346 Dr. L. T. More on the Changes in Length 



Historical. 



That magnetizing an iron rod causes it to alter its length 

 was first discussed by Joule * in 1847. His attention was 

 called to the phenomenon by a machinist of Manchester, who 

 imagined that the volume of a mass of iron was increased by 

 magnetizing it. Joule, to test the opinion of the machinist, 

 immersed a mass of iron in a closed vessel full of water in 

 which stood a fine capillary tube. When the iron was 

 strongly magnetized the height of the column of water in the 

 tube remained unaltered, showing that within the limits of 

 accuracy of his apparatus, for the intensity employed, the 

 volume of the iron was unchanged. Bidwellf has also 

 investigated this subject and found, on the contrary, that the 

 volume was affected by magnetization. The volume dimi- 

 nishes at first and attains a minimum. It then increases, 

 until with sufficiently intense fields the original size is re- 

 gained. After reaching this point the volume continues to 

 increase. As a consequence of this relation, if Joule had used 

 an intensity either greater or less, he probably would have 

 noticed a change in the volume. Joule afterwards, by means 

 of a system of levers, found that the length of a rod was 

 increased by the magnetizing force, and gave as a result of 

 his observations the following laws : — 



1st. When soft iron rods are magnetized their length is 

 increased, and the elongation is approximately proportional 

 to the square of the magnetizing force. 



2nd. Tension applied to the rod diminishes the elongating 

 effect, and ' ' In the case of a bar one foot long and one-quarter 

 inch in diameter, a tensile force of about 600 pounds caused 

 all the phenomena of changes of length to disappear." 



3rd. " That the elongation is, for the same intensity of 

 magnetism, greater in proportion to the softness of the metal. 

 It is greatest of all in the well-annealed iron bars, and least 

 in hardened steel. This circumstance appears to me to favour 

 the hypothesis that the phenomena are produced by the attrac- 

 tions taking place between the magnetized particles of the bar; 

 an hypothesis in perfect accordance with the law which I have 

 pointed out,'" that the elongation was proportional to the 

 square of the intensity of magnetization. 



The first two laws pointed out by Joule have been often 

 confirmed, but the third seems to rest on a single experiment, 

 and until very recently there have been no published records, 



* Phil. Mag. [3] vol. xxx. pp. 76 & 225. 

 t Proc. Roy. Soc. vol. hi. p. 94. 



