-1830] 



WRITINGS OF JOSEPH HENRY. 



represents a coil of only two turns of wire': on the left side 

 of the figure the plates are soldered directly to the ends of 

 the wire of the coil ; on the right, the plate of zinc Z, is 

 attached to the part of the wire ending with copper on the 

 other side, while the plate of copper on the right corresponds 

 to the zinc on the left. By this arrangement, we can in- 

 stantly reverse the direction of the currents, and deflect the 

 needle either to the right or left, by merely holding a tumbler 

 of acidulated water so as to immerse one or the other of the 

 double plates into the fluid. The arrows at B, formed of 

 two pieces of card, are intended to show the direction of the 

 currents, and they should point in the course of the wires 

 going from the copper. N S, is the needle, about nine and 

 a half inches long, made by binding together several watch 

 springs, touched separatel3^ so as to form a compound mag- 

 net; at the ends are two balls of pith, to show the movement 

 of the needle more plainly. This instrument is complete in 

 itself, and we receive the full effect of the instantaneous im- 

 mersion of the galvanic element. 



Fig. 3, represents a modification of 

 De la Rive's ring on a large scale. A B, 

 is a coil about nine inches by six, with 

 a small cylinder of copper, enclosing 

 another of zinc, without bottoms, sol- 

 dered to its extremities, which end at c, 

 the whole being suspended by a fibre of 

 raw silk, so as to swing freely in a cup 

 of acidulated water. When this appa- 

 ratus is made sufficiently light, it inva- 

 riably places itself, after a few oscilla- 

 tions, at right angles to the magnetic 

 meridian. W and E, are two pieces of 

 card, with letterson them, to show which 

 side of the coil will turn to the east or 

 west : they may be properly placed by 

 recollecting that the current from the 

 copper to the zinc has a tendency to circulate in a direction 

 contrary to that of the sun. 



E] ^ 



^SEQSS 



"^ 



