OF THE MAGNETIC PERMEABILITY OF MAGNETITE. 577 



to an ellipsoid three diameters long, we obtain the theoretical value 



JV= 1-2165. 



Hence we see that N appears to have a rather larger value in this case than theory 

 would predict for a similar ellipsoid. 



Employing, again, the value of T as before, we find that, for the fragment of mag- 

 netite used for the temperature variation, the value of k' for ordinary temperatures is 



given by 



k =0151 (13) 



This fragment was part of a prism 1 '3 cm. square, with one end at right angles to its 

 long edges, whose lengths were respectively 1*9, 2*2, 3'2, and 2"8 cm. long, and is repre- 

 sented in figure 5. 



Its mean length is 1*76 times its equivalent diameter (i.e., the diameter of a cylinder 

 of equal cross-sectional area). Now for an ellipsoid of two diameters long we get, from 

 equation (12), 2V=2*13. By analogy from the case of the larger piece, N might be 



Table showing Values of k and pfor Various Temperatures. 







K 



K 



j" 



Temperatures 

 in Degrees 



Ordinates for I 

 of 



ragment 

 of 



K 



= l + 47nc = 

 Ordinates of 



1-2/c' 



Centigrade. 



Curve (vi.). Ma, 



?netite. 



Ordinates of 

 (vii.). 



(viii.). 



20° C. 



233 



151 



0-216 



3-71 



50 



243 



■158 



•231 



3-90 



75 



249 



T61 



•237 



3-97 



100 



252 



•163 



•242 



4-04 



125 



256 



■166 



•249 



4-12 



150 



260 



169 



•255 



4-20 



175 



265 



•172 



•262 



4-29 



200 



270 



■175 



•269 



437 



225 



279 



•181 



•283 



4*55 



250 



288 



•187 



•299 



4-75 



275 



296 



•192 



•312 



4-91 



300 



305 



198 



•328 



5-12 



325 



314 



203 



•342 



5-29 



350 



282 



183 



•289 



4-63 



375 



272 



176 



•272 



4-41 



400 



267 



173 



•265 



4-32 



425 



265 



172 



•262 



4-29 



450 



263 



171 



•260 



4-26 



475 



257 



167 



•251 



4-15 



500 



246 



159 



•233 



3-92 



525 



181 



117 



•153 



2-92 



540 



47 



030 



•032 



1-40 



557 







000 



o-ooo 



1-00 



