250 MEMORIAL OF JOSEPH HENRY. 



rent from the jar.* By moans of a second glass cylinder similarly 

 provided with helical tin-foil ribbons in suitable connections, a ter- 

 tiary current of induction was obtained, analogous to that derived 

 from galvanism. "Also by the addition in the same Avay of a third 

 cylinder, a current of the fourth order was developed." 



Similar as these successive inductions from an electrical discharge 

 were to those previously observed in the case of the galvanic cur- 

 rent, they presented one puzzling difference in the direction of the 

 currents of the diflPerent orders. "These in the experiments with 

 the glass cylinders, instead of exhibiting the alternations of the gal- 

 vanic currents, were all in the same direction as the discharge from 

 the jar, or in other words they were all plus. On substituting for 

 the tinned glass cylinders, well insulated copper coils, "alternations 

 were found the same as in the case of galvanism." The only differ- 

 ence apparently between the two arrangements, v/as that the tin-foil 

 ribbons were separated only by the thin glass of the cylinders, while 

 the copper spiral coils were placed an inch and a half apart. By 

 varied experiments, the direction of the induced currents was found 

 to depend notably on the distance between the conductors; — the 

 induction ceasing at a certain distance, (according to the amount of 

 the charge and the characters of the conductors,) and the direction 

 of the induced current beyond this critical distance being contrary 

 to that of the primary current.* "With a battery of eight half- 

 gallon jars, and parallel wires about ten feet long, the change in the 

 direction did not take place at a less distance than from twelve to 



* About a year later, the distinguished German electrician Peter Riess, appa- 

 rently unaware of Henry's researclics, discovered tlie secondary current induced 

 from mechanical electricity, by a very similar experiment. (Poggeudorff's 

 Annalcn dcr Phijsik viul Chcmie, 1839, No. 5, vol. xlvii.jip. 5;j-7G.) 



tTlic variation in the direction of polarization (without reference to induction 

 currents) appears to liave l)een first noticed by Felix Savaky, some dozen years 

 before. In an important memoir communicated to the Paris Academy of Sciences 

 July 31, lS2f), iSl. Savary announced that "The direction of tlie magnetic polarity of 

 small needles exposed to an electric current directed along a wire stretched longi- 

 tudinally, varies with the distance of the wire:" — the action being found to be 

 l)eriodical with the distance. JI. Savary observed three periods, and also the fact 

 that tlie distances of maximum effect and of the nodal zeros " vary with the length 

 and diameter of tlie wire, and with the intensity of the discharge." He also found 

 that "when a helix is used for mngnctizing, the distance at wliich the needle placed 

 within it is from the conducting wire, is indiflTerent; but the direction and the de- 

 gree of magnetization dei)ends on the intensity of the discharge, and on the ratio 

 between the length and size of the wire." (Brewster's Edinburgh Jour. Set. Oct. 

 J826, vol. V. p. 309.) 



