Lurch presented the following explanation :
On Sat, 10 Mar 2007 20:54:17 GMT, "The Medway Handyman"
mused:

Will a normal dimmer switch work on a two way circuit? e.g. 3 core & earth.

Define 'normal'.

If by normal you mean '2 way', then yes. If by normal you mean '1 way'
then no.

It could be done by making the dimmer an extra switch position, in
circuit after the two way circuit, in other words - in the switch wire
to the light(s). That would provide lighting level control providing
the circuit was turned on first at one of the 2 way switches first.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk