http://www.nobelprize.org/nobel_prizes/physics/laureates/1928/richardson-lecture.pdf
OWEN W. RICHARDSON
Thermionic phenomena and the laws which govern them
Nobel Lecture, December 12, 1929
In its broadest aspect this subject may be summarized as the branch of Physics which deals with the effect of heat on the interaction between electricity and matter. It is not altogether new. Nearly 200 years ago it was known that air in the neighbourhood of hot bodies conducted electricity. In 1873 Guthrie showed that a red-hot iron ball in air could retain a negative but not a positive charge. In a series of researches extending from 1882 to 1889, Elster and Geitel examined the charge collected on an insulated plate placed near various hot wires in diverse gases at different pressures. The observed effects were very specific and varied, but there emerged a general tendency for the plate to acquire a positive charge at low temperatures and high pressures, and a negative charge at high temperatures and low pressures. The matter became really interesting in 1899 when J. J. Thomson showed that the discharge from an incandescent carbon filament in a vacuum tube was carried by negative electrons. In 1900 McClelland showed that the currents from a negatively charged platinum wire were influenced very little, if at all, by changes in the nature and pressure of the surrounding gas, if the pressure were fairly low. These facts seemed to me to be highly significant, and I resolved to investigate the phenomenon thoroughly.