Mass, Energy'^ and Radiation. 



687 



positive half -of the pulse has just passed P and the negative 

 half is just arriving* j At first the secondaries which arrive 

 at P will be those excited by the front and positive half of 

 the pulse, and the force on them will be in the negative 

 direction, i. e. in the same direction as that in the part of 

 the primary pulse which is now arriving at P, and thus the 

 secondaries will increase the magnitude of the electric force 

 in the pulse. After a time the secondary radiation excited 

 by the negative part of the pulse will begin to come up ; the 

 force on this will be in the positive direction, and will 

 diminish the intensity of the force in the primary pulse. 

 The secondary radiation from outlying regions will continue 

 to arrive at P after the primary pulse has passed, so that the 

 primary pulse will have developed a tail. Before passing- 

 through the slab the distribution of electric force in the 

 pulse would be represented by a graph like a. (fig. 2), while 

 after passing through the slab it will be represented by '/3. 



Fin-. 2. 



(X 



We see that the result of passing through the slab has 

 been to diminish the energy in the front half of the pulse, 

 to increase it in the rear half, and to develop a positive 

 tail 7. Now let this modified pulse go through a second 

 slab ; the energy in the front half will be still further dimin- 

 ished, the energy in the rear half and in the tail 7 -vvill be 

 increased, and another tail of negative force 8 will be 

 •developed. This process will go on as the pulse passes 

 through other slabs until the energy in the front part is 

 reduced to insignificance and the second half of the pulse 

 will be the active front ; this w r ill in its turn be worn down 

 by the same process and the tail 7 will take its place, this 

 will be succeeded by the tail e>, and so on. In this way the 

 virtual front of the pulse is continually falling behind the 

 place which the pulse would have reached if it had not been 

 passing through the slabs of electrons, and the amount by 

 w r hich it lags behind will depend on the density of the electrons 



