538 



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



theory is only an approximation, for it is hardly imaginable that 

 the velocity should be greater than in free space. 



FIG. 96. 



The current is obtained, according to (7), by differentiating 

 with regard to as. We must therefore differentiate the integral 

 by its upper limit, multiplying this by its derivative by x. 

 Accordingly 



(15) 



/-FA 



L " n \ I ' ini ;= 



FIG. 97. 



