Robert11 ) said...

... I measured (with an analog voltmeter) the
voltage between the white neutral, and the bare copper ground wire in the
box.

Was very surprised to see that it was about 2 V AC.

This may or may not be a problem, and this is why the neutral should never
be used to ground an item, and also why the neutral is only bonded to
ground at the service entrance (not at sub-panels within the same building).

The neutral is known as the groundED conductor as it is grounded at the
service entrance; the ground is known as the groundING conductor as it
provides grounding throughout the building. The neutral is meant to carry
current and the ground is meant to not normally carry current.

As such, with no current through the grounding conductor, it should be
at ground potential throughout the building. Since the neutral carries
current, and since our conductors have some, albeit very small, resistance
they will develop a voltage drop. The voltage drop is a product of the
current passing through it and its resistance (which is proportional to its
length). So, it is not unheard of to find a point where there could be
2 volts difference between the neutral and ground.

Now, having said that, I would say that it is a little surprizing to find
this difference on a circuit like smoke detectors (the exception would be
where this circuit were fed from a sub panel that had some fairly heavy
circuits run off of it, mainly on one hot).

The other issue is related to the meter used to measure the voltage. Fairly
high impedance meters could be measuring noise or capacitively-coupled
signals from the hot in the same cable. A lower impedance (under 50k)
would elimnate this possibility. If you don't have a lower impedance
meter, you could try using a resistor of about that value (47k is the
closest) -- all you do is connect the resistor between the two points
(neutral and ground here) and measure the voltage across the resistor.

--
Calvin Henry-Cotnam
"Never ascribe to malice what can equally be explained by incompetence."
- Napoleon
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