354 Prof. A. M. Mayer on the Effects of Magnetization 



the elongution is in the duplicate ratio of the magnetic intensity of 

 the bar, both when the magnetism is maintained by the influence 

 of the coil, and in the case of the permanent magnetism after 

 the current has been cut off. The discrepancies observable 

 will, I think, be satisfactorily accounted for when we consider 

 the nature of the magnetic actions taking place. When a bar 

 experiences the inductive influence of a coil traversed by an 

 electrical current, the particles near its axis do not receive as 

 much polarity as those near its surface, because the former have 

 to withstand the opposing inductive influence of a greater num- 

 ber of magnetic particles than the latter. This phenomenon 

 will be diminished in the extent of its manifestation with an in- 

 crease of the electrical force, and will finally disappear when the 

 current is sufficiently powerful to saturate the iron. Again, 

 when the iron, after having been magnetized by the coil, is aban- 

 doned to its own retentive powers by cutting off the electrical 

 current, the magnetism of the interior particles will suffer a 

 greater amount of deterioration than that of the exterior par- 

 ticles. The polarity of the former may indeed be sometimes 

 actually reversed, as Dr. Scoresby found it to \q in some exten- 

 sive combinations of steel bars. Now, whenever such influences 

 as the above occur, so as to make the different parts of the bar 

 magnetic to a various extent, the elongation will necesarily bear 

 a greater proportion to the square of the magnetic intensity 

 measured by the balance than would otherwise be the case. 



"For similar causes the interior of the bar will in general 

 receive the neutralization and reversion of its polarity before the 

 exterior ; and hence we see in the Tables that there is a consi- 

 derable elongation of the bar after the reversion of the current, 

 even when the effect upon the balance has become imperceptible, 

 owing to the opposite effects of the interior and exterior mag- 

 netic particles." 



Joule now experimented on a bar of unannealed iron, and on 

 three bars of soft steel. As these bars had considerable degrees 

 of retentive power, the anomalies occasioned by the above- 

 described actions did not exist to any considerable extent, and 

 they gave a confirmation of the law that the elongation is pro- 

 portional, in a given bar, to the square of the magnetic intensity. 



The next bar he experimented with was of moderately har- 

 dened steel. This bar was slightly increased in length every 

 time that contact with the battery was broken, although a con- 

 siderable diminution of the magnetism of the bar took place at 

 the same time. He says : — " I am disposed to attribute this 

 effect to the state of tension in the hardened steel, for I find that 

 soft iron wire presents a similar anomaly when stretched tightly." 



In a subsequent communication, contained in the same vo- 



