78 Mr. J. P. Joule on the Effects of Magnetism 



observed to take place with extreme suddenness, as if it had 

 been occasioned by a blow at the other extremity. The ex- 

 pansion, though very minute, is indeed so very rapid that it 

 may be felt by the touch ; and if the electro-magnet be placed 

 perpendicularly on a hard elastic body, such as glass, the ear 

 can readily detect the fact that it makes a slight jump every 

 time that contact is made with the battery. 



" When one end of the electro-magnet is applied to the ear, 

 a distinct musical sound is heard every time that contact is 

 made widi, or broken from, the battery ; another proof of the 

 suddenness with which the particles of iron are disturbed." 



In another part of the lecture I stated my reasons for sup- 

 posing that whilst the bar of iron was increased in length by 

 the magnetic influence, it experienced a contraction at right 

 angles to the magnetic axis, so as to prevent any change taking 

 place in the bulk of the bar. I intended as soon as possible 

 to bring this conjecture to the test of experiment, and I pre- 

 pared some apparatus for the purpose : but owing to other 

 occupations I was obliged to relinquish the experiments until 

 the beginning of last summer. In the meantime the inquiry 

 has been taken up by De la Rive, Matteucci, Wertheim, 

 Wartmann, Marrian, Beatson and others, whose ingenious 

 experiments have invested the subject with additional interest. 

 The researches of Beatson have taken a similar direction to 

 mine; and he appears also to have employed a somewhat 

 similar apparatus to that which I shall presently describe. I 

 have confirmed several of the results at which this gentleman 

 has arrived, and have added new facts, which I hope will 

 throw further light upon this rather obscure department of 

 physics. 



In order to ascertain how far my opinion as to the invaria- 

 bility of the bulk of a bar of iron under magnetic influence was 

 well-founded, I devised the following apparatus. Ten copper 

 ■wires, each 110 yards long and one-twentieth of an inch in 

 diameter, were bound together by tape so as to form a good, 

 and at the same time very flexible conductor. The bundle of 

 wires thus formed was coiled upon a glass tube 40 inches long 

 and \h inch in diameter. One end of the tube was herme- 

 tically sealed, and the other end was furnished with a glass 

 stopper, which was itself perforated so as to admit of the in- 

 sertion of a graduated capillary tube. In making the experi- 

 ments, a bar of annealed iron, one yard long and half an inch 

 square, was placed in the tube, which was then filled up with 

 water. The stopper was then adjusted, and the capillary tube 

 inserted so as to force the water to a convenient height within it. 

 The bulk of the iron bar was about 4,500,000 tin)es the 



