100 



THE BELL SYSTEM TECHNICAL, JANUARY 1952 



the output signal would be at maximum amplitude due to addition of 

 energy coming from the two paths. This amplitude is, of course, twice 

 that of the signal from one branch alone. In the experiment the plunger 

 in one branch was left fixed and the attenuator in that branch left set 

 at zero. The path through this branch then represented the normal 

 transmission path for an actual system. The path through the other 

 branch could be made to correspond to spurious paths ha^ing different 

 amounts of delay and attenuation simply by adjusting the position of the 

 reflecting plunger and the setting of the attenuator. A series of photo- 

 graphs were taken of pulses resulting from these different amounts of 

 delay and attenuation. 



The first three pictures of Fig. 10 were taken with the path lengths 

 exactly equal. When the amplitudes were also equal there was complete 

 cancellation. As the signal in one branch was attenuated the amplitude 

 of the resultant pulse increased until it became equal to that of the 

 original pulse as shown in the third pictiu'e. Increasing one path by one- 

 half wavelength brought the signals from the two branches into phase 

 and they added up to double amplitude as seen in the fourth picture. It 

 should be pointed out that although in our experiment we changed delay 

 by 0.36 millimicroseconds in going from the fii'st minimum to the first 

 maximimi, in free space a change of delay of only 0.125 millimicroseconds 



SIDE ARM A--> 



\/W^ ATT E N U ATOR 

 E PLANE ARM J Nk. H PLANE ARM 



PLUNGER A 



PULSE INPUT 



OUTPUT 

 TO DETECTOR 



-/\/V\/ AT T E N U ATO R 



SIDE ARM B--> 



PLUNGER B 



Fig. 9 — Apparatus to simulate two-path transmission. 



