1854-1920

German physicist who helped to develop a simple thermionic device that converts heat energy to electrical energy. Elster's apparatus consisted of a sealed tube that contained an electrode, or electrical conductor, at either end. One electrode could be heated while the other was cooled. The heated metal emitted electrons, which passed through the tube to the cool electrode. In other words, an electric current flowed through the tube as a result of temperature differences between the electrodes.