CHANGE OF ELASTICITY BY MAGNETIZATION. 9 



Since the resistance of the coils did not exceed 30 ohms, no 

 heating* of the core due to current was observed during the time, in 

 which the deflection was taken ; hence we dispensed with water- 

 jacketing arrangement. 



The lateral contraction or elongation due to magnetization was 

 at most of the order 2 x lO"*' cm for iron and 7 x 10"" cm for nickel. 

 For, the maximum elongation or contraction per centimeter in the 

 field strength used in the present experiment was about 4 x lO"*^ and 

 27 X 10"" cm for iron and nickel respectively. Hence, assuming the 

 change of volume to be negligibly small compared with that of length, 

 the maximum lateral contraction or elongation was approximately 

 2 X 10"^ and 7x10"" cm for the three metals respectively. But in our 

 experiments, the displacement of 1mm of the vertical scale in the field 

 of the telescope corresponds to a change of depression of 1.72 x 10"' cm. 

 Thus the lateral change of dimensions due to magnetization is within 

 the limit of experimental errors. The disturbance of the results due 

 to magnetic elongation nv contraction in the longitudinal direction 

 was eliminated by means of the rotating cylinder, which served as 

 one of fulcrums. 



The bar bent slightly downwards if loaded ; hence when it was 

 magnetized, it would strive to make itself straight. This may cause 

 an apparent increase of elasticity ; but it was confirmed by a direct 

 experiment that the eltect was negligibly small. For, the reading 

 obtained by inclining the two coils with respect to the bar to a degree 

 o-reatr than the actual case was almost the same as in the case when 



c 



the coils rested in a coaxial line. 



Since the bar was considerably shorter than the whole length 

 of the coils, it lay nearly in a uniform field, except at the middle. 

 The eftect of the air gap between the coil was also studied, varying its 



