English: Demonstration in 1938 of the waveguide before the Institute of Radio Engineers by the inventor, Bell Telephone Laboratory scientist George C. Southworth (left). Behind the blackboard (right) were several vacuum tube oscillators producing 1.5 GHz (8 inch, 20 cm) microwaves of different modes. He demonstrated that they could pass through the 25 ft flexible metal waveguide and be detected by a receiver consisting of a silicon diode coupled to an amplifier and galvanometer. He demonstrated further properties of the waves:

• Using a metal paddle reflector he created standing waves.
• Using an electric field probe he demonstrated the E field structure of the different modes.
• By using progressively smaller diameter pipe (seen at right on floor) he showed that waves would not propagate through a waveguide that was under a certain diameter, that is, each waveguide had a cutoff frequency.
• A brass grating in one orientation could block the waves but in perpendicular orientation allowed them through
• The propagation velocity was less than that of waves in open air.
• A soft rubber rod could also conduct the waves, forming a dielectric waveguide.
  

Caption: Dr. George C. Southworth and his assistant A. E. Bowen of Bell Telephone Labs show how extremely short electric waves can be guided through a flexible metal pipe