A thermocouple is a sensing device that use used to determine temperature. One type of thermocouple consists of a thermally conductive bulb (usually copper) and capillary tube that contains an inert gas that when heated, expands. The expanding gas physically moves something else in a very controlled manner. The amount of
movement is directly related to the temperature - expansion is heating and contracting is cooling. These are the types most often found in appliances with "standing" pilot flames. If the pilot goes out, the thermocouple senses this and closes the pilot valve by moving an electrical switch operator and prevent gas from filling the space.
A Capillary Tube type T/CThe second type consists of two different metal wires that are joined at
one end and sheilded from shorting out in an insulator and a protected from physical damage inside a hardened case such as stainless steel. These two dissimilar metals produce a small voltage due to
galvanic action. When the junction of the two wires is heated or cooled, the
resistance changes which in turn changes the output voltage. The
voltage
is very specific and can easily be converted to a temperature. These are mostly found in very high temperature commercial and industrial furnace and kiln applications.
An electric T/C