John White wrote:

I've tried it with minor success. A friend of mine can do it
consistently though.
http://www.randi.org/library/dowsing/

Thanks for the link. I do believe that people like the James Randi
Educational Foundation are coming at this from the right angle.

That may surprise you, but if you start off by believing that a thing
is true, then the tendency is to do experiments which will prove your
theory. If however you start on the basis that the claims are NOT
true, then by definition the standard of proof has to be much higher.

That is true. Carl Sagan put it well : "extraordinary claims require
extraordinary proof".

On the other hand, as my history teacher used to say, "Absence of
evidence is not evidence of absence".

It works both ways round.

I do not know how it works, I have only seen it done as I described
earlier in this thread. I would however like to know.

There are several theories on Wikipedia about why it appears to work,
sometimes. In practice under controlled conditions it seldom works.