Thermo-electric Call.
A thermostat arranged to ring a bell or to give some indication when the
temperature rises or falls beyond certain points. It may be a compound
bar of brass and steel fixed at one end and free for the rest of its
length. Its end comes between two adjustable contacts. As the
temperature rises it bends one way (away from the brass side) and, if
hot enough, touching a contact gives one signal. If the temperature
falls it curves the other way, and if cold enough touches the other
contact, giving another signal. (See Thermostat, Electric.)
Thermo-electric Couple.
If two dissimilar conductors form adjacent parts of a closed circuit,
and their junction is at a different temperature than that of the rest
of the circuit, a current will result. Such pair of conductors are
called a thermo-electric couple. They may be joined in series so as to
produce considerable electro-motive force. (See Thermo-electricity and
other titles in thermo-electricity.)
The efficiency of a thermo-electric couple according to the second law
of thermo-dynamics is necessarily low--not over 10 per cent.
Thermo-electric Diagram.
A diagram indicating the change in potential difference for a fixed
difference of temperature between different metals at different
temperatures. It is laid out with rectangular co-ordinates. On one axis
temperatures are laid off, generally on the axis of abscissas. On the
other axis potential differences are marked. Different lines are then
drawn, one for each metal, which show the potential difference, say for
one degree centigrade difference of temperature between their junctions,
produced at the different temperatures marked on the axis of abscissas.
Fig. 333· THERMO-ELECTRIC DIAGRAM, GIVING POTENTIAL DIFFERENCE IN
C. G. S. UNITS.
Thus taking copper and iron we find at the temperature 0° C. (32° F.) a
difference of one degree C. (1.8° F.) in their junctions will produce a
potential difference of 15.98 micro volts, while at 274.5° C. (526.1°
F.) the lines cross, and zero difference of potential is indicated.
Taking the lead line on the same diagram it crosses the iron line a
little above 350° C. (662° F.), indicating that if one junction is
heated slightly above and the other is heated slightly below this
temperature no potential difference will be produced. Lead and copper
lines, on the other hand, diverge more and more as the temperature
rises.