

Width of 



Thickness of 



Length of 





Main part. Arms. - 



effective part. 



Main part. Arms. 



1 



5'0 mm l-gmm 



. 80 mm 



Tomm omm 



2 



0*5 0'7 



0-77 



12 5 



278 K H. Hall— Effect of Magnetic Force on the 



It remains to give an account of experiments with other 

 metals in which the effect of shape of cross-section has been 

 in some measure tested. With these experiments will be de- 

 scribed certain others closely related to them. 



Cobalt — Wide and narrow : Two crosses of cobalt were cut 

 from a rolled bar of this metal furnished by the courtesy of 

 Mr. Joseph Wharton of Philadelphia. In each cross a line 

 which had been transverse to the original bar became the 

 direction of the main current. The approximate dimensions 

 were as follows : 



No. 



These crosses were compared in a magnetic field of intensity 

 about 10700. The visible effect was large and the test was 



easilv made. The result was ' ' — „ _._ " ■ = 1*12. 

 J R. P. of No. 1 



The question arose whether any large part of the superiority 

 shown by the narrow cross could be attributed to a greater 

 heating of this cross by the direct current. Experiment con- 

 vinced me that this was not the case. 



The R. P. of No. 1 appeared from a rough test to be about 

 -J times as great as that of a very thin cross of cobalt taken 

 from the same bar, and to be described hereafter. This dis- 

 crepancy is hardly significant in view of the roughness of the 

 comparison and the uncertainty as to the thickness of the thin- 

 nest cross. 



Thin cross ; field of varying strength : A part of the cobalt 

 bar already mentioned was heated and hammered to reduce its 

 thickness. The reducing process was continued by grinding, 

 etc., until finally a cross was obtained, the dimensions of which 

 were approximately as follows : 



Thickness. 



Width of 

 Main part. Arms. 



Length of 

 Main part. Arms. 



•048 mm 



ll-5 mra 2-0 ram 



ogmm ^mm 



The value given for the thickness is a rough estimation of 

 the average thickness between the arms. It may be wrong by 

 10 or 15 per cent in either excess or defect. 



This piece of metal was, like most others that I have used, 

 imbedded in a cement of bees-wax and rosin on a glass plate. 

 It was placed between the poles of the magnet in a narrow 

 tank through which water flowed to control the temperature 

 of the metal. The character and results of the examination 

 are set forth in the following table, where M is the intensity of 

 the magnetic field in absolute units : 



