"Wouter van Ooijen" writes:
>they don't. they need temperature *difference*.

	That does make more sense.  Some thermostat and safety control
systems on gas-powered appliances use what is called a millivolt
system in which a probe sticks right in to the flame of a gas jet,
usually the pilot light, and appears to generate about 750 millivolts
as long as it is red hot.

	I always thought the voltage came from two pieces of different
metal inside the probe but it must come from the temperature
difference between the part of the probe in the flame and the rest of
it which is much cooler and possibly made of a different metal.

	For those interested, the power both controls the solenoid to
the main burner and a small solenoid that acts to shut off the pilot
light jet if it goes out so that gas won't escape.  The thermocouple
thus powers the system and acts as a fail safe.

	The only such system I have ever personally examined at all
was about 30 years old and I read about .3 volts across the open
thermostat contacts.  A technician told me that they do produce less
output with age until the thermocouple fails and it can't keep itself
going any longer.

Martin McCormick WB5AGZ  Stillwater, OK 
OSU Information Technology Division Network Operations Group
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