660 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



In all work with short bars of iron or steel, the " demagnetizing 

 force due to the ends " becomes very important. The outer portions 

 of a very short magnet often reverse the direction of the polarization 

 in the inner portion so that most of the lines of polarization are closed 

 within the metal and the effect of the magnet upon a magnetometer 



FiGUEE 16. A shell and core of soft Bessemer steel about 12 cm. long 

 were exposed to a magnetizing field of given strength in a long solenoid, and 

 the exciting current was then suddenly broken. The ordinates of the curves 

 OSU and OWQ represent the remanent induction flux through the central 

 cross section of the specimen, when the solenoid current was slowly built up 

 and suddenly appUed, respectively. A positive ordinate represents a reversal 

 of magnetism in these curves. 



needle is often very slight indeed. If a cylindrical shell with a core 

 of loose steel wires be placed inside a solenoid of stout wire and a 

 powerful current be then suddenly sent through the coil, the wires will 

 be thrown violently out of the shell if the latter be short and stout, in 

 a direction which shows that the moment induced in them by the ex- 

 citing current was opposite in sign to that of the shell. This experi- 

 ment is sometimes very striking. 



If, in the experiments upon the combination Z, the exciting current 

 was suddenly destroyed, the flux in the shell became instantly negative, 



