ON MAGNETOSTEICTIOX. 21 



behaviour of cast and annealed cobalt stands in reciprocal relation 

 with that of the change of length produced by magnetization, as will 

 be clear in the following parallel statements. 



Cdst cobalt. 

 Magnetization pi-oduces diminution Mechanical elongation produces 



of length in low fields, which after diminution of magnetization in low 

 reaching a maximum gradually lessens, fields, which after reaching a maximum 

 and finally produces increase in strong gradually lessens, and finally produces 

 fields. increase in strong fields. 



Annealed cobalt. 

 Magnetization produces diminution Mechanical elongation produces 



of length, which gradually increases diminution of magnetization, which 

 with the strength of the field. gradually increases with the strength of 



the field. 

 The effect of mechanically elongating nickel steel always results 

 in the increase of magnetization, as will be seen in Fig. G, PI. II. 

 The change caused by stretching depends on the strength of the field, 

 and is generally greater in the weak than in the strong. The 

 correlation between the elongation due to magnetization and the 

 effect of stretching on magnetization may be expressed in fallowing 

 words. 



Magnetization produces increase of Mechanical elongation produces in- 



leugth in nickel steel. crease of magnetization. 



§ 6. Change of Volume by Magnetization. 



In spite of the well known experiments of Joule showing that 

 iron does not change in bulk by magnetization, experiments by 

 Knott, and ours sufticiently prove that this is by no means the case. 

 It was suggested by Rhoads, that the change of volume may in a 

 great part be due to the heterogeneity of the material under exami- 

 nation, while in some the arrangement of the experiment was not 



