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PROCEEDINGS OF THE AMERICAN ACADEMY. 



When the circuit of the exciting coil of a large electromagnet is sud- 

 denly broken, the induced current in a test coil wound around the core 

 rises very quickly to a maximum value and then falls away gradually : 

 indeed the form of the current is usually much like that in the second- 

 ary circuit of an induction coil with air core when the primary current 

 is suddenly interrupted. Such a current is shown by curve P of Figure 



Figure I. 



G, which is drawn for the case M= L/2 when the self-inductances of 

 the two circuits are equal. If, after the current in the exciting coil of 

 an electromagnet has been running steadily, its circuit be broken and 

 after a short interval closed again, the induced current in the test coil 

 will be very different according to the direction of the current in the 

 main circuit. If the new direction is the same as that of the current 

 before the break, the new current is called " direct," but if the new di- 

 rection is opposed to the old, the new current is said to be " reversed." 

 The curves M, N in Figure H, which are reproduced to scale from the 

 records of an oscillograph, show the manners of growth of reversed and 

 direct currents, respectively, in the exciting circuit of a certain electro- 

 magnet ; and the boundaries of the shaded portions of the diagram 

 show the forms of the induced currents. The shaded areas give the 



