126 ANNUAL OF SCIENTIFIC DISCOVERY. 



cause. Heiice the conception of the celebrated Ampere that a mag- 

 net is simply an assemblage of electric currents. Round the atoms 

 of a magnet Ampere supposed minute currents to circulate inces- 

 santly in parallel planes ; round the atoms of common iron he also 

 supposed them to circulate, but in all directions, thus neutralizing 

 each other. The act of magnetism he supposed to consist in the ren- 

 dering of the molecular currents parallel to a common plane, as they 

 are supposed to be in a permanent magnet. This is the celebrated 

 theory of molecular currents propounded by Ampere. 



Tke Lent/then ing of Iron by Magnetism. Is it a fact that an iron 

 Star is shortened by the act of magnetization ? It is not. And here, 

 ts before, we enter into the labors of other men. 



Mr. Joule was the first to prove that the bar is lengthened. Mr. 

 Joule rendered this lengthening visible by means of a system of levers 

 and a microscope, through which a single observer saw the action. 

 The experiment has never, I believe, been made before a public audi- 

 ence, but the instrument referred to at the commencement of this 

 lecture, will, I think, enable me to render this effect of magnetization 

 visible to everybody present. 



Before you is an iron bar, two feet long, firmly screwed into a solid 

 block of wood. Sliding on two upright brass pillars is a portion of 

 the instrument which you see above the iron bar. The essential parts 

 of this section of the apparatus are, first, a vertical rod of brass, 

 which moves freely and accurately in a long brass collar. The lower 

 end of the brass rod rests upon the upper flat surface of the iron bar. 

 To the top of the brass rod is attached a point of steel ; and this point 

 now presses against a plate of agate, near a pivot which forms the 

 fulcrum of a lever. The distant end of the lever is connected by a 

 very fine wire with an axis on which is fixed a small circular mirror. 

 If the steel point be pushed up against the agate plate, the end of the 

 lever is raised ; the axis is thereby caused to turn, and the mirror ro- 

 tates. I now cast a beam from an electric lamp upon the mirror ; it is 

 reflected in a luminous sheaf, 15 or 16 feet long, and it strikes our 

 screen, there forming a circular patch of brilliant light. This beam 

 is to be our index ; it will move as the mirror moves, only with twice 

 its angular velocity ; and the motion of the patch of light will inform 

 us of the lengthening and shortening of the iron bar. 



I employ one battery simply to ignite the lamp. I have here a 

 second battery to magnetize the iron bar. At present no current is 

 passing. I make the circuit, and the bright image on the screen is 

 suddenly displaced. It sinks a foot. I break the circuit ; the bar 

 instantly shrinks to its normal length, and the image returns to its 

 first position. I made the experiment several times in succession; 

 the result is always the same. Always when I magnetize, the image 

 instantly descends, which declares the lengthening of the bar; always 

 when I interrupt the current the image immediately rises. A little 

 warm water projected against the bar causes the image to descend 

 gradually. This, I believe, is the first time that this action of mag- 

 netism has been seen by a public audience. 



I have employed the same apparatus in the examination of bismuth 

 bars ; and, though considerable power has been applied, I have hith- 

 erto failed to produce any sensible effect. It was at least conceivable 



