240 ADVANCED ELECTRICITY AND MAGNETISM. 



of one of the dotted curves in the upper half of the figure; and a 

 quarter of a cycle later, when the voltage across the line is 

 everywhere zero, the currents in the line are represented by the 

 ordinates of one of the dotted curves in the lower half of the 

 figure. 



voltage node voltage antinode voltage node voltage antinode 



1 ,.---* 



voltage distribution (second mode) 



current antinode current node current antinode current node 



i . \ 



Closed current-distribution (second mode) 



Fig. 180. 



The dotted curves in Fig. 180 represent current and voltage 

 distribution over a transmission line when it is oscillating in its 

 second mode (next to lowest frequency) , the oscillations being set 

 up by the alternator A of proper frequency. 



The time of one complete oscillation of a transmission line is 

 the time required for an electric wave to travel (at a speed of 

 186,000 miles per second) over one whole wave-length of the 

 standing wave train, and the wave-length of a standing wave 

 train is twice the length of one vibrating segment, from one cur- 

 rent node to the next current node. Thus the whole length of 

 the line in Fig. 179 is one quarter of a wave-length of the standing 

 wave train, the whole length of the line in Fig. 1 80 is three 

 quarters of a wave-length of the standing wave train.* 



* The student should be able to make diagrams somewhat like Figs. 179 and 180 

 but showing current and voltage distributions for the first and second modes of 

 oscillation of a transmission line which is short-circuited at one end and connected 



