study of the thermoelectric properties of metals due to Seebeck, Peltier, and William Thomson. Petrographers had been in the habit of examining the structure of rocks by cutting sections thin enough to be transparent and examining them under the microscope. Sorby in 1861 found it was not possible to examine metals thus and developed the art of polishing the surface and etching it with suitable chemicals, thus bringing out the internal structure. Its application to engineering problems passed unnoticed until the method was independently revived by Osmond in France and Martens in Germany. Seebeck discovered that when in a circuit of two metals a difference of temperature exists between the junctions, an electric current is produced in the circuit. The strength of this current is a measure of the difference in temperature, and this discovery was applied many
