285 Messrs. H. Nagaoka and K. Honda on Magnetostriction. 



It must not be forgotten that these coefficients are, strictly 

 speaking, functions of the strain caused by mechanical 

 action on nickel. Taking Prof. E wing's* experiments on 

 the magnetization of nickel under various loadings, we find 

 the following values of k' on the supposition that 



3k' + k" = 0. 



1 1 



H 5 ' 5 U 



1 kg. sq. mm. J kg. sq. mm. 



16-5 

 kg. sq. mm. 



22 



kg. sq. mm. 



27-5 

 kg. sq. mm. 



33 



kg. sq. mm. 



30 -4250 ; -3320 



-2950 



-2350 



-1680 



-1170 



50 -2320 , -2140 



-1930 



-1630 



-1260 



- 900 



100 - 950 ! - 850 



- 810 



- 800 



- 670 



- 520 



The above values will probably not be far from those 

 obtained by actual determination. The coefficients k' and k" 

 are thus functions of the strains of the magnetized body. In 

 nickel, k" diminishes as the longitudinal pull is increased. 

 In calculating the coefficients k' and k" from the stress-effect, 

 we have taken care to use such values of SI as are due to 

 very small loading, in order that the result may be com- 

 parable to those obtained from the strains produced by 

 magnetization. The diminution of k" with increased loading 

 is greater in the weak than in strong fields. Applying 

 equation (d) for measuring the length-change produced by 

 magnetization, we notice that hl/l diminishes with the coefficient 

 k", so that we expect, from the above result, diminution in 

 the contraction of nickel wire with increased longitudinal 

 pull ; but as the rate of diminution of k" becomes less as 

 the field-strength is increased, the lessening of contraction 

 will not be so marked in strong fields as in weak. This 

 theoretical conclusion is borne out by the experiments of 

 Bidwell on the effects of longitudinal stress on the length- 

 change of nickel wire. The change of volume due to mag- 

 netization will somewhat diminish for nickel wire under 

 longitudinal pull, but the difference will not be so pronounced 

 as for the length-change. The experimental verification of 

 these conclusions will be attended with considerable difficulty. 



k' and k" for Iron. — Making use of the measurement of 

 strains in the ovoid produced by magnetization, we find the 

 following numbers for k [ and k". 



* Phil. Trans, clxxix. A, p. 325 (1888). 



