Duncan Di Saudelli expressed precisely :
I noticed that a light switch back box measured around 100V ac w.r.t. the
cold water tap in our recently-purchased house. I checked the earth bonding
on the metal pipework for the hot and the cold tap and the radiator; all are
at the same zero volt potential. I checked the earth of a nearby mains socket
and saw no p.d. between it and the pipework mentioned. I looked around the
house and I see lots of earth bonding that has been carried out within the
last 3-4 years.

You really need to make all measurements from one reference point and
that should be the consumer unit earth, so you probably need a long
cable of some sort to do that.

The sort of numbers you are quoting suggests the voltage is simply
induced from the live running adjacent to the earth. I would probably
be prudent to check the resistance of the earth from all lighting
switches and lights, to gain a better idea of what is wrong.

If the wiring dates back to the 1960's, it was not unusual for the
lighting circuits not to be provided with an earth. Later mods and
improvements to the circuits might mean metal boxes installed, but with
no earth.

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