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.