480 MEMORIAL OF JOSEPH HENRY. 



While Ampere, in 1820, was pursuing his -researches, Sehweigger, 

 af Halle, invented his galvanometer. This he formed by wrapping 

 an insulated wire in several turns and layers around a suspended 

 magnetic needle. This instrument excited a powerful influence in 

 electrical researches, and the contemplation of its action led Henry 

 to make his first trials as an original experimenter. 



The history of another research is now in order as bearing directly 

 on one of Henry's investigations — and one which he ever regarded 

 with considerable pride. In 1827 Savary began experiments on 

 the magnetizing actions of the discharge of the Leyden jar on steel 

 needles. These needles, of various lengths, diameters, and degrees 

 of hardness, were placed at right angles to the wire conveying the 

 electric discharge. They were also put in the interior of Ampere's 

 helices, after the manner of Arago's original experiments. The 

 phenomena thus observed were found to be of the most complex 

 characters. It was found that the direction of the polarity in the 

 needle and the intensity of its magnetization depended on its distance 

 from the wire, on the diameter of the needle, on the potentiality of 

 the discharge, and on the resistance of the wire through which the 

 discharge took place. Similar phenomena were observed when the 

 needles were placed in one of Ampere's helices, through which the 

 discharge was thrown. After a long and tedious research Savary 

 concluded that these facts could only be explained by the supposition 

 that the discharge of a Leyden jar was not continuous, but consisted 

 of a series of rebounds or reflections to and from the two coatings 

 of the jar. In 1842, Henry, apparently ignorant of this research 

 of Savary, went over the same ground, and arrived independently 

 at the same inference which Savary had formed fifteen years before — 

 an inference directly confirmed by the experiments of Feddersen, 

 who, in 1862, got the life history of the electric spark of the Leyden 

 jar by photographing its image reflected from a concave mirror 

 revolving 800 to 1,000 times in a second. 



Two years previous to Savary's work, i. e. in 1825, William 

 Sturgeon, of Woolwich, England, improved on Arago's experiment 

 of magnetizing steel and iron with the voltaic current. Sturgeon's 

 improvement consisted in bending the straight rods used by Arago 

 into U-shaped pieces, and then, coating them with shellac varnish, 



