Re: bimetallic plate

There are devices called bimetallic strip thermometers used to measure
temperature.  Two strips of dissimilar metals are bonded together, each of 
which has it's own thermal expansion rate.  The thermal expansion coefficient 
is the amount of expansion a material undergoes as you increase temperature, 
and this value is different for different metals.  At some temperature, the
strip will be flat due to both materials being the same size.  At a 
different temperature, however, the strips are of different sizes, so the 
strip curls up or bends.  This mechanical behavior may be calibrated and
measured by pointer, which then enables us to obtain the temperature.  In 
this device, no electrical current is measured between the plates, only 
a mechanical motion.

There is another device called a thermocouple, which does measure 
temperature based on an electrical current.  When two dissimilar metals 
are placed in intimate contact (they are usually alloyed together by 
melting), this makes a junction.  At the junction interface, a double 
charge layer of electrons forms due to a transfer of electrons from one metal 
to the other.  To measure temperature accurately, you make two junctions 
which are connected together by the same wire.  One junction is placed in 
a known temperature, such as an ice bath; the other is placed wherever you 
want to measure the temperature.  The voltage difference between the two
junctions can be converted to temperature via a reference table, which 
lists voltages and temperatures for various different combinations of 
metals.  Some metal combinations are more accurate at higher temperatures, 
and others at lower temperatures.