242 MEMORIAL OF JOSEPH HENRY. 



tion of intensity. From various trials the inference was drawn 

 that the length .required for maximum effect varied with the size of 

 the galvanic element. Thicker wires of the same length produced 

 greater effect, depending in some degree on the size of the battery. 

 A wire of forty feet when coiled into a cylindrical helix "gave a 

 more intense spark than the same wire uncoiled. " A ribbon of 

 sheet copper about an inch wide and twenty-eight feet long, being 

 covered with silk and coiled into a flat spiral like a watch 

 spring (after the plan of Dr. Ritchie) gave a vivid spark with a 

 loud snap. When uncoiled, it produced a much feebler spark. 

 With the insulated copper ribbon folded in its middle, and the 

 double thickness coiled into a flat spiral, there was no spark what- 

 ever, although the same ribbon unrolled gave a feeble spark : thus 

 showing that the induction of the current upon itself was neutral- 

 ized by flowing equally in opposite directions in the double spiral. 

 With a larger copper ribbon one inch and a half wide, and 96 feet 

 long (weighing 15 pounds), spirally coiled, the snap of the spark 

 could be heard in an adjoining room with the door closed. W r ant 

 of material prevented the result being pushed further, so as to 

 ascertain the range of maximum effect with this form of conductor. 

 With increased battery surface, the effect was also increased ; so that 

 with eight elements of his battery arranged as a single pair (of 12 

 square feet) the spark on breaking contact "resembled the discharge 

 of a small Leyden jar highly charged." With the flat spiral, no 

 increase of effect was observable on the introduction of a soft iron 

 core into the axis of the spiral, forming a magnet. With a helical 

 or cylindrical coil about nine inches long, enclosing an iron core, 

 "the spark appeared a little more intense than without the iron." 

 The inference is also drawn " from these experiments, that some of 

 the effects heretofore attributed to magneto-electric action are 

 chiefly due to the reaction on each other of the several spirals of 

 the coil which surround the magnet." 



In these researches it was found that when the two plates of a 

 single pair were placed even fourteen inches apart in an open trough 

 of diluted acid, "although the electrical intensity in this case must 

 have been very low, yet there was but little reduction in the appar- 

 ent intensity of the spark." It was also shown that "the spiral 



