Final answer:
The simplest form of a plain vacuum tube is known as a diode, which consists of two electrodes: a cathode and an anode. It was instrumental in early electronic developments and laid the foundation for the evolution of many technological devices.
Step-by-step explanation:
The plain vacuum tube, in its simplest form, is referred to as a diode. This type of vacuum tube contains two electrodes: the cathode and the anode. One of the earliest prototypes of the cathode ray tube, which is a precursor to vacuum tubes, was developed by Heinrich Geissler and later improved by William Crookes. The cathode ray tube and other vacuum tubes have been instrumental in various electronic developments such as the triode tube, which was the foundation of early radio transmitters and receivers, and have led to advances in technology like radar, computers, and television.