270 BULLETIN or THE 



capacity of the needle at a giveu position, the return wave might 

 be just sufiQcient to completely reverse its polarity, and the 

 diminished succeeding wave insufficient to restore it to its former 

 condition; while at a greater distance, the primitive wave might 

 be so far reduced as to just magnetize the needle fully, and the 

 second wave, being still more enfeebled, would only partially de- 

 magnetize it, leaving still a portion of ihe original polarity ; and 

 so for the following diminished oscillations. 



In the course of these extended researches the presence of in- 

 ductive action was traced to most surprising and unimagined 

 distances. "A single spark from the prime conductor of the 

 machine, of about an inch long, thrown on the end of a circuit of 

 wire in an upper room, produced an induction sufficiently power- 

 ful to magnetize needles in a parallel circuit of wire placed in the 

 cellar beneath, at a perpendicular distance of thirty feet, with 

 two floors aud ceilings — each fourteen inches thick intervening." 



"The last part of the series of experiments relates to induced 

 currents from atmospheric electricity. By a very simple arrange- 

 ment, needles are strongly magnetized in the author's study, even 

 when the flash is at the distance of seven or eight miles, and 

 when the thunder is scarcely audible. On this principle he pro- 

 poses a simple self registering electrometer, connected with an 

 elevated exploring rod." For obtaining the results above alluded 

 to, a thick wire was soldered to the edge of the tin roof of his 

 dwelling and passed into his study through a hole in the window 

 frame ; while a similar wire passing out to the ground, terminated 

 in connection with a metal plate in a deep well close by. Between 

 the wire ends within his study, various apparatus, including mag- 

 netizing helices of different sizes and characters could be attached, 

 so as to be within the line of conduction from the roof to the 

 ground. The inductions from atmospheric discharges were found 

 to have the oscillatory character observed with the Leyden jar; 

 and by interposing several magnetizing helices wn'th few and with 

 many convolutions, he was able to get from a needle in the former 

 the polarity due to the direct current, and in the latter, that due 

 to the return current; thus catching the lightning (as it were) 

 upon the rebound. 



In examining the "lateral discharge" from a lightning-rod in 

 good connection with the earth, he had often observed that while 

 a spark could be obtained sufficiently strong to be distinctly felt, 

 it scarcely affected in the slightest degree a delicate gold-leaf 

 electroscope. How explain so incongruous a phenomenon ? 

 Henry detected the very simple solution, by a reference to the 

 self-induction of the rod, — a negative wave passing followed im- 

 mediately by a positive wave so rapidly as to completely neutralize 

 the effect upon the electroscope before the inertia of the gold leaf 

 could be overcome, while actually producing a double sj^ark (sen- 

 sibly co-incident) to aud from the recipient. 



44 



