PEIRCE. 



RESISTIVITY OF HARDENED CAST IRON. 



19,'] 



of ascending reversals with the aid of a small test coil about its centre 

 and a ballistic galvanometer of period sufliciently long for the purpose. 

 Each rod was about 50.9 of its own diameters long and, according to 



TABLE VI. 



the formula of Dr. Shuddemagen for the end corrections of rods of 

 these dimensions, the actual magnetic intensity (//) inside the metal 

 at the centre is equal to //' — 0.00107 B, where //' is the force inside 

 the solenoid when the rod is removed. It is possible, therefore, to 



TABLE VII. 



determine very approximately the relative values of // and B from the 

 observed values of H' and B, and the computation has been made by 

 Mr. Coulson for these rods. The results of this work show that though 

 the moments of the magnets differed so much among themselves, the 

 permeabilities of the pieces of metal for excitations up to H = 50, at 

 least, are much the same. Magnet B4. for instance, had a moment 



VOL. XLVI. — 13 



