136 DOVE ON THE ELECTRICITY OF INDUCTION. 



The phcenomena essentially differ from these when the pri- 

 mary current is that produced by the discharge of a Leyden jar. 



IV. Currents induced by iron which ivas magnetized by the 

 discharge of an electrical battery. 



43. If a battery*, to which a constant charge has been com- 

 municated by means of a unit jar, is discharged through the 

 inner spirals of the differential inductor for frictional electricity 

 (described § 8, fig. 3), the shock of the induced current in the same 

 direction with the primary current is obtained from the secon- 

 dary spirals, which are connected together in the same direction. 

 This has been shown, independently of each other, both by 

 Henry f and by RiessJ. 



This shock is modified when metallic substances are placed in 

 the previously empty tubes. Whether the change which then 

 occurs is due to an increase or deci'ease of power it is difficult to 

 determine, when the modification is but slight, and other methods 

 of testing it are requisite in order to arrive at a safe conclusion. 

 When the secondary spirals are connected crossways, an equili- 

 brium of the currents is estabhshed for all the methods of test- 

 ing which are here applicable, and this is immediately destroyed 

 by the insertion of a metal into one of the compensating spirals. 

 But the current which then appears does not deflect the needle 

 of the galvanometer ; for, when the revolutions of the wire which 

 are wound upon glass are insulated in the most careful manner 

 from each other, hj varnishing the wire already covered with 

 silk, sparks will constantlj' pass from one coil to the other; 

 the current tested with iodide of potassium exhibits no chemical 

 decomposing power, nor does it magnetize soft iron in such a 

 manner, that a magnetic needle placed by the side of it is de- 

 flected, or that iron filings stand on end when sprinkled over it. 

 For ascertaining the direction of the current, therefore, there 

 remains no other means than the process proposed by M. Riess, 

 by which resinous figures ai"e obtained^, or that by means of the 



* Different batteries were emploj'ed in the experiments, some consisting of 

 smaller, some of larger jars : the number of the jars was also changed, but not 

 in the same series of experiments. The results obtained from these different 

 batteries were all in unison. 



t Transactions of the American Pliilosophical Society, vol.vi.p. 17. 



*; Poggendorff's Jiuialen, 1. p. 1. § lOiil. 11. p. o53. 



