144 



1'KoCKKHiNGS (iF Till: AMEBIGAH A.OADEMY. 



After a curve had been drawn on a very large scale to represent the 

 numbers of Table III, a zinc template was made from it, by aid of 

 which and a long "straight-edge" the slopes of the curve could be 

 determined with some accuracy. The next diagram (Figure 37) shows 

 the slope as a function of the strength of the current. 



When the slope for any point of the curve is multiplied by 

 (13.94) / (E — ri), where E and r are given, the result is the value of 

 dt/di for the reverse current curve when the applied voltage is E 

 and the resistance r, for the given value of i. Figure 38 exhibits 

 dt/di for E= 19.5, and r = 15. 



The actual curve was drawn on a large scale, and the area X from 

 x = to x = i, for a number of different values of i were measured by a 

 planimeter in terms of the unit square of the figure ; this area ex- 

 pressed in tenths of seconds the time required for the reverse current 

 to attain the strength i. A few values of X are shown in the next 

 table. 



TABLE IV. 



Every form of current curve which I have met in practice can be 

 closely imitated by a theoretical curve ; but all these curves have at 

 the outset a direction differing widely from the horizontal. Dr. 

 Thornton, however, shows a beautiful curve which at the beginning is 

 convex downward and has at the start a direction not very different 

 from that of the axis of abscissas. 



Before one uses an oscillograph for purposes of accurate measure- 

 ment, one must make sure that the instrument has been properly set 

 up. When the drum which carries the sensitive film or paper is at 



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