http://www.flickr.com/photos/g3zvt/s...7633875041535/

The first image is an old plug with an aperture at the top to make
separate contact with the earth pin. Useful for an earth on your
crystal set, or to resolve the hum-loop in your record deck. Not a lot
more to be said I suspect, but I wonder what regulatory change put an
end to this feature?

The other two images are more intriguing. It's 2 pin adaptor, I think
the fuse might be original, it's 13A!

But what is the purpose of the groove and hole that communicates to
the inside of the plug?



--
Graham.

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