EXPLICATION OF ARAGO’S MAGNETIC PHENOMENA. 
159 
magnets. This is perfectly in harmony with the results relative to volta- 
electric and magneto-electric induction described in this paper. And by using 
flat spirals of copper wire, through which electric currents were sent, in place 
of ordinary magnetic poles (111.), sometimes applying a single one to one side 
of the rotating plate, and sometimes two to opposite sides, I obtained the 
induced currents of electricity from the plate itself, and could lead them away 
to, and ascertain their existence by, the galvanometer. 
130. The cause which has now been assigned for the rotation in Arago’s 
experiment, namely, the production of electrical currents, seems abundantly 
sufficient in all cases where the metals, or perhaps even other conductors, are 
concerned ; but with regard to such bodies as glass, resins and, above all, gases, 
it seems impossible that currents of electricity capable of producing these 
effects should be generated in them. Yet Arago found that the effects in 
question were produced by these and by all bodies tried (81.). Messrs. Babbage 
and Herschel, it is true, did not observe them with any substance not me¬ 
tallic, except carbon, in a highly conducting state (82.). Mr. Harris has ascer¬ 
tained their occurrence with wood, marble, freestone and annealed glass, but 
obtained no effect with sulphuric acid and saturated solution of sulphate of 
iron, although these are better conductors of electricity than the former sub¬ 
stances. 
131. Future investigations will no doubt explain these difficulties, and decide 
the point whether the retarding or dragging action spoken of is always simul¬ 
taneous with electric currents *. The existence of the action in metals, only 
whilst the currents exist, i. e. whilst motion is given (82. 88.), and the explica¬ 
tion of the repulsive action observed by M. Arago (82. 125), are the strong 
reasons for referring it to this cause ; but it may be combined with others 
which occasionally act alone. 
132. Copper, iron, tin, zinc, lead, mercury, and all the metals tried, pro¬ 
duced electrical currents when passed between the magnetic poles: the mercury 
was put into a glass tube for the purpose. The dense carbon deposited in 
* Experiments which I have since made convince me that this particular action is always due to the 
electrical currents formed; and they supply a test by which it may be distinguished from the action of 
ordinary magnetism, or any other cause, including those which are mechanical or irregular, producing 
similar effects. 
