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



the iron at right angles to the direction of its polarity. For 

 this purpose I took a piece of drawn iron gas-piping 1 yard long, 

 ■fg of an inch in bore, and f^ of an inch in thickness. A 

 piece of thick covered copper wire was inserted into this tube 

 and bent over the outside of it. The lower extremity of the iron 

 tube being fixed, and the upper end being attached to the mi- 

 crornetrical apparatus, each division of which corresponded to 

 — t of an inch, I obtained .... results which show that 



■J Q Q ^ O O J 



the length of the tube was diminished in order to make up for 

 the increase of its diameter, which in this instance was in the 

 direction of the polarity. The quantity of the shortening effect, 

 viz. 3*4, is, however, only one third of that due to the maximum 

 elongation of soft iron bars as observed in the first section. This 

 is probably owing to the grain of the iron being in cross direc- 

 tions with respect to the polarity in the two cases, and partly 

 perhaps to the iron tube not being fully saturated with mag- 

 netism. The experiment is worth repeating, especially as it affords 

 a means of studying the magnetic condition of closed circuits." 



Remarking on the cause of the phenomena of elongation, Dr. 

 Joule says : — " The law of elongation naturally suggests the joint 

 operation of the attractive and repulsive forces of the constituent 

 particles of the magnet as the cause of the phenomena. On the 

 other hand, the fact that the shortening effect is proportional to 

 the magnetic intensity of the bar multiplied by the current tra- 

 versing the coil seems to indicate that in this case the effect is 

 produced by the attraction of the magnetic particles by the coil. 

 But then it will be asked why so remarkable an augmentation 

 of the effect is produced by the increase of tension in the case 

 of the soft iron bars. When we are able to answer this question 

 in a satisfactory manner, we shall probably have a much more 

 complete acquaintance with the real nature of magnetism than 

 we at present possess." 



This full account of Dr. Joule's remarkable research is here 

 presented in order to give an exposition of our present know- 

 ledge of this subject, and clearly to set forth the relations which 

 my own attempts bear to his labours. Here Joule, the disco- 

 verer of these phenomena, has given us almost all the knowledge 

 we have up to this time possessed in reference to their charac- 

 teristics and their laws. That a subject so fascinating should 

 not have been eagerly followed up appears strange, especially so 

 when it seems highly probable that the faithful study of these 

 actions may one day give us an insight into the dynamic nature 

 of electro-magnetization, and thus lead the investigator into a 

 fruitful field of research. 



No one can duly appreciate this work of Joule's until he 

 attempts the confirmation of his results ; then the difficulties of 



