in changing the Dimensions of Iron, 357 



the research and the skill and acumen of this eminent physicist 

 will be properly estimated. 



Although the cognate discovery by our countryman Page, in 

 1837, that iron bars produce sound on their magnetization, has 

 been carefully studied by Delezenne, De la Rive, Eeatson, Mar- 

 rian, and Wertheim, yet in the annals of science I have found 

 only two experimental investigations, in addition to the one by 

 Joule, on the phenomena of the elongation produced in iron rods 

 on their magnetization. The first is by Wertheim, in the Ann, 

 de Chim. et de Phys. 3 ser. vol. xxiii. ; the second by Tyndall, 

 contained in a paper entitled " On some Mechanical Effects of 

 Magnetization," published in his 'Researches on Diamagnetism 

 and Magnecrystallic Action/ London, 1870. 



In Wertheim^s memoir " On the Sounds produced in Mag- 

 netized Iron," all we find on the subject of the elongation of 

 magnetized iron rods is the following : — u Here are the results 

 of these experiments : the helix being placed so that its axis 

 coincides with that of the bar, we do not observe any lateral 

 movement, but only a very small elongation ; this elongation 

 rarely surpasses *002 millim. [in rods about 970 millims. long], 

 and although visible is bearly measurable ; it is most pronounced 

 when the helix [whose length was a litttle over \ of that of 

 the rod] encloses the extremity of the bar; it diminishes as the 

 helix approaches the point [the centre] where the rod is clamped ; 

 and it is probable that when it is quite close to this point the 

 elongation changes into a retraction ; but I have never been able 



to observe the motion in this direction with any certainty 



I have already remarked that it was not possible for me to mea- 

 sure this longitudinal traction ; happily Mr. Joule has supplied 

 that omission." 



Dr. Tyndall opens his paper thus: — "Wishing, in 1855, to 

 make the comparison of magnetic and diamagnetic phenomena 

 as thorough as possible, I sought to determine whether the act 

 of magnetization produces any change of dimensions in the case 

 of bismuth, as it is known to do in the case of iron. The action, 

 if any, was sure to be infinitesimal ; and I therefore cast about 



for a means of magnifying it I consulted Mr. Becker ; and 



thanks to his great intelligence and refined skill, I became the 



possessor of the apparatus now to be described The same 



apparatus has been employed in the examination of bismuth bars ; 

 and though considerable power has been applied I have hitherto 

 failed to produce any sensible effect. It was at least conceivable 

 that complementary effects might be here exhibited, and a new 

 antithesis thus established between magnetism and diamag- 

 netism." 



The apparatus used by Dr. Tyndall consisted of two vertical 



