The Effect of Light on the Thermo-Electric Power of Selenium

Abstract

Thermo-electric power of pure selenium.—(1) Effect of light. Pure selenium was melted on ground glass slides provided with nickel electrodes 6 cm long and 2 mm apart. The amorphous material was converted into the grey crystalline form. A temperature gradient was established in the selenium by means of two copper blocks clamped to the nickel electrodes and kept at different constant temperatures. The thermal e.m.f. and resistance were measured for various illuminations with white light, the former by a compensation method using an electrometer as a null instrument. Light caused a decrease of about 5 percent in thermo-elecric power, corresponding to an increase of about 500 percent in conductivity. These effects of light may be explained according to the electron theory by assuming an increase in the mean free path of conducting electrons caused by a decrease in the potential energies of electrons in interatomic space. It is pointed out that in a circuit entirely of Se, homogeneous in every respect except that two sections have temperature gradients of opposite sign, if one section is illuminated a large thermal e.m.f. will be developed in the circuit. (2) The dark values for different samples were between 0.99 and 1.14 millivolts per degree against copper.