ELECTRICAL WAVE. 225 



234. Let us suppose that the potential at the origin V has 

 alternately constant values which are equal, and of opposite signs 

 during the very short and equal times r, and that the operation is 

 repeated an uneven number of times, 2n+ i for instance that is 

 to say, 



+ V from to T, 



- V from T to 2T, 



+ V from 2T to 3T, 



+ V from 2m to (272+1)1-. 

 We have then 



TT T 2nr 



The different values of the function <f> (t - 0) are : 



For the first contact - + <(/- 0) =+</>(/), 



For the second contact - -</>(/- T) = - <j>(t) 



For the third contact - + <(/- 2r) = + <.(/) - 2r</>'(/). 



For the ( 2 n + i ) th contact +</>(/- 2 nr) =+</,(/)_ 

 Adding these equations, we have 



and, therefore, 



The maximum value of U at the distance x is produced after a 

 time T n defined by the condition 



which gives 



/ \ r~ / 



Q 



