OVER-VOLTAGE PROTECTION 



597 



in hcnrys per unit length (c.m.), of the circuit. These two quan- 



T "2 /"* 9 



tities are equal or - = -^ and the relation between the voltage 

 and current at a certain point of the traveling wave is, therefore, 



li. 



-^ is termed the " natural impedance " of the circuit, and is of 



great value in the study of transient phenomena. 



If the line is open-circuited at the farther end, it is obvious 

 that when the wave reaches this point it cannot flow any further, 



FIG. 381. Reflection of a Travel- 

 ing Wave at the Open-circuited 

 End of a Line. 



FIG. 382. Reflection of a Travel- 

 ing Wave at the Short-circuited 

 End of a Line. 



but is reflected, the voltage and current of the reflected wave 

 being of the same values as in the original waves because the 

 energy remains constant. The total current of the incoming and 

 reflected wave must, however, be zero on account of the open- 

 circuited line, and the whole energy is, therefore, stored at this 

 point in the electrostatic field. The reflected current wave must 

 therefore be reversed and its value equal i, while the value of the 

 voltage wave at the end of the line where the original and reflected 

 waves overlap is, therefore, equal to 2e, as shown in Fig. 381. 



When the end of the line is short-circuited, however, the con- 

 ditions are entirely reversed. In that case the voltage at this 

 point must be zero, and all the energy is stored in the electro- 



