195 



this was detected by attaching to the ends of the open circuit, a quan- 

 tity of insulated metal, or by connecting one end with the earth. 



The next part of the research relates to a new examination of tho 

 phenomena of the change in the direction of the induced currents 

 with a change of distance, &c These are shown to be due to the 

 fact that the discharge from a jar does not produce a single induced 

 current in one direction, but several successive currents in opposite 

 directions. The effect on the needle is principally produced by two 

 of these : the first is the most powerful, and in the adverse direction 

 to that of the jar ; the second is less powerful, and in the same di- 

 rection with that of the jar. To explain the change of polarity, let 

 us suppose the capacity of the needle to receive magnetism to be 

 represented by ±10, while the power of the first induced current to 

 produce magnetism is represented by — 15, and that of the second by 

 + 12; then the needle will be magnetized to saturation or to — 10, 

 by the first induced current, and immediately afterwards all this mag- 

 netism will be neutralized by the adverse second induction, and a 

 power of +2 will remain; so that the polarity of the needle in this 

 case will indicate an induced current in the same direction as that of 

 the jar. Next, let the conductors be so far separated, or the charge 

 so much diminished, that the power of the first current to develope 

 magnetism may be reduced to — 8, while that of the second current 

 is reduced to -f-6, the magnetic capacity of the needle remaining the 

 same. It is evident, then, that the first current will magnetize the 

 needle to — 8, and that the second current will immediately after- 

 wards neutralize 6 of this ; and consequently the needle will retain a 

 magnetism of — 2, or will indicate an induced current in an opposite 

 direction to that of the jar. 



In extending the researches relative to this part of the investiga- 

 tion, a remarkable result was obtained in regard to the distance at 

 which inductive effects are produced by a very small quantity of elec- 

 tricity ; 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 up- 

 per room, produced an induction sufficiently powerful to magnetize 

 needles in a parallel circuit of wire placed in the cellar beneath, at a 

 distance of thirty feet perpendicular, with two floors and ceilings, 

 each fourteen inches thick, intervening. The author is disposed to 

 adopt the hypothesis of an electrical plenum, and from the foregoing 

 experiment it would appear, that the transfer of a single spark is suf- 

 ficient to disturb perceptibly the electricity of space throughout at 

 least a cube of 400,000 feet of capacity; and, when it is considered 



