516 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Table IV gives to four decimal places the values of S', for fx = 40 and 

 r = 1.25, at a number of different epochs after the coil circuit has been 

 broken. 



TABLE IV. 



If a small closed testing coil of fine insulated wire could be so imbedded 

 in the iron as to surround a portion of the core coaxial with the whole 

 and of one tenth the whole diameter, the intensities of the current in the 



Figure 6. 



coil at different times should be proportional to >S". Figure 6 shows 

 four curves 9 in which S' is plotted against t for ll = 20, 40, 80, and 160. 

 The maximum value of q at r = a/10 may be found by equating to 

 zero 3 q / 3 t obtained from (19) 



9 q = 2.56 ff ^ (n„ a) 2 J x ( T V n p a) • er-v* 

 9t iTLia ^4 J(n p a) 



(20) 



If for r = a/10, q be plotted against t for a number of values of li, t/fA, 

 will have the same value at the highest point of all the curves. 



9 Each curve practically coincides with the axis of abscissas for a time, and 

 then suddenly bends sharply away from the axis. It is not easy to indicate these 

 sharp bends in a small figure. 



