1838] WRITINGS OF JOSEPH HENRY. 183 



discharge is passed through a long wire. Whether if it 

 happen accurately at the same moment they would entirely 

 neutralize each other, or whether they would not still give 

 some definite peculiarity to the discharge, is a matter re- 

 maining to be examined." 



99. The discovery of the fact that the secondary current, 

 which exists but for a moment, could induce another cur- 

 rent of considerable energy, gave some indication that simi- 

 lar effects might be produced by a discharge of ordinary 

 electricity, provided a sufficiently perfect insulation could 

 be obtained. 



100. To test this a hollow glass cylinder. Fig. 11, of about 



e 



Fig. 11. — a glass cylinder, b Leyden jar, c magnetizing spiral. 



six inches in diameter, was prepared with a narrow ribbon 

 of tinfoil, about thirty feet long, pasted spirally around the 

 outside, and a similar ribbon of the same length, pasted on 

 the inside ; so that the corresponding spires of the two were 

 directly opposite each other. The ends of the inner spiral 

 passed out of the cylinder through a glass tube, to prevent 

 all direct communication between the two. When the ends 

 of the inner ribbon were joined by the magnetizing spiral 

 (11), containing a needle, and a discharge from a half gallon 

 jar sent through the outer ribbon, the needle was strongly 

 magnetized in such a manner as to indicate an induced cur- 

 rent through the inner ribbon in the same direction as that of 

 the current of the jar. This experiment was repeated many 

 times, and always with the same result. 



101. When the ends of one of the ribbons were placed 

 very nearly in contact, a small spark was perceived at the 



