650 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



gausses and the vertical unit very nearly 14,000 maxwells. Of 42,000 

 maxwells which the 8 centimeter long piece had under an exciting cur- 

 rent of 16.4 amperes, only about 55 maxwells remained when the current 

 had been gradually destroyed, and this for a cross section of about 7 

 square centimeters. 



Figure 10. Residual magnetism of normal sign in rods of very soft mild 

 steel of different lengths. 



It is usually difficult to get a piece of Bessemer steel 3 centimeters 

 in diameter and even 30 centimeters long so £oft, magnetically consid- 

 ered, that it will show abnormal residual magnetism, and the metal just 

 mentioned is exceptional, as has already been said. 



Pieces cut from a certain round rod of soft steel, of lengths in centi- 

 meters 30, 15, 8, 6.8, and of diameter 1.6 centimeters, all refused to 

 reverse when tested, but reversal finally appeared in a piece about 6.4 

 centimeters long. 



It is often impossible to make a stout piece of hardened tool steel 

 reverse unless its length be made so smaU that the observations become 



