Magnetisation in the Length of Metal Rods. 117 



Table I. 



Metal. 



Diameter in 

 millimetres. 



Observer. 



Magnetising 

 force. 



Total , 

 elongation in 

 fractions of 

 the length. 



Soft iron .... 

 Iron 



6-35 

 12-7 



25-4 



2 65 



3 65 

 25 4 



9x0-75 



J oule 

 Mayer \ 



Barrett -| 

 Bidwell 



Barrett 

 Bidwell 



64 

 Between 

 75 and 118 

 " Maximum 

 magnetisation" 

 45 

 73 



Unknown 

 300 



•00000562 



X -00000457 

 J 



} -00000385 



•00000450 

 •00000389 

 •00000769 

 •00001000 





Nickel 







These figures include the total elongation of the rods, i.e., that due 

 to the permanent as well as to temporary magnetisation. 



The magnitude of the effect undoubtedly varies considerably with 

 the quality of the iron employed ; that used in my own experiments 

 was ordinary commercial iron wire annealed by being heated red hot 

 and slowly cooled. The permanent elongation was, in both cases, 

 rather more than one-third of the whole. For reasons already given, 

 it was much easier to measure with certainty the temporary than the 

 permanent effect. 



By using thinner rods and greater magnetising forces than those 

 previously employed, I have arrived at the curious and interesting 

 fact which it is the main purpose of this paper to describe. If the 

 magnetisation be carried beyond a certain critical point, the conse- 

 quent elongation, instead of remaining stationary at a maximum, 

 becomes diminished, the diminution increasing approximately with 

 the magnetising force. If the force be sufficiently increased a point 

 is arrived at, varying according to the dimensions and quality of the 

 iron, where the original length of the rod is totally unaffected 

 by magnetisation. And if the magnetisation be carried beyond this 

 point, the original length of the rod will be reduced. To take 

 a concrete example : the maximum temporary elongation of my 

 thinnest iron rod occurs when the external magnetising force is about 

 45 ; an external force of about 212 has no effect whatever upon the 

 length, which remains exactly the same as when the rod is unmagne- 

 tised ; a force of about 300 causes the length of the rod to diminish, 

 the amount of the retraction thus produced being about one-half that 

 of the maximum elongation. Here I had exhausted the capability of 

 my battery power — seven Grove cells — but so far I could detect no 

 indication that a limit of retraction was being approached. 



