Peirce — Permeabilities and Reluctivities for Steel. 275 



iron magnetized lengthwise, has, near the surface of the rod, 

 lines parallel to the rod's axis, we may inquire whether the 

 lines of force and of induction within the rod are not in this 

 neighborhood all parallel to the axis, so that the value of H 

 throughout a cross section of the rod is the same as the value 

 just outside the iron. 



The assumption that the value of PL in the air just about a 

 slender neck of iron of proper form held between the jaws of 

 a highly excited electromagnet is the same as the value in the 

 neck itself, lies at the foundation of the " Isthmus Method " 

 of determining permeabilities under very high excitations 

 introduced by Ewing and Low.* According to my some 

 what extended experience the so called " maximum value of 

 /" may be determined by the Isthmus Method to within 1 

 or 2 per cent of the truth if the poles and the test-piece are 

 of the proper shape and are properly connected, and if the 

 jaws as well as the isthmus are fairly soft; but these conditions 

 are not always easy of attainment, and if one assumes them to 

 be satisfied without investigating each case by itself, one may 

 be led into grievous error. The field about a hard steel 

 isthmus between soft jaws is usually far from uniform, and for 

 some specimens of hard steel which I have studied I have not 

 yet succeeded in obtaining by the Isthmus Method trustworthy 

 determinations of 7^ . In some instances the values of Pi in 

 the air were manifestly smaller or larger than in the isthmus, 

 and sometimes they were smaller for one excitation and larger, 

 for the same isthmus, for another excitation. Nevertheless 

 the method is, of course, a most valuable one. 



This paper describes a long series of determinations of the 

 permeabilities of normal brands of compressed steel, Bessemer 

 steel, and Norway iron in the form of half-inch rods, over a 

 wide range of excitation, and it considers especially a method 

 of measurement in a massive yoke in the interesting region 

 from H = 400 to PL = 2500 which lies above the limits of 

 most permeameter observations and below those of the Isthmus 

 Method. The work was undertaken in order to determine the 

 magnetic behavior, in the region just mentioned, of an impor- 

 tant specimen of soft iron of which only a single short piece 

 was available, and it was necessary to test the trustworthiness 

 of the method to be used by applying it to some soft metals 

 which could be obtained in large pieces and the permeabilities 

 of which could be otherwise found, at least approximately. 



If the rod to be experimented upon can be kept cool artifi- 

 cially, it is not very difficult to determine accurately in a very 



* Ewing, Magnetic Induction in Iron and other Metals ; Ewing and Low, 

 Philosophical Trans., clxxx, 1889. 



