Magic Eye Tube: The First RF 'Debugger'
Before LEDs and digital meters, radios used a glowing green vacuum tube called a magic eye to show signal strength. Invented by Allen B. DuMont in 1932 and popularized by RCA's 6E5 in 1935, these tubes helped users tune in stations precisely by watching a shadow close as the signal peaked. They served as cheap voltage indicators in radios, tape recorders, and test equipment until transistors made them obsolete.
The magic eye tube was the first in a line of development of cathode ray type tuning indicators developed as a cheaper alternative to needle movement meters.