ON ELECTRO-DYNAMIC INDUCTION. 331 



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 op- 

 posite current. To satisfy myself on this point a series of charges, of 

 various intensity and quantity, from a single spark of the large conduc- 

 tor 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 dis- 

 charge, 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 esta- 

 blished. 



119. With a single half gallon jar, and the conductors separated to 

 a distance less than ^Vth of an inch, the induced current is always in 

 the same direction as the primary. But when the conductors are gra- 

 dually 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 larger battery and longer conductors, no 

 change was found, although the induction was produced at the dis- 

 tance of several feet. 



120. The facts given in the last paragraph relate to the inductive 

 action of the primary current ; but it appears from the results detailed 

 in paragraphs 110 and 114, that the currents of all the other orders 

 also change the direction of the inductive influence with a change of 



VI. — 4 H 



