The problem with true rheostats (as opposed to the variable resistor
controlling a solid state device such as a transistor) is that just as you
approach the "bright" end there is a large amount of current and a
significant resistance, with the result being a maximum of power dissipated.
At "full on" there is maximum current but zero resistance, hence zero power
dissipated in the rheostat. At "full off" there is a maximum of resistance
but zero current, hence zero power dissipated in the rheostat. Somewhere in
between is the max power dissipated in the rheostat point.

Having said all that good stuff, maximum power means maximum heat means
maximum oxidation of the rheostat wire. Either a GOOD cleaning with a
toothbrush and contact cleaner or a little judicious application of very
fine (400-600 grit) wetordry sandpaper followed by a good cleaning to get
the sandpaper and oxide particles out of the rheostat will probably solve
the problem.

Jim


"Jonathan Goodish" wrote in message
...
This evening, I noticed a problem with the nav/instrument lighting and
panel intercom in my Cherokee that I haven't noticed before.