Peirce — Induction Coefficients of Hard Steel Magnets. 347 



Akt. LI. — On the Induction Coefficients of Hard Steel 

 Magnets ; by B. O. Peirce. 



In preparing a large number of deflecting magnets for the 

 use of students in measuring by Gauss's method the intensity, 

 H^ of the horizontal component of the earth's magnetic field, 

 I have had occasion to make several hundred measurements of 

 the induction coefiicients of seasoned magnets of different 

 sizes and shapes. The results of this work have enabled me 

 to predict with considerable accuracy what measurement would 

 show the induction coefficients of various other magnets to 

 be, and, since an approximate knowledge of the induction 

 coefficient of a magnet is sufficient for many practical pur- 

 poses, I hope that these results may prove useful to others. 



A short account of the way in which the deflecting magnets 

 were made will serve to show the scope of the observations 

 and to explain some terms used in the sequel. 



The fact that a great many sets of apparatus were required, 

 each capable of giving accurate results, determined in some 

 degree the character of this apparatus. It was desirable that 

 no two of the deflecting magnets should be just alike, but it 

 was essential that every one of them should be homogeneous 

 and of such a form that its moment of inertia could be com- 

 puted with an error less than \ per cent, corresponding to an 

 error from this cause of -^-^ per cent in a determination of H. 



Notwithstanding their advantages, extremely slender magnets 

 hardened in bundles like those which led to excellent results 

 in the hands of Professor T. Gray,"^ were for several reasons 

 unsuited to my purpose ; accurately made hollow magnets 

 would have been expensive, and it seemed best to use solid 

 round rod magnets from 0*8*^'^ to 0-95^°^ in diameter. Extremely 

 homogeneous, round, polished drill rod of Stubs or Crescent 

 steel was easily obtainable, and, to avoid as far as possible the 

 necessity of grinding the hardened steel to form, the magnets 

 were generally cut from long perfect specimens of this drill 

 rod and the ends of the pieces were ground flat. The steel 

 was then heated uniformly to a bright red by aid of a special 

 gas-heater devised by Mr. G. W. Thompson, the mechanician 

 of the Jefferson Laboratory, and immersed in a large bath of 

 rapid 1 J -stirred, ice-cold, acidulated water. In this way great 

 uniformity was obtained in the hardening. The ends of the 

 hardened pieces were very slightly ground again under water 

 in a special holder on a Brown and Sharpe grinder. This 

 gave a mirror surface at each end accurately perpendicular to 

 the axis of the magnet without disturbing the temper in the 



* A. Gray: Absolute Measurements in Electricity and Magnetism, ch. 2. 



Am. Joub. Sci. — Fourth Series, Vol. II, No. 11. — Noyember, 1896. 

 25 



