ELECTRIC OSCILLATIONS AND ELECTRIC WAVES. 209 



aries to the region. Thus the wave, Fig. 143, which shoots out 

 on the line when the varying voltage of Fig. 142 is connected 

 across the end of the line satisfies the differential equations (3) 

 and (4) because travel, pure and simple, is all that these equa- 

 tions demand; and it satisfies the boundary conditions at the 

 end of the line as is evident from the identity of the curves in 

 Figs. 142 and 143. It must be remembered that the effects of 

 wire resistance and of imperfect insulation between line wires are 

 ignored in this discussion. 



When a periodic electromotive force acts across the end of the 

 line, for example when an alternator is connected to the line, 

 then what is called a wave-train passes out along the line, and the 

 state of affairs (before matters are complicated by the reflection 

 of the waves from the distant end of the line) is shown in Fig. 

 144. This figure shows a wave-train which is produced by a 



Fig. 144. 



harmonic alternating voltage, and the curve WW is a curve of 

 sines. The short horizontal arrows represent the current at various 

 places in the wires, the fine vertical lines represent the lines of 

 force of the electric field, and the dots represent the lines of 

 force of the magnetic field as in Fig. 141. 



The ribbon wave. When a battery of constant voltage and 

 negligible resistance is connected across the end of a transmission 

 line, a wave shoots out on the line, the voltage e in the wave is 

 everywhere of the same value and equal to battery voltage, and 

 the current is everywhere the same in value and equal to 

 15 



