PEIRCE. — BEHAVIOR OP THE CORE OF AN ELECTROMAGNET. 121 



effects of eddy currents in them are practically negligible for the 

 purposes of this paper. 



The Demagnetizing of the Core of -a Large Electromagnet. 



In order to be able to study satisfactorily the magnetic properties of 

 a given piece of iron or steel, it is usually necessary that one should 

 know with some accuracy the magnetic state of the specimen at the 

 outset, and, especially when the metal has the form of a closed ring or 

 frame, the previous history of which is unknown, the only safe pro- 



FlGUKE 15. 



The electromagnet Q, which has a laminated core made of sheet iron one 

 third of a millimeter thick and weighs about 300 kilograms. 



cedure is to demagnetize the iron as completely as possible before one 

 makes any experiments upon it. If the metal has the form of a long 

 rod in a solenoid, or of a slender ring wound about uniformly with 

 insulated wire and magnetized in the direction of its circumference, it 

 is easy to send through the coil which surrrounds the iron a long 

 series of currents alternately in opposite directions, which, starting with 

 a value that shall subject the core to a magnetic field at least as 

 strong as any to which it has been previously exposed, gradually de- 



