262 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



carrying of energy backwards into the tail of the wave. After 

 a given interval of time the tail of the wave has a total length 

 2 Vt where V is the velocity of progression of the wave. 



If a canal is filled brimful of water so that the elevation of the 

 water level causes an overflow, or spill, the tendency is for a 

 wave to remain pure, and therefore to be propagated without 

 change of shape, because the elevation is reduced by spill and 

 the velocity of flow v is reduced by friction. This is precisely 

 analogous to the action which takes place on a poorly insulated 

 telephone line and which causes such a telephone limit to transmit 

 speech more distinctly than if it were thoroughly insulated. 



145. The electromagnetic wave. An electromagnetic wave 

 consists of a state of ether distortion and a state of ether motion 

 traveling along together and mutually sustaining each other. 

 The ether distortion is electric field and the ether motion is mag- 

 netic field. A layer of electric field unsustained breaks up into 

 two electromagnetic waves just as the elevated portion of water 

 in Fig. 192 breaks up into two water waves. 



The action which takes place in an electromagnetic wave may 

 be clearly understood with the help of Maxwell's conception of 

 the electromagnetic field. It is desirable to consider the case of 

 an electric wave which moves along between two wires (or broad 

 sheets of metal) which bound the electric wave very much as a 

 speaking tube bounds a sound wave which passes through it. 



Figure 195 shows two broad sheets of metal with an electro- 

 magnetic wave pulse traveling along between them at velocity 

 V. The fine vertical lines represent the electric field which is 

 towards the top of the page, and the dots represent the lines 

 of force of the magnetic field which is perpendicular to the plane 

 of the paper and directed towards the reader. A single chain 

 of geared cells is shown in the figure, although a complete repre- 

 sentation of what takes place in the wave would necessitate the 

 showing of great numbers of horizontal chains of geared ether 

 cells every one of which would be exactly similar to the one 

 shown in Fig. 195. Within the region of the wave the ether 



