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strip were connected to a small magnetizing helix, while the ends 
of the outer strip were arranged so that the discharge of a half- 
gallon Leyden jar could be passed through the strip. The magneti- 
zation of a needle in the helix indicated an induced current through 
the inner tin-foil ribbon, and the direction of this magnetization 
showed the direction of this current. By means of a second and a 
third cylinder, provided with heliacal tin-foil ribbons, Henry was 
able to show the production of induced currents of the third and 
even of the fourth order. 
While the results in general were quite similar to those obtained 
with the voltaic current, a puzzling difference was observed with 
reference to the direction of the currents of the different orders, as 
shown by the magnetized needles. ‘‘ These currents,’’ he says, 
‘in the experiments with the glass cylinders, instead of exhibiting 
the alternations of the galvanic currents, were all in the same direc- 
tion as the discharge from the jar; or, in other words, they were 
all plus.’’? By suitably varying the 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 
(depending upon the amount of the charge and the character 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,’’ he says, ‘‘and parallel 
wires about ten feet long, the change in direction did not take place 
at a less distance than from twelve to fifteen inches, and, with a still 
larger battery and longer conductors, no change was found, although 
the induction was produced at the distance of several feet.’’ Using 
Dr. Hare’s battery of thirty-two one-gallon jars and a copper wire 
about one-tenth of an inch thick and eighty feet long, stretched 
across the lecture room and back on either side towards the battery, 
a second wire stretched parallel with the former for about thirty- 
five feet and extended to form an independent circuit (its ends con- 
nected with a small magnetizing helix) was tested at varying dis- 
tances, beginning with a few inches until they were twelve feet 
apart; at which distance the induction in this parallel wire, though 
enfeebled, still indicated, by its magnetizing power, a direction 
corresponding with the primary current.* 
Continuing his researches in this direction, Henry presented a 
paper to the American Philosophical Society, in June, 1842, giving 
* Trans. Amer. Philos. Soc., vi (N.S.), Art. ix, p. 303, 1838. 
