160 



PROCKKKINiiS OF THK AMERICAN ACADEMY. 



The Growth 01 the Induction Flux in the Core of an Elec- 

 tromagnet while the Exciting Current is Temporarili 

 Constant. 



It Bometimes happens that if a number of secondary coils of Low 

 resistance, wound upon the core of an electromagnet, are closed mi 

 themselves, the building-up curve of a current in the exciting coil is 

 for a comparatively long time almost exactly parallel to the time a.\i~. 

 During this time it is difficult to detect any change in the intensity 

 of the current, and yet the. flux of magnetic induction through the 

 core is increasing at a very nearly constant rate. This fact, which 

 has a certain pedagogic interest, is easily illustrated. The curve 



SECONDS. 



FlGUBE 64. 



Direct and reverse current curves for a transformer with a laminated core, 

 The existence of eddy currents is clearly Bhown. 



OPQU (Figure 55) shows a nearly typical case, and the line OKLG 

 represents on a different scale the induced current in one of the second- 

 ary circuits. To a person watching an amperemeter in the primary 

 circuit, the current seems to have attained its final value in less than 

 a second, and if he leaves the instrument at the end of, say, five sec- 

 onds, he feels sure that the current has become steady. Meanwhile the 

 induction flux, as measured on the scale of the diagram by the area 

 1>' 'tween the curve and the line YU(ot, on a different scale, by the 

 area under the curve <>KUi\ is constantly growing. < )f course if the 



e is very Large, the whole building-up time maybe a minute or 

 more, and the phenomenon may then become very striking. 



The magnet T has three coils. The first (A) has 750 turns, the 



