220 MEMORTAT. OF JOSEPH HENRY. 



■with a plate of zinc 12 inches long and 6 wide, and surrounded by- 

 copper, ^yas substituted for the galvanic element used in the former 

 experiments; the weight lifted in this case was 750 pounds."* In 

 illu^^tration of the feeble power of the magnetic poles when exerted 

 sej^arately, it was found that with precisely the same arrangements 

 giving a holding power of 750 pounds to the double contact arma- 

 ture, either pole alone was capable of sustaining only 5 or 6 

 pounds; "and in this case we never succeeded in making it lift the 

 armature — weighing 7 pounds. We have never seen the circum- 

 stance noticed of so great a difference between a single pole and 

 both." 



Henry^s "Quantiti/'' Magnet compared with MoWs. — About the 

 same time that Henry was developing this wonderful jjower in the 

 electro-magnet, Dr. Gerard INIoll, Professor of Natural Philosophy 

 in the University of Utrecht, was engaged in a similar research. 

 In a paper published in the latter part of 1830, he states that his 

 attention was drawn to the electro-magnet of Sturgeon in 1828, 

 during a visit to London.f "This apparatus I saw in 1828 at Mr. 

 Watkins's, curator of philosophical apparatus to the London 

 University; and the horse-shoe with which he performed the experi- 

 ment, became capable all at once of sujjporting about nine pounds. | 

 I immediately determined to try the effect of a larger galvanic 

 apparatus on a bent iron cylindrical wire, and I obtained results 

 which appear astonishing, and are — as far as the intensity of mag- 

 netic force is concerned, altogether new. I have anxiously looked 

 since that time into different scientific continental and English jour- 

 nals, without finding any further attempt to extend and improve 

 Mr. Sturgeon's original experiment." Moll's first magnet, a 

 horse-shoe formed of a round bar of iron about one inch thick, was 

 about eight and one-half inches in height, and had a wrapped cop- 

 per wire of about one-eighth inch diameter coiled eighty-three 

 times around it. The weight of the horse-shoe and wire was about 



♦SiHiman's Am. Journal of Science, Jan. 1831, vol. xix. pp. 404, 405. 

 iliibliothf'que Univer.ielle des Sciences, etc. Sept. 1830, vol. xlv. i)p. 19-35. Also Edin- 

 burgh Journal of Science, Oct. 18.30. 



J [At the date referred to, Henry had already exhibited before the Albany Insti- 

 tute, a much more powerful magnet.] 



