216 MEMORIAL OF JOSEPH HENRY. 



cians, laboring to attain the result, had for six years failed to make 

 such an extension. Nor was the result by any means antecedently 

 assured by Schweigger's success with the galvanometer. If Stur- 

 geon's improvement of economizing the battery size and consump- 

 tion, by increasing the magnet factor (in those few cases where 

 available), was well deserving of reward, surely Henry's improve- 

 ment of a far greater economy, by increasing the circuit factor 

 (entirely neglected by Sturgeon) deserved a still higher applause. 



In a subsequent communication to Silliman's Journal, Henry 

 remarks on the results announced in October, 1827: "Shortly 

 after the publication mentioned, several other applications of the 

 coil, besides those described in that paper, were made in order to 

 increase the size of electro-magnetic apparatus, and to diminish 

 the necessary galvanic power. The most interesting of these was 

 its application to a development of magnetism in soft iron, much 

 more extensive than to my knowledge had been previously effected 

 by a small galvanic element." And in another later paper, he 

 repeated to the same eifect : " After reading an account of the gal- 

 vanometer of Schweigger, the idea occurred to me that a much 

 nearer approximation to the theory of Ampere could be attained 

 by insulating the conducting-wire itself, instead of the rod to be 

 magnetized; and by covering the whole surface of the iron with a 

 series of coils in close contact." 



The electro-magnet figured and described by Sturgeon (in his 

 communication of November, 1825,) consisted of a small bar or 

 stout iron wire bent into a n or horse-shoe form, having a copper 

 wire wound loosely around it in eighteen turns, with the ends of 

 the wire dipping into mercury-cups connected with the respective 

 poles of a battery having 130 square inches of active surface. 

 This was probably the only electro-magnet then in existence. 



In June of 1828, Henry exhibited before the Albany Institute a 

 small-sized electro-magnet closely wound with silk-covered copper 

 wire about one-thirtieth of an inch in diameter. By thus insulat- 

 ing the conducting wire, instead of the magnetic bar or core, he 

 was enabled to employ a compact coil in close juxtaposition from 

 one end of the horse-shoe to the other, obtaining thereby a much 

 larger number of circuits, and with each circuit more nearly at 



