222 ADVANCED ELECTRICITY AND MAGNETISM. 



negligibly small value over the whole line, and a large current / 

 being established in the line. The generator is to be thought of as 

 having a large inductance so that the generator current cannot 

 change perceptibly during the very short time required for the 

 characteristic line surges. When the distant end of the line is 

 opened, a ribbon wave shoots towards the generator, is completely 

 reflected at the generator with reversal of current, again com- 

 pletely reflected at the opened end D of the line with reversal 

 of current, and so on. The first lap of this ribbon wave wipes 

 out the current in the line and lays down a certain voltage E 



(= ^ times the initial value of the current). The second lap 



of the ribbon wave lays down a double voltage, and the original 

 current. The third lap wipes out the current again, and lays 

 down a voltage equal to 3-E, and so on. 



Note. The character of the reflection at the generator in Fig. 

 158 is determined by the condition that the sum of the currents 

 in the successive laps at A must always be equal to the initial 

 current I because the current in the highly inductive generator 

 does not have time to change perceptibly. 



Note. It must not be imagined that the sending out of a 

 ribbon wave depends upon a continued supply of energy at the 

 point where the ribbon wave originates. Thus in Fig. 158 the 

 ribbon \yave is superposed upon the initial current /, and the 

 first lap of the ribbon wave wipes out this current. Therefore 

 since the current is zero, there is no energy flow at all from the 

 opened end. The second lap of the ribbon wave lays down the 

 original current / and a doubled voltage 2 E, and this combina- 

 tion of voltage and current represents a flow of energy from the 

 generator into the line. 



121. Impure wave and pure wave. A wave of the type which 

 is represented by equations (5) and (7) of Art. 116 travels along 

 a transmission line without any change of shape, and in such a 

 wave the magnetic energy per unit length of line (JZ^" 2 ) is 



