160 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



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

 tromagnet WHILE THE Exciting Current is Temporarily 

 Constant. 



It sometimes happens that if a number of secondary coils of low 

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

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

 for a comparatively long time almost exactly parallel to the time axis. 

 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. 



Figure 54. 



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

 The existence of eddy currents is clearly shown. 



OPQU (Figure 55) shows a nearly t)^ical 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 

 between the curve and the line YU (or, on a different scale, by the 

 area under the curve OKLG), is constantly growing. Of course if the 

 core is very large, the whole building-up time may be 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 



