538 K. Honda, S. Shimizu, and S. Kusakabe on Change of 



that the torsion of an iron wire was diminished by magneti- 

 zation. C. Barus* hung two identical iron wires in the 

 same vertical line, separated by a rigid piece of brass which 

 carried the index mirror ; to the lower end of the wire a 

 weight was attached. The wire was twisted, and then either 

 the upper or the lower end of the system fastened. If both 

 ends were twisted equally in opposite directions, the direction 

 of the mirror remained unchanged. A magnetizing coil was 

 placed coaxially with the upper wire. If the rigidity of the 

 wire be changed by magnetization, the mirror would rotate 

 in either direction when magnetized. Barus found that for 

 soft iron and steel the increase of rigidity was 0*24 and 0*08 

 per cent, respectively. In his later experiments he observed 

 an increase amounting to 1 per cent, for soft iron. With a 

 similar arrangement H. D. Dayt investigated the same sub- 

 ject in iron. He found that the change of rigidity increased 

 with field and that it became generally less as the initial 

 torsion was increased. The maximum value obtained was 

 0*8 per cent. The experiments of J. S. Stevens J for iron and 

 steel rods gave an increase of rigidity. The change amounted 

 to 2*3 per cent, for soft iron and 0*4 per cent, for steel in a 

 field of 138 c.G.s. units. In his experiments the magnetizing 

 coil was much less than the length of the rod, so that the 

 magnetization was far from being uniform. 



2. Our method of twisting the ferromagnetic rod was the 

 same as that of Prof. Nagaoka§, used for studying the elastic 

 constants of rocks; but the sensibility of the apparatus for 

 measuring the angle of torsion was 106 times greater. 



The front and side views of our apparatus are given in the 

 accompanying figure (fig. 1) . 



AA is the lateral view of a stout wooden frame rectangular 

 in shape. BB are the projections protruding from the frame; 

 to the one, a brass rod, to which a ferromagnetic rod is 

 soldered, is clamped by means of a screw H, and to the other 

 a screw Gr is clamped, which carries an agate cup in one of 

 its extremities. F is a double pulley whose axis is a thick 

 brass cylinder ; a point made of non-magnetic nickel steel is 

 firmly fixed to one of its extremities, while the ferro- 

 magnetic rod is soldered to the other, as shown in fig. 2. 

 The inner pulley serves to twist the specimen to be tested, 

 and the outer to increase the sensibility of the apparatus. 



* Amer. Journ. of Sci. xxxiv. p. 175 (1887) ; Phys. Rev. xiii. p. 257 

 (1901). 



t Day, ' Electrician,' xxxix. p. 480 (1897). 



X J. S. Stevens, Phys. Kev. (3) x. p. 161 (1900). 



§ H. Nagaoka, Phil. Mag. 1. p. 53 (1900). 



