264 Messrs. H. Nagaoka and K. Honda on Magnetostriction. 



The demagnetizing factor for the rod was calculated on 

 the supposition that N was equal to that of a circular cylinder 

 of the same cross-section. 



The magnetizing coil was 30 cm. long, and wound in 12 

 layers ; its resistance was 0'63 ohm. and gave the field of 

 37*97 C.G.S. units at the middle of the coil due to a current 

 of one ampere. 



The magnetometer consisted of a small bell-magnet sus- 

 pended in a thick copper case by a quartz fibre and provided 

 with a plane mirror. It was placed due magnetic east of the 

 coil, and its deflexion read by means of scale and telescope. 



The following table gives the magnetization in different 

 fields : — 



H. 



I (Iron Ovoid). 



I (Iron Cylinder). 



I (Nickel). 



5 



660 



158 



47 



10 



1020 



380 



100 



20 



1220 



770 



175 



30 



1270 



900 



240 



40 



1310 



980 



280 



50 



1340 



1030 



308 



75 



1390 



1100 



358 



100 



1440 



1143 



392 



125 



1480 



1180 



414 



150 



1500 



1210 



432 



200 



1550 



1270 



455 



250 



1600 



1315 



469 



300 



1640 



1350 



477 



350 







1380 



482 



Change of Volume produced by Magnetization. — Before we 

 proceed to the description of the method employed in the 

 present experiment, it will be worth while to compare the 

 results of several previous investigators on the change of 

 volume produced by magnetization. 



It was generally admitted that there is no change of volume 

 by magnetization, but it will be easily seen that most of these 

 experimenters tried to increase the volume of the magnet by 

 unusually increasing the thickness instead of length, thus 

 incurring the risk of increasing the demagnetizing factor. 

 They did not therefore arrive at a field-strength sufficient to 

 produce appreciable change of volume. 



Joule * was the first to call attention to the change of 

 volume which may accompany the magnetization of iron. 

 The result was in the negative ; but as he gave neither the 

 strength of the magnetizing current nor the intensity of 

 magnetization, it is difficult to compare his result with that 



* Joule, Phil. Mag. vol. xxx. p. 76 (1847). 



