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 ^^^th 

 of an inch ; the current was still in the same direction, or 

 plus. When the distance was increased to about |^th of an 

 inch, no induced current could be obtained ; and when they 

 were still further separated the current again appeared, but 

 was nowjbimd to liave a different direction, 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 this 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 charge 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. 



119. With a single half-gallon jar, and the conductors se- 

 parated to a distance less than ^V^h of an 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 



