HEXRY AND THE TELEGRAPH. 285 



it may be added here that in the following year, 1831, Henry constructed 

 for the laboratory of Yale College a magnet about one foot high from a 

 three inch octagonal bar of iron thirty inches long, which wrapped with 

 twenty-six strands of copper wire and excited by a battery surface of 

 aJjoutfive square feet, supported 2,300 pounds. Professor Silliman wrote 

 on this occasioD, " He has the honor of having constructed by far the 

 most powerful magnets that have ever been known ; and his last, weigh- 

 ing (armature and all) but 82^ i)Ounds, sustains over a ton. It is eight 

 times more powerful than any magnet hitherto known in Europe."* 

 And Sturgeon (if not the real father, at least the true foster-father, of 

 the electro-magnet), with a generous enthusiasm, remarked : " Professor 

 Henry has been enabled to produce a magnetic force which totally eclipses 

 every other in the whole annals of magnetism ; and no parallel is to be 

 found since the miraculous suspension of the celebrated oriental imx)ostor 

 in his iron coffin."! 



But to return to his investigations of 1830, Henry, after finding that 

 the highest attractive power of the magnet was developed by his novel 

 artifice of multiple coils, proceeded to exjjeriment with the simjjle sj^ool 

 magnet of long continuous single coil; and his researches were rewarded 

 by a new discovery, namely that though the former method of winding 

 the magnet produced tlio strongest attraction, the latter arrangement 

 (under special conditions) permitted the weaker attractive power to be 

 exercised at a far greater distance ; that is through a much greater 

 length of conducting wire. 



Employing his earlier and smaller magnet of 1829, formed of a quar- 

 ter-inch rod, but wound with about 8 feet of copper wire, he tried the ef- 

 fects of different battery powers, of different length of circuits, and of dif- 

 ferent lengths of coil uj^on the magnet. Excited with a single pair, " com- 

 l>osed of a i)iece of zinc plate 4 inches by 7, surrounded with copi)er'' 

 (about 56 square inches otzinc surface), the magnet sustained four and 

 a half pounds. (Experiment 4.) With about 500 feet of insulated copper 

 wire (.045 of an inch in diameter) interposed between the battery and 

 the magnet, its lifting-power was reduced to two ounces; (Experiment 5;) 

 or about 30 times. With double this length of wire (or a little over 1,000 

 feet) interposed, the lifting-power of the magnet was only half an ounce • 

 (Experiment 4;) thus fully confirming the results obtained by Barlow 

 with the galvanometer ; and showing that the same conditions of en- 

 feebled action with increasing length of circuit applied equally to the 

 magnet. With a small galvanic pair 2 inches square, acting through 

 the same length of wire, (over 1,000 feet,) '• the magnetism was scarcely 

 observable in the horse-shoe." (Experiment 3.) 



Employing next a trough battery of 25 pairs, having the same zinc 

 surface as previously, the magnet in direct connection, (which before had 

 supported four and a half pounds,) now lifted but seven ounces; not 



*Sillimaii's Am. Jour. Sci. April, 1831, vol. xx, j). 201. 

 \PliilosopMcal Magazine; ajid Annals, March, 1832, vol. xi, p. 199. 



