Thermo-electric Inversion.
The thermo-electric relations of two conductors vary at different
temperatures. Sometimes at a definite point they have no electro-motive
force and after passing this point the positive plate becomes a negative
one and vice versa. This is inversion, or reversal. (See Thermo-electric
Diagram.)
Synonym-- Thermo-electric Reversal.
Thermo-electricity.
Electric energy, electro-motive force or electrification produced from
heat energy by direct conversion. It is generally produced in a circuit
composed of two electric conductors of unlike material, which circuit
must possess at least two junctions of the unlike substances. By heating
one of these to a higher temperature than that of the other, or by
maintaining one junction at a different temperature from that of the
other a potential difference is created accompanied by an electric
current.
In many cases differential application of heat to an identical material
will develop potential difference. This effect, the converse of the
Thomson effect, is not used to produce currents, as in a closed circuit
the potential differences due to differential heating would neutralize
each other.
Thermo-electric Junction.
A junction between two dissimilar conductors, which when heated or
cooled so as to establish a differential temperature, as referred to the
temperature of the other junction, produces potential difference and an
electric current.
Thermo-electric Pile, Differential.
A thermo-electric pile arranged to have opposite faces subjected to
different sources of heat to determine the identity or difference of
temperature of the two sources of heat. It corresponds in use to a
differential air thermometer.
Thermo-electric Power.
The coefficient which, multiplying the difference of temperature of the
ends of a thermo-electric couple, gives the potential difference,
expressed in micro-volts. It has always to be assigned to a mean or
average temperature of the junctions, because the potential difference
due to a fixed difference of temperature between two metals varies with
the average temperature of the two junctions. (See Thermo-electric
Diagram.)
For bismuth and antimony at 19.5° C. (67.1° F.) it is 103 microvolts per
degree Centigrade (1.8° F.). This means that if one junction is heated
to 19° C. and the other to 20° C. (66.2° F. and 68.0° F.) a potential
difference of 103 micro-volts will be produced.
The potential difference is approximately proportional to the difference
of temperature of the two junctions if such difference is small. Hence
for large differences of potential the thermo-electric power coefficient
does not apply.
As a differential function it is thus deduced by Sir William Thomson,
for expressing the E. M. F. in a thermo-electric circuit: If a circuit
is formed of two metals with the junctions at indefinitely near
temperatures, t and t + dt, and dE is the E. M. F. of the circuit, then
the differential coefficient dE/dt is called the thermo-electric power
of the two metals for the temperature t.