550 



THE ELECTROMAGNETIC FIELD. [PT. III. CH. XIII. 



time, the dotted lines showing the wave as it would be if there were 

 no resistance. The last figure,/ Fig. 101, shows the effect of short- 

 ening the duration of a signal, the tail left being noticeably smaller. 



a 



X 



X 



X 



\ d 



- X 



r T 

 : i 



f 



FIG. 101. 



From these figures we may obtain an idea of the distance to 

 which telephony is possible, if we know the relaxation-distance of 

 the line. With ordinary land-lines the relaxation-distance is of 

 the order of several hundred kilometers. This has made speaking 

 possible between Boston and Chicago. Obviously it is of importance 

 to make the relaxation- distance d = v . 2L/R as great as possible 

 by making the distance between the wires great, and using large 

 copper wires. 



256. Terminal Conditions. In the preceding examples we 

 have considered the line to be without end. In many practical 



