250 MEMORIAL OF JOSEPH HENRY. 



rent from the jar.* By means 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 thai derived 

 from galvanism. "Also by the addition in the same way 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, tiny presented one puzzling difference in the directi f the 



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

 the class 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 tiny were all 'plus. < )n substituting for 

 the 1 tinned glass cylinders, well insulated copper coils, "alternation- 

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

 ence apparently between the two arrangements, was 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 

 tin' 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 ;i year later, the distinguished German electrician Peter Riess, appa- 

 rently unawai f Henry's researches, discovered the s ndary current induced 



from mechanical electricity, by :i very similar experiment. Poggendorff's 

 Annalcn <!• r Physik inn I Chcmic, 1839, No. 5, vol. xlvii. pp. 55 76.) 



f The variation in the direction of polarization (without reference to Induction 

 currents) appears t<> have been first noticed by Felix Savary, some dozen years 

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

 July 31, 1826, M. Savarj announced that "The direction of the magnetic polarity "i 

 small needles '\|n>s«-<l t-> an electric current directed alongawire stretched longi- 

 tudinally, varies with tin' ilistai of the wire:"— the action being found to be 



periodical with the distance. M. Savary observed three periods, ami also tin' fact 

 that the distances of max i mum effect ami of the nodal zeros "vary with the length 

 ami diameter of tin' wire, ami with tin- intensity of the discharge." 11'- also found 

 that "when a helix is used for magnetizing, the distance at which the needle placed 

 within it is from the conducting wire, is indifferent; hut the direction ami tin- de- 

 gree of magnetization depends on the intensity of the discharge, and on the ratio 

 betweeD the length ami size of the wire." (Brewster's Edinburgh Jour. Sci. Oct. 

 1820, vol. v. p. 369.) 



