138 WRITINGS OF JOSEPH HENRY. [1838 



were still further separated the current again appeared, but 

 was now found to have a different direction, or to be minus. No 

 other change was observed in the direction of the current; 

 the intensity of the induction decreased as the ribbons were 

 separated. The existence and direction of the current, in 

 this experiment, were determined by the polarity of the 

 needle in the spiral attached to the ends of one of the ribbons. 



117. The question at this time arose, whether the direc- 

 tion 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 cir- 

 cuit. 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 

 separated to a distance less than gV^^^ of an inch, the in- 

 duced 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 



