Telephone amplifier with Lieben tube

A tube developed by Robert von Lieben in 1910 now meant that speech signals could be amplified for the first time. Three electrodes are placed inside an evacuated flask. An electron current flows between 2 of these, and a grid is located in the middle, the electrical potential of which is controlled by a speech signal and influences the current. The amplifier tube made compact speech and radio devices possible for the first time.