Max Christian wrote:

I've just replaced plastic light switches with ones that have metal
faceplates, and I *just* can feel electricity when I brush my fingers
against them lightly. I'm certain it's not static -- I can feel the
50hz cycle.

It only happens on one lighting circuit; on other circuits with the
same new switches you can't feel anything.

Likeliest cause is that the earth connector isn't connected to earth
somewhere further back than the switch, and that the 50Hz you can detect
is capacitively coupled into the free-floating earth conductor which is
correctly connected to the switch faceplate. You can check this out
if you have a multimeter - you should find that the resistance between
the faceplate and something-else-which-has-a-good-earth (e.g.: socket
screw, socket earth pin, outer case of a plugged-in non-double-insulated
metal appliance, etc.) is Huge, while the resistance from the faceplates
of the other, non-tingling faceplates to the same point is Negligible
(probably down at 0 ohms unless your meter has an accurate "low ohms"
range). Turn off the power to the lighting circuit while you do this -
less because of the risk of shock (you've shown the available current
at the suspect faceplate is not currently (ha) dangerous), but becuase
it'll louse up your resistance measurement.

But it *is* worth tracking down where the earth connection's missing,
since without an effective earth at your metal faceplate you (and those
who share the house) are at risk in the event of a "hard" fault where
the live directly comes into contact with the faceplate; at that point
what you want to happen is for a brief flash behind the faceplate and
the MCB/fuse for the lighting circuit to trip, EEBADS-style; *not* for
the faceplate to stay live, now with plenty of heart-stopping current
available, to give a nasty (or worse) belt to the victim.

Your previous installation - with plastic faceplates - masked the fault
I'm guessing you have; tens of years ago it wasn't even a requirement
for lighting circuits to have an earth-continuity conductor. Until you
fix the underlying fault (which you should do ASAP), replacing the
metal-faced switch with the previous plastic one is a Good Move - but is
no substitute for the proper fix, mind you...

(While your laptop might give you a similar weak tingle, we Presume it's
a double-insulated/Class-II design where any such current is restricted;
while the tingle you say you're getting suggests the absence of an
effective earth in an arrangement where such an earth *is* a
requirement.)

HTH - Stefek