Turn a calculator into a metal detector?

[Lake]

Interesting video here:
http://www.diylife.com/2007/11/29/ra...etal-detector/

What's the depth that can be reached?
I remember there were some older designs where radio was also used as a part of metal detector, but the sender circuit didn't have to be that close to radio..(?)
Perhaps it's idea was something else.. Is the calculator crystal causing the interference to radio circuit or something?

[ToyMaker]

I think I saw this or a similar lash up on instructables.com
recently. It might be handy for finding a medium size auto
at 250mm.
Like Ambrose Bierce said about dogs walking on two legs,
"One is not surprised at how well it is done, but that it is
done at all".



robotic regards,

Tom

[Fred]

A better one:

http://www.youtube.com/watch?v=nyXQbcEnUzE&feature=fvw

Looks like an advanced version of a comecial design.