In a message dated 96-03-27 11:38:00 EST, rainer@geophys.washington.edu
(Rainer Heller) writes:

>There's also a phenomenon wit h a fancy name I forget right now, 
>works like this:
>Put two metals together that need different energies to get rid of
>one electron. Temperature dependence of the get-rid-of- the-electron
>stuff leads to a voltage.
>Think it was the Zeeman effect.
>Don't know how realible and technically applicable that effect is.
>My guess is that there should be some voltage supply and you have
>some temperature dependent resistance.
>Rainer
>85 Vanagon

It's called a "Thermocouple".  The VDO cylinder head temparature gauges use
this effect.  The voltage and currrent created is enough when the metals are
heated to operate a senstive meter without an external power source.  Most
gauges use an RTD,(resistance temparature device).  This type sensor has a
resistance that changes proportionally with temparature.  Both VW and VDO use
an NTC,(negative temparature coeffecient), type sensor.  This works the same
as the RTD, except the resistance goes down as the temparature goes up.  Hope
this helps clear up some of the confusion.