472 ELECTRO-MAGNETIC EXPERIMENTS AND OBSERVATIONS. 



inches long by 9 wide, which has a short pipe of the same me- 

 tal soldered to its centre, for supporting one end of the con- 

 necting wires, while the other is inserted in a similar pipe sol- 

 dered to the copper-tray, as represented at C. When this ap- 

 paratus was used, nothing more was necessary than to separate 

 the zinc and copper by a few slips of window-glass, or by thick 

 paper, and then pour the acid into the tray. The effect of 

 changing the relative positions of the metallic plates, as in the 

 former experiments, could be instantly produced, by moving 

 one end of the connecting wire to either side of the copper- 

 tray. On this account, it is the most convenient form of the 

 apparatus, and is sufficiently powerful to give sparks, and to 

 magnetise small sewing needles, if previously softened by 

 heat. 



With this apparatus all our former experiments were repeat- 

 ed, and the following are the general results which were ob- 

 tained. 



The effect of the simple and conducting bent wires differed 

 only in energy from what was before observed. The deflec- 

 tions, when the acid was fresh, was — 90° to either hand; and 

 in those positions where there was no deflection, there was 

 evidently very strong electro-magnetic action. 



When helices were employed, we found that the needle in- 

 troduced invariably arranged itself parallel to the axis of the 

 helix, whatever might be its direction. This law was striking- 

 ly illustrated by the combination represented in Fig. 8. Here 

 a very long left helix was employed, portions of which were 

 bent toward the four cardinal points ; and on successively in- 

 troducins the needle into each, it assumed the direction of the 

 axis of that portion of the helix ; but in such a manner, that 

 the north pole of the needle is always directed, so as to meet 



the 



