JOSEPH HENRY 5 



also did another thing in his actual construction: lie adopted what 

 may be called the spool construction, the placing of the windings on 

 spools, and then the sliding of the spools on the core. That is a 

 standard method of building electromagnets today. 



Soon after doing this work Henry built a magnet to be used at 

 Yale University, which was in its time a wonder and would even 

 today be considered very powerful. He also built a series of magnets 

 in which the emphasis was placed upon the lifting power in relation 

 to the weight of the magnet and succeeded in designing one which, when 

 energized by a single small cell, could support 420 times its own weight. 



The improvements which Henry made in magnets suggested to him 

 applications of magnetic attraction to the production of mechanical 

 motion. He realized that electromagnets such as he built were easy 

 to control, and believed that he could design a machine by w'hich he 

 could get power from an electric current and this at a time when the 

 only source of current were primary batteries as the dynamo did not 

 yet exist. 



His electric motor was the first ever built to use electromagnets;^ 

 it was extremely simple consisting of an electromagnet supported at 

 its center by a pivot so that it could rock back and forth under the 

 alternating pulls of two permanent magnets. To efTect the reversal of 

 magnetization of the electromagnet and hence the alternation of pulls, 

 mercury cups were arranged so that wires would dip in them as the 

 suspended magnet rocked to and fro. These contacts were the proto- 

 type of the commutator which is found in every direct current motor 

 and dynamo today. It is interesting to note the words in which 

 Henry described this invention. In Silliman's American Journal of 

 Science for 1831 he wrote, "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 attached to the invention since 

 the article in its present state can only be considered a philosophical 

 toy; although in the progress of discovery and invention it is not 

 impossible that the principle or some modification of it on a more 

 extended scale may hereafter be applied to some useful purpose." 



The modesty of this statement and Henry's vision of the future 

 possible applications of the principle there shown cannot fail to com- 



^ Faraday has some years before shown that a wire carrying a current could be 

 caused to revolve continuously around the pole of a permanent magnet. Henr\'s 

 advance over this was considerable in that he materially increased the force causing 

 motion by employing the attraction lietween two magnets, one permanent and one 

 generated by current. The motor using electromagnets throughout did not come 

 until later. 



