Warning.
Here be dragons and black magic. Sort of.

> What is the difference between a Peltier device and any other
> type of thermocouple?  I guess a Peltier device is a special
> thermocuple, but not all thermocuples are Peltier devices.

A Peltier device is a thermocouple intended to be used to generate 
heating and cooling when current flows through the junction.
Siebeck discovered that when you differentially heat two different 
metals a voltage differential occurs. peltier "discovered" the 
converse effect that when you apply a voltage difference atemperature 
differential occurs.

So all thermocouples are notioonally Peltier devices BUT they are not 
all mechanically arranged to make use of this effect.

Modern peltier devices use semiconductopr thermocouples arrnaged so 
the same face in all couples face uin the sdame direction so that use 
may be made of the hot and cold generated (or heat pumped).

> I knew that Peltier devices worked on temperature difference

no - they GENERATE temperature difference. All peltier devices can 
also be used as Siebeck effect theromocouples and generate voltage and 
power. How good they are depends largely on size of the Siebeck 
coeffricient for the materials used, and the electrical and thermal 
conductivity (Physics) plus, of cousrse, how well you utilise the 
material you have (Engineering).



> but thought that other thermocuples worked on dissimilar metal 
> contact
> since thermocuple wire looks a lot like zip cord except that one
> conductor is coper and the other is something else such as 
> constantan.
> Now, I am confused.  Someone once gave me a short length of
> thermocuple wire to play with.  The insulation on one conductor was
> blue and the other was red.  I stripped a bit of each and twisted 
> the
> two bare ends together.  I could get a few tenths of a volt if I 
> held
> the twist junction in the stream of hot air from a heat gun while
> reading the open end of the thermocuple wire with a volt meter.

To be more confused - the overall result is the sum of the voltage due 
to the temperature differential between two dissimilar materials at 
one temperature and the differential effect of the other end at some 
other temperature at the other end of the wire :-). Change materials 
that you join to the cold side with and you'll see  a small change. 
Higher temperature gives greater difference.

>  I think in a proper lab setting, they have a spot welder to
> bond the two conductors together but I don't understand how the
> differential is formed.

Google knows.
My internet is downish (barely getting email - ISP switch problem they 
say) so I'll leave you search. Will be lots.

> One conductor is going to be pretty much the
> same temperature as the other because they are tacked or bonded
> together.

Yes. The differential is material related and temperature related.


        RM

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