Current Rectification at Metal Contacts

Six different contacts of dissimilar metals, namely Cu-Fe, Cu-Sn, Sn-Zn, Zn-Fe, and Pb-Sn, were studied for their rectifying properties. As far as possible small lengths of cylindrical rods with circular section were used. Their tips barely touched each other producing an imperfect contact at which rectification was found to take place best. In all cases, static characteristics showed that when the thermopositive element was given a positive polarity, the characteristic reached saturation conditions, while with reversed polarities the contact behaved like an ohmic resistance. On the positive side the characteristic starts with being straight for a distance and then with a sudden rise, reaches saturation. The effect of varying contact area, pressure, and temperature were next studied. Rectification improved with diminution of contact area, point-to-point contact being the best. The rectification vanished on either side of the limiting added weights (1 to 2 grams). External heating of contact renders the characteristics almost a straight line up to the saturation bend. An attempt is made to explain the rectifying property by assuming that a thermo e.m.f. develops at the contact and that the resistance of the contact itself varies with terminal voltage.