232 



ADVANCED ELECTRICITY AND MAGNETISM. 



distant radial rods (arrows) set into it in a helical row; the cylin- 

 der with its attached arrows is supposed to be rotating at a speed 

 of / revolutions ( = 2irf radians) per second in a counter-clock- 

 wise direction as seen from the end B\ and the projections of the 

 attached arrows upon the plane of the paper represent the instan- 

 taneous values at the various points of the transmission line of 

 the alternating current i which is associated with a simple wave 

 train of electromagnetic waves moving along the transmission 



transmission line 



direction of 

 clock-diagram rotation 



side view of clock-diagram model end view of model 



Fig. 169. 

 Clock diagram model of a simple wave train moving to the right. 



line from left to right. In a moving wave train, current i and 

 voltage e are in phase with each other everywhere, so that the 

 projections of the radial arrows in Fig. 169 represent the values 

 of e as well as the values of i. 



We have chosen to consider both i and e positive (or both 

 negative) in an electromagnetic wave moving to the right along 

 the transmission line in Fig. 169. Therefore, in an electromag- 

 netic wave moving to the left, the current and voltage must 



