Spark plug electrode - what size are they?

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My C felt a bit underpowered recently - and a belt along the dual carraiageway it mis-fired like it was only firing on 3 cylinders.

 

I took out the sparkplugs to give them the once over - and came across this:

 

 

The central electrode is missing! the insulator moves up and down - you can see the difference in the 2 pics.

 

I've searched around in the cylinder with a magnetic probe, but nothing appears to in there, i.e. no metal particles.

A cold compression test gave 160 lb/ft which if I remember correctly is the same as a year ago.

 

The question is - what size are the central electrodes and has this happened to anyone else on here? I still haven't ruled out the possibility that is welded to the pistopon or valve!!

I don't have a spare plug to dismantle!