The chief difficulty, as usual, was the determination of the gradient, which depended on a difference of potential of the order of 20 microvolts between two junctions inserted in small holes 2 cms. apart in a bar 1 .
Unlike the frictional generation of heat due to the resistance of the conductor, which Joule (1841) Table I.-Thermoelectric Power, p=dE/dt, IN Microvolts At 50° C. Of Pure Metals With Respect To Lead.
The Peltier coefficient may also be expressed in volts or microvolts, and may be regarded as the measure of an E.M.F.
The value found at a temperature of 150° C. was +2.5 microjoules per ampere-second per degree, or +2.5 microvolts per degree in the case of copper, which agrees very fairly with the value deduced from thermoelectric tests.
The value found by Batelli for iron was - 5 -o microvolts per degree at 108° C., which appears too small in comparison.
Taking the lead-iron couple as an example, the value of dE/dt at the hot junction too° C. is 10.305 microvolts per degree, and the value of the Peltier coefficient P = TdE/dT is +3844 microvolts.
If the circuit is open, as represented in the diagram, the flow will cease as soon as it has raised the potential of the iron 3844 microvolts above that of the lead.
Due to the Thomson effect of about 10 microvolts per degree tending to drive positive electricity from hot to cold, and raising the cold end of the iron 989 microvolts in potential above the hot end on open circuit.
If the circuit is cut at this point, there remains a difference of potential E =1184 microvolts, the resultant E.M.F.