Centurion "isolating spark gap"?

[Malcolm Towrie]

Hi,
The wiring diagrams for the Mk 3 Centurion show an "isolating spark gap" installed between the booster coil HT output and the one magneto the coil feeds. The manual says this mysterious piece of equipment is mounted on the rear fighting compartment wall beside the booster coil. I don't think our Mk 5 has this as the HT cable attached to the booster coil is about 10' long so would have fed the mag directly. Am I right in this assumption? What did this isolating spark gap do?

Malcolm

[David Dunlop]

Hello Malcolm.

It wouldn’t happen to be a large electrical clip holding what looks like an oversized glass fuse, by any chance?

David

[Malcolm Towrie]

It may well be, David. There's no picture or description in the manuals I have. They just say that no maintenance is required on it. The wiring diagram shows it installed between the booster coil and the magneto so I assume it sparks when the booster coil is firing.
It puzzles me what it would be isolating, unless it stops the magneto back feeding to the booster coil when the engine is running.
Can you post a picture?
Malcolm

[David Dunlop]

Hi Malcolm.

There is a photo on Post #24 of my Wireless Set No. 52 Thread and further comments up to Post #27.

May or may not be the same sort of thing in the Centurion. In a way, it is sort of a self resetting fuse.

David

[Malcolm Towrie]

That's interesting, David. If you google the specific term "isolating spark gap", you find descriptions of a modern device that limits the voltage of a lighting strike on pipelines by arcing across the device to ground. These are used on pipelines where each section is electrically isolated from the next, for corrosion resistance I think. A lighting strike would damage the isolating gaskets, bolt sleeves, etc., so the "gap" device limits the voltage to what the isolation can handle.

So in the case of a radio antenna, maybe that glass "fuse" is to limit the voltage fed back down the antenna by a lighting strike to what the circuitry can handle?

Rather reminds me of the "safety gap" built into the booster coil. To prevent HT coil damage during the times the magneto isn't supplying a spark plug, the generated voltage sparks across the safety gap to ground.

Malcolm

[eddy8men]

the isolating spark gap does look like a fuse but it is there to improve the spark, i have no idea how, as electrickery is not my thing but that's what i have been told. i'm sure i have some nos ones in the yard i'll have a look tomorrow and put up a pic.