On Sat, 16 Nov 2013 20:18:17 +0000 (UTC), Danny D'Amico
wrote:

On Sat, 16 Nov 2013 12:05:12 -0500, Ralph Mowery wrote:

As most dryers use the 120 volts from one leg to power the
control circuits and light , the third wire is acting as a neutral.

Well, that answers my question as to whether the neutral is carrying
current.

Since there are 120v "things" on that dryer (e.g., the timer, the
bulbs, etc.), there *must* be some current in that neutral (unless
the loads are perfectly balanced).

Yes, though it's possible to balance the two, it would be quite
expensive to perfectly do so under all conditions. The fourth wire is
cheaper.

So, in the case of my 3-wire dryer, the neutral is always carrying
current, whereas a ground wire shouldn't normally be carrying current
(because once you carry current, there is a chance that there will
be resistance, and if there is resistance, you get a potential,
whether you like it or not).

Sure.

So, maybe, just maybe, my spark, is due to a high-resistance neutral?
Geez. How do I check for a high-resistance neutral?

It would have to be a very high resistance neutral to get a spark.
Even if there is no neutral, the voltage on the common point (the
neutral on the dryer) would be very close to ground. The imbalance is
proportional to the ratio of the current in the heater and the timer.
You would measure resistance with an ohm meter. ;-)