In presenting to you this youngest member of the electron tube family, I am both aided and embarrassed by its family history. I am aided by the fact that you are already acquainted with electrons, so that I need waste no time in explanation or argument regarding their existence. You believe in these little cannon balls which jump out of the hot filament, fly across the vacuum, and plunge into the anode. You have seen them heat the tungsten anode of an X-ray tube to its melting-point in a fraction of a second. Most of you believe that when a current flows through a wire it is these same little electrons, and nothing else, that stream through the wire, like water flowing through a pipe.