Messrs. H. Na°r;ioka and K. Honda on Magnetostriction. 273 



thus giving the position of maximum deflexion. It was 

 necessary to place the magnet in the above position, owing to 

 the slight displacement due to the strain caused by strong- 

 pressure. For a small vertical motion, the magnetometer could 

 remain practically unaffected in the position above chosen. : 



It is clear from the arrangement for compensation that not 

 only is the effect of magnetization on the magnetometer to a 

 great extent compensated, but the effect of temperature rise 

 due to the magnetizing current is also compensated, as the 

 auxiliary magnet is enclosed in a similar brass tube and 

 placed in a coil of the same dimension and resistance. It 

 is to be remarked that the compensation was never exact, for 

 though the ovoids or rods were made of the same material, 

 there was some difference of qualify as regards magnetization. 

 Thus the compensation, though exact in certain fields, was 

 not fulfilled throughout the whole range of fields; neverthe- 

 less the difference was not very great, and we believe that 

 the influence of the rise of temperature or that of change of 

 position due to the strain caused by pressure would not be so 

 large as to materially deteriorate the experimental results. 

 In spite of this, care was taken to keep the field during the 

 experiment constant by watching the indication of the deci- 

 ampere balance, by which the current was measured ; further, 

 it was generally possible to perfect the compensation for 

 feeble change of current by slightly shifting the auxiliary 

 magnet or rhe coil. 



The horizontal component of the terrestrial magnetic force 

 was slightly affected by thus placing the coils very near the 

 magnetometer, so that it was necessary to measure the period 

 of vibration of the magnetometer magnet by means of a chro- 

 nograph, and to apply the correction to the observed intensity 

 of magnetization. 



The change in the intensity of magnetization due to alte- 

 ration of volume is evidently nearly equal to — I8v/v. The 

 diminution of volume will therefore produce increase of 

 magnetization which is generally of the same order of magni- 

 tude as the change in magnetization wrought by compression. 



Results in Iron. — A few of the observed results with iron 

 ovoid or cylinder are given in figs. 8 and 9 (Pi. II.). The 

 dotted lines indicate the correction due to change of volume 

 by compression, which must be added to the apparent change. 

 The inspection of these figures shows minute diminution of 

 magnetization by the application of hydrostatic pressure : in 

 fact, the apparent change measured in C.G.S. units does not 

 even amount to 01 with the pressure of 250 atmospheres. 

 At the above-mentioned pressure the change of intensity for 



