164 REPORT OF NATIONAL MU8EUM, 1891. 



machine, which was coiistruced by liis own hands, is one of tlie earliest 

 applications of magneto-electricity to the prodnction of power. Pro- 

 fessor Henry calls it "reciprocating motion produced by magnetic at- 

 traction and repulsion." 



In July, 1831, writing to the editor of Silliman's Journal, he says: 



I have lately succeeded in producing motion in a little machine by a power which, 

 I believe, has never before been applied in mechanics — by magnetic attraction and 

 repulsion. 



Not much importance, however, is attai^hed to the invention since the article in its 

 present state can only be considered a philosophical toy, although in the discovery 

 and invention it is not impossible that the same principle, or some modification of 

 it on a more extended scale, may hereafter be applied to some useful purpose. But 

 without reference to its practical utility, and A'iewed only as a new eftect produced 

 by one of the most mysterious agents of nature, you will not, perhaps, think the fid- 

 lowing account of it unworthy of a place in the Joui'nal of Science. 



It is well known that an attractive or repulsive force is exerted between two mag- 

 nates, according as poles of different names, or poles of the same name, are pre- 

 sented to each other. 



In order to understand how this i)riucii)le can be apjilied to produce a reciprocat- 

 ing motion, let us suppose a bar magnet to be supported horizontally on an axis 

 jjassiug through the center of gravity, in jirecisely the same manner as a dipping 

 needle is poised ; and suppose two other magnets to be placed perpendicularly, one 

 under each pole of the horizontal magnet, and a little below it, with their north poles 

 uppermost; then it is evideut that the south pole of the horizontal magnet will be 

 attracted by the north pole of oue of the perpendicular magnets, and its north pole 

 repelled by the north pole of the other; in this state it will remain at rest, but if 

 by any means we reverse the polaritj^ of the horizontal magnet, its position will be 

 changed and the extremity, which was before attracted, will now be repelled. If 

 the polarity l»e again reversed, the ijosition will again be changed, and so on indefi- 

 nitely. To produce, therefore, a continued vibration, it is only necessary to intro- 

 duce into this arrangement some means by which the p<darity of the horizontal 

 magnet can be instantly changed, and that, too, by a cause which shall be put in 

 operation by the motion of the magnet itself; liow this can be effected will not be 

 difficult to conceive, when I mention that instead of a permanent steel magnet in 

 the movable part of the a)»i)aratus, a soft iron galvanic magnet is used. 



The motion here described is entirely distinct from that produced by the electro- 

 magnetic combination of wires and magnets; it results directly from the mechanical 

 action of ordinary magnetism, galvanism being only introduced for the purpose of 

 changing the poles. 



At the end of sixty years the same principle, that led the great physi- 

 cist (who is known to the world as a discoverer rather than an inventor) 

 to make this " little machine," is used in constructing the electrical 

 devices which may be x^roperly classed among the mechanical triumphs 

 of this century of invention.* 



A deposit was made of two original letters written to S. Vail & Son, 

 in 1838, by Alfred Vail, while associated with Prof. Morse. They 

 describe Mr. Vail's relations with Prof. Morse, and the operations of the 

 first practical electro magnetic telegraph ma<hiue then being tested 

 before the Committee on Commerce at the United States Capitol. This 



* .See article " Henry's Electric Motor ( 'onstructe<l in 1831," by .1. Elfreth Watkius, 

 Electrical World, May 9, 1891. 



