Convection currents; changing electrical current flow; dimmers; rheostats and pots.

From: Nicholas Bodley (
Date: Wed Mar 28 2001 - 10:33:19 PST

Hey! No way! A dimmer switch just *will not work* on low-voltage DC!
It contains a triac, essentially a bidirectional SCR, a variable
resistor, and some simple electronics. Basic principle is pulse-width
modulation, at 120 pulses per second. It *has* to be used with AC
power, and probably *much* more than 6 volts. If you do use AC, use a
stepdown transformer and a lamp or resistor in series to limit the
current. Pay attention to current ratings! Triacs can blow out in a

Better yet, buy a power resistor for a heater, at Radio Shack, if you
do go the AC route. Do Ohm's Law to figure out the maximum voltage
you can apply. A water-cooled resistor can dissipate much more power
than its free-air rating, maybe 5x or 10X, maybe more.

Use a rheostat *as a heater*; makes more sense. Oil the shaft bushing
a little, to exclude water. Dry it out afterward; better to connect
it to a low-voltage DC "power supply" than a battery to dry it.
Naturally, set it to maximum resistance, first.

(For the uninformed: A rheostat may be called a wirewound pot.
(potentiometer), although a WW pot is likely to be a different animal
from the typical rheostat, and can't get as hot without damage.) The
speaker faders (volume controls) that you connect directly to the
speakers (argh!) are rheostats. Pots and rheostats are electrically
identical, except that rheostats often are made to get hot enough to
boil water, while pots may not be. A pot has connections to both ends
of the resistance element, while a rheostat has connections to only
one end. Both are accurately and properly called/classified as
variable resistors.

"Rheostat" comes from a Greek root, rheo--, meaning flow; implication
is that it controls the amount of flow. See "rheology". It's an
adjustable device that wastes power by converting it to heat; they
used to be used in theaters as light dimmers. With a fixed-voltage
source, a rheostat varies the current, which flows through the
controlled device (such as a sewing-machine motor).

Nicholas Bodley |@| Waltham, Mass.
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