1842] WRITINGS OF JOSEPH HENRY. 203 



In extending the researches relative to this part of the 

 investigations, a remarkable result was obtained in regard 

 to the distance at which inductive effects are produced by a 

 very small quantity of electricity; 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 upper room, 

 produced an induction sufficiently powerful to magnetize 

 needles in a parallel circuit of wire placed in the cellar 

 beneath, at a perpendicular distance of thirty feet 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 sufficient to disturb per- 

 ceptibly the electricity of space throughout at least a cube of 

 400,000 feet of capact}^; and when it is considered that the 

 magnetism of the needle is the result of the difference of two 

 actions, it may be further inferred that the diffusion of 

 motion in this case is almost comparable with that of a spark 

 from a flint and steel in the case of light. 



The author next alludes to a proposition which he 

 advanced in the second number of his Contributions, namely, 

 that the phenomena of dynamic induction may be referred 

 to the known electrical laws, as given by the common theories 

 of electricity ; and he gives a number of experiments to illus- 

 trate the connection between statical and dynamical induc- 

 tion. 



The last part of the series of experiments relates to induced 

 currents from atmospheric electricity. By a very simple 

 arrangement, needles are strongly magnetized in the author's 

 study, even when the flash is at the distance of seven or eight 

 miles, and when the thunder is scarcely audible. On this 

 principle, he proposes a simple self-registering electrometer, 

 connected with an elevated exploring rod. 



