556 
Prof. J. Henry’s Contributions 
same direction was obtained from the other. The ribands 
were then separated, by plates of glass, to the distance of 
of an inch ; the current was still in the same direction, or 
"plus. When the distance was increased to about Jth of an 
inch, no induced current could be obtained ; and when they 
were still further separated the current again appeared, but 
was now found to ham a different directioUy or to he minus. 
No other change was observed in the direction of the cur- 
rent ; the intensity of the induction decreased as the ribands 
were separated. The existence and direction of the cur- 
rent, in this experiment, were determined by the polarity of 
the needle in the spiral attached to the ends of one of the 
ribands. 
117. The question at tliis time arose, whether the direction 
of the current, as indicated by the polarity of the needle, was 
the true one, since the magnetizing spiral might itself, in 
some cases, induce an opposite current. To satisfy myself 
on this point, a series of charges, of various intensity and 
quantity, from a single spark of the large conductor to the 
full chai-ge of nine jars, were passed through the small spiral, 
which had been used in all the experiments, but they all gave 
the same polarity. The interior of this spiral is so small, 
that the needle is throughout in contact with the wire. 
118. The fact of a change in the direction of the induced 
current by a change in the distance of the conductors, being 
thus established, a great number and variety of experiments 
were made to determine the other conditions on which the 
change depends. These were sought for in a variation of 
the intensity and quantity of the primary discharge, in the 
length and thickness of the wire, and in the form of the circuit. 
The results were, however, in many cases, anomalous, and 
are not sufficiently definite to be placed in detail before the 
Society. I hope to resume the investigation at another time, 
and will therefore at present briefly state only those general 
facts which appear well established. 
1 ] 9. With a single half-gallon jar, and the conductors se- 
parated to a distance less than inch, the induced 
current is always in the same direction as the primary. But 
when the conductors are gradually separated, there is always 
found a distance at which the current begins to change its 
direction. This distance depends certainly on the amount of 
the discharge, and probably on the intensity; and also on the 
length and thickness of the conductors. With a battery of 
eight half-gallon jars, and parallel wires of about ten feet 
long, the change in the direction did not take place at a less 
distance than from twelve to fifteen inches, and with a still 
