196 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



comes incapable of retaining the charge given it in the solenoid and 

 the resulting magnet is perhaps only half as strong as the steel magnet. 



Figure 2. 



The same phenomenon appears in the case of tool steel, though it is not 

 very easy to harden a piece of tool steel glass-hard a number of times in 



succession without working it un- 

 l_ der the hammer to avoid the ap- 



.•**.., ^ pearance of minute cracks in the 



♦ '".i,. .4"»* _ metal. 



• For many years small magnets 



• made of cast iron as it comes from 

 • ' the founder have been used in toys 



and in small "magnetos," but such 

 magnets are not nearly permanent 

 and are not so strong at the outset 

 as similar magnets made of prop- 

 erly chilled iron. A certain an- 

 nealed rod which I tested had when 

 magnetized to saturation a moment 

 of 605 on a certain scale, but a few 

 minutes in boiling water reduced this to i'>f) ; when the rod had been 

 hardened and again magnetized, its moment on the same scale as 



Figure 3. 



