Note on the Input Impedance of Vacuum Tubes at Radio Frequency

It is shown experimentally that the input impedance of a three-element vacuum tube at radio frequency is not infinite, as heretofore supposed, but that because of the capacitive coupling between input and output circuits within the tube itself, the input impedance greatly depends on the character of the output circuit. Experiments with resistance in the output circuit are given and a theory is advanced to explain the results observed. The general effect of inductive and capacitive output circuits is mentioned.