152 PROCEEDINGS OF THE AMERICAN ACADEMY. 



dN 



E ~ n Tt =r% - P) 



Let the slope ^ ( T) be determined graphically from the curve B A 

 described above, as a function of T. In practical units 



„ ndN 



E -mi = rt - » 



or 



naV(T) dT __r T 

 Wm dt n' 



whence 



naV(T)dT 



— at 



10 8 m ' ^ 



(-?) 



Call the left hand side of equation (4) Q (T) dT, then 



Q(T)dT= dt. 



If now ft (T), computed from the known quantities of equation (4), 

 be plotted as a function of T with one horizontal unit representing a 

 ampere turns and one vertical unit v units of 12, the area under the 

 curve, thus plotted, from 7\ to T% when multiplied by av is equal to 

 the time in seconds which elapses while the excitation is increasing 

 from T\ to TV It is evident that by taking successive values of T 

 as T h Ti, Tz etc., respectively equal to ni\, ruk, ni s , etc., it is easy to 

 obtain a graphical representation of T (and therefore of i) in terms of t. 

 This gives in the absence of eddy currents the current curve. 



We may illustrate the working of the foregoing theory by applying 

 it to the electromagnet J, the form of which is shown in Figure 1 . 



Application to Magnet J. — With the core of ./ is magnetically 

 neutral at the outset, the application of a series of currents, each a 

 little stronger than the last, gave the numbers of Table I. With 

 these results carefully plotted the slopes of the resulting curve fur- 

 nished the numbers of Table II. 



