SUDDEN REVERSAL OF CURRENTS. 



On gradually lifting the iron bar, s, to the 

 sloping position of the dip 

 of a dipping-needle, the 

 circulating currents about 

 the iron bar, induced by 

 the terrestrial currents, 

 are suddenly intensified 

 by the percussion of the 

 hammer, and turn the 

 compass needle rapidly 

 to the position indicated 

 by the dotted lines. The 

 end of the needle, which 

 was previously attracted 

 toward the piece of iron, 

 is suddenly repelled for- 

 cibly. 



This phenomenon 

 shows an analogy to the sudden reaction devel- 

 oped between the particles of percussion powder, 

 commonly used for firing gunpowder. The pres- 

 ence of molecules of nitrogen, combined with 

 molecules of mercury, silver, potash, glycerine, 

 cotton, &c., reduces the compound to an unstable 

 condition of circulating molecular currents, corre- 

 sponding with those about the iron in the experi- 

 ment described. The blow of the hammer of a 

 percussion lock suddenly determines the reversal 

 of the molecular currents, with the result of a 

 violent repulsion between them, and explosive 



fig. 34- 



