1835] WRITINGS OF JOSEPH HENRY. 91 



a small battery is used with a large conductor, it must bo 

 charged with strong acid. 



The action of the spiral conductor depends on the induc- 

 tive principle of an electric current discovered by Mr. Fara- 

 day, and is consequently intimately connected with the 

 whole subject of magneto-electricity. 



If a magnet be fitted up in the ordinary manner, with a 

 spool of wire covered with silk around the keeper, the in- 

 tensity of the shock will be astonishingly increased if the 

 current generated in the spool be transmitted through a coil 

 of several hundred feet of fine wire surrounding the legs of 

 the magnet. To produce this effect however it is necessary 

 that the wire on the spool and that around the magnet 

 should at first form a continuous closed circuit, and that 

 this be interrupted at the same instant that the keeper is de- 

 tached, so that the induced current may pass entirely 

 through the body. 



The intense shock may also be given by generating a cur- 

 rent with one magnet, and accelerating it by passing it 

 around a second magnet. 



Professor Emmet, of the University of Virginia, more 

 than two years since, made the interesting discovery that 

 the magneto-electric current is much increased in intensity 

 by passing it through a portion of the generating magnet. 

 This interesting fact, which he has applied with much suc- 

 cess to improve the magneto-electric machine, may un- 

 doubtedly be referred to the same cause as the action of the 

 spiral, and I have succeeded in modifying the application of 

 it in several ways. 



These magnetic experiments were made on the first or 

 second day of May last, while on a visit to Philadelphia, 

 with the large magnet belonging to the museum, and kindly 

 lent to me for the purpose by Mr. Peale. They were made 

 with the assistance of my friend, Mr. Lukens; but as I have 

 not had an opportunity of verifying them, I cannot at pres- 

 ent give a more detailed account. I have also made some 

 preparations for applying the same principle to increase 

 the action of a thermo-electric current. 



