210 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



A well annealed isthmus of this iron cut lengthwise of the bar gave for 

 /, under excitations as high as 18000 gausses, a final value of 1795, 

 and an unannealed rod tested in a yoke gave 1730. These remarkable 

 values point to a much higher permeability at medium excitations than 

 the rings just mentioned show. 



Ring of Annealed 



TABLE II. 



American Ingot Iron." Measurements made by the 

 Method of Ascending Reversals. 



Table IV shows corresponding values of ZTand B for a ring of annealed 

 "American Ingot Iron " cut out by Mr. Thompson from a large plate 

 of the metal. The outside diameter of the ring was 5.012 cm., its 

 thickness was 0.283 cm., and its breadth about 2.116 cm. There were 

 112 turns in the testing coil and 74 turns in the exciting coil. 



Table III gives for comparison the results of the determinations of 

 the permeabilities of a number of different specimens of soft iron by 

 different observers. Some of the numbers which I have obtained graph- 

 ically from the published figures are only approximately correct. 



Columns 7 and 9 give the records of observations made upon two small 

 rings of very pure iron given by Colonel Dyer of the Elswick works to 



Wilson, Proc. Roy. Soc, 62, 1898; du Bois, Ann. d. Physik, 51, 1894 (537); 

 Taylor Jones, Ann. d. Physik, 54, 1895 (641) ; Phil. Mag., 39, 1895 (254) ; 

 Stefan, Wiener Berichte, 81, 1880 (89) ; Ewing, Roy. Soc. Trans. 



