HVPEK-FREQUENCV WAVE GUIDES 



293 



electric constant were pro.u:ressively lowered the velocity even at the 

 highest frequencies would approach that of light in free space and the 

 curve shown would become progressively flatter. In the limit the 

 dielectric constant would he unity and the velocity ratio also would be 

 unity. Under this circumstance the dielectric wire having a constant 

 substantially the same as that of the surrounding medium would 

 cease to function as a guide. 



The experimental points of Fig. 4 were obtained by transmitting 

 waves at each of several frequencies ranging from 100 mc. (X = 300 

 cm.) to 400 mc. (X = 75 cm.) through columns of moderately pure 

 water. The distances between nodes and loops of the standing waves 

 gave data for the velocity of propagation. The method therefore 

 utilized, in a modified form, a technic sometimes invoked for deter- 

 mining the velocity of electric waves on wires or the velocity of sound 

 in air columns. The columns were supported in thin walled bakelite 

 cylinders each about three feet long. Two diameters were used, 

 6 inches and 10 inches respectively. 



Figure 5 covers the case of the same type of waves and the same 



I —^ 



RATIO OF WAVELENGTH TO DIAMETER 



Fig. 5 — Velocity ratio for the £o type of wave in a metal pipe filled with an insulator 



{K = 81). 



