Robbie Rhodes wrote:

> If the resistor isn't working the motor current flows through the
> points alone.  When the resistor fails an oscilloscope will then
> indicate greatly increased voltage across the points, showing that
> the points are dissipating the power that normally is dissipated
> by the resistor.  Therefore I presume that the expensive ceramic
> wire-wound resistor was installed to increase the service life of
> the points, perhaps to two lifetimes.  ;-)

Sort of.  The resistor helps to dampen the back emf that occurs when
the points open by reducing the size of the voltage transient.  When
you build up a magnetic field around an inductor (the motor windings)
and then open the circuit, the field collapses, generating a reverse
current in the motor windings.  This leads to a huge voltage transient
of the opposite sign (back emf).  That is how spark coils in cars work.
The resistor reduces the size of this voltage transient, thereby
protecting the points.

In my case, I had replaced the resistor with a Dale power resistor
(which should last forever), but the brushes were stuck in their
holders because the pot metal had swollen.  This resulted in poor brush
contact with the armature, leading to a lot of circuit "opening" (just
as though you open a switch, repeatedly and rapidly), which led to big
voltage transients (sparking).  The points would go bad within minutes.
I got a new set of brass brush holders from the Ampico machinist guy,
Bob Streicher, which after a bit of fiddling solved the problem.

The armatures of these motors are also held together with kind of a
tarry substance, which flows if the rotor is left in the same position
for a long time.  This also can cause excessive arcing and should be
machined flat (carefully!) on your South Bend.

A. B. Bonds