SHUDDEMAGEN. — DEMAGNETIZING FACTORS FOR IRON RODS. 205 



the same value at any point whatever in the iron slab. The value of 

 N, the demagnetizing factor, is 4tt throughout the slab. As in soft 

 iron a negative force of H' less than 10 cg.s. units of field intensity is 

 sufficient to demagnetize the remanent magnetization which exists in 

 the iron after the original magnetizing field is withdrawn, and the 

 value of 4^/^ is about 200,000 of cg.s. units, it is easily seen that on 

 removing the infinite field the demagnetizing field Hi would instantly 

 demagnetize the slab completely. 



A diagram of the apparatus and its arrangement, as used prac- 

 tically throughout the present investigation, is shown in Figure 5. 



Figure 5. 



Diagram of apparatus used in the Jefferson Physical Laboratory in obtaining 

 magnetization curves for the present investigation. 



Experimental Methods and Apparatus. 



G is a Thomson four-coil ballistic galvanometer with astaticised mag- 

 netic suspension, controlled by a permanent magnet S-N, and not 

 shielded at all magnetically, for it was found that when shielded with 

 three large cylindrical iron shells and heavy iron plate tops and 

 bottom, certain unknown magnetic disturbances were caused in these 

 shields, and effectually prevented the needle, which was then non- 

 astatic, from coming to rest. E is the storage battery of from 5 to 20 

 cells, giving about 2 volts each, for furnishing the current in the 

 primary coil. S is a large solenoid of the following dimensions: 



