have a screw terminal as the other two, just a 2mm cross hole. it's also very close to one of the terminals.

The guy at the store said he thought it had been used for ignition coil testing, but it doesn't strike me as a mechanic's tool.

Well that's exactly what it is !

Plenty of further examples and discussion here:
https://www.eevblog.com/forum/projects/high-voltage-vintage-equipment-question-for-the-physicists/

[T3sl4co1l]

Oddly, the pointy electrode design predisposes it to corona discharge, making it less reliable as a measurement tool (besides all the problems with atmospheric discharge in the first place).

Tim

[floobydust]

In one science paper, the teaser electrode arc was said to emit UV which would then start the main arc.
So I tried pointing a UV LED at a spark gap and it hardly makes a difference in breakdown voltage. Thought maybe one could make a light activated spark gap.
Something good for transient generators, ESD guns etc.

[T3sl4co1l]

Or if not the air, then the work function of the electrodes.  Which will be pretty high if made from common metals.

Hm, I don't know offhand if there are even any air-stable metals, intermetallic alloys, or compounds (oxides, sulfides, etc.) that have a work function lower than the ionization potential of molecular oxygen (i.e., ca. 250nm UV).

Tim

[MagicSmoker]

In one science paper, the teaser electrode arc was said to emit UV which would then start the main arc.
So I tried pointing a UV LED at a spark gap and it hardly makes a difference in breakdown voltage....

The wavelength needs to be closer to the mercury line (254nm, UV-C) to reliably initiate ionization in the gap and almost all UV LEDs emit in the long end of UV-A (~400nm) which is only good for making fluorescent things glow in the dark.

[Gyro]

UHP Mercury vapour projector lamps use an optically coupled separate UV (<270nm) corona discharge cavity to initiate the discharge in the main (>200 bar pressure) chamber... https://www.lightsearch.com/pdf/uhp.pdf

[Zapro]

If you look up the part number molded into the bakelite base, you will find that it's part of this contraption:

http://www.bosch-classic.com/media/bosch_classic/wissen_1/prueftechnik/EFMZ1.pdf

I work in the store and researched the thing before it was sold

History of it coming to us: My colleague used to work in a company called Autometer that started out making test equipment / electronic tools for auto repair. He had this thing sitting around for ages, and it apparently appeared in a tidy up at home, and so it sought a new home.

// Per.