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.