CHANGE OF ELASTICITY BY MAGNETIZATION. 



13 



of elasticity, M^hen the load exceeds about 1.5 kilograms. Iron 

 contracts laterally when magnetized by weak currents, and this 

 contraction may produce such an apparent decrease of elasticity ; but 

 the calculation shows that it is more than can be accounted for by the 

 lateral contraction. When the üeld increases beyond this region, the 

 change of depression increases rapidly and soon reaches its asymptotic 

 value, after which the increase takes place quite slowly. When the 

 weight is added under a given field, the change of depression is 

 increased. The rate of increase is laro^e wnth a small weijïht, and 

 decreases in amount as the w^eight is increased, approaching an 

 asymptotic value. 



From the change of depression, we may calculate the ratio of the 

 change to the modulus itself. The depression due to the suspended 

 weight as well as to its own weight in an unmagnetized bar is given 

 by the approximate formula* .s— ,-p,'^,., (T + |-W), 



where /, a, h are the length, breadth and thickness of the bar, T and 

 W are the suspended weight and the weight of the bar itself 

 respectively, / and W refer to the part of the bar lying between two 

 fulcrums. The observed change of depression divided by this is the 

 ratio in question, that is, — =p—. Some of our results of calculation are 



*) Clebsch's ElasticitJit 375 ; Winkelmann's Physik I, 2G6. 



