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TRANSACTIONS OF THE SECTIONS. 17 
goes variations of intensity as soon as there is a relative displacement of the parts 
of the apparatus which attract or repel each other under the influence of the elec- 
tricity. This phenomenon may be explained by the production of currents of induc- 
tion in the conductor itself, in which the principal current is propagated. M. Jacobi 
has been the first to insist upon this point in different memoirs. The simplicity of 
the formule at which he arrived has been contested by M. Marié Davy (Comptes 
Rendus de |’Acad. des Sciences, 1855). I was of course obliged to attend to this 
subject, and I shall give a brief summary of the results of this investigation, results 
which are for the most part well known or easy to foresee, but which have not per- 
haps been exhibited in a sufficiently general manner, and of which all the conse- 
quences have not been examined. 
Part J.—The phenomena which are here in question may be summed up in the fol- 
lowing manner:— When an electric current causes the attraction of two pieces of an appa- 
ratus, if these two pieces, yielding to this attraction, are set in motion, a diminution of the 
intensity of the current is observed ; and inversely, when these two pieces which mutu- 
ally attract each other are compelled to move in a direction opposed to that which the 
attraction tends to give them, we observe an augmentation of the intensity of the current 
whilst this movement is being effected. This law appears to flow from the principle 
of Lery upon induction. I have verified it experimentally in four principal cases; 
in which a movement may be produced by the action of the current. I have also 
examined two other cases which do not. answer directly to the enunciation of the 
law. 
’ Most frequently these variations of intensity cannot be proved by the direct 
Measurement of the current by means of a galvanometer, for they are too small in 
proportion to the total intensity ; it is necessary to have recourse to a more delicate 
method, such as that employed by Wheatstone, Svanberg, &c., which has been 
sometimes designated by the name of the method of the galvanic bridge; at other 
times, especially when permanent magnets entered into the composition of the 
apparatus, it was necessary to be satisfied with proving the presence of a current 
of induction in a conducting wire suitably arranged. 
The four principal cases which I have investigated are the following :— 
' 1. Attraction of a piece of soft iron by an electro-dynamic coil ; each time that 
the piece of soft iron penetrates into the interior of the coil, or emerges from it, a 
diminution or an augmentation of the current is observed. 
2. Attraction of a piece of soft iron by an electro-magnet. The law is easily 
verified, even without the employment of the galvanic bridge, if a strong electro- 
magnet be used. 
3. Mutual attraction of two coils, of which one can penetrate into the interior of 
the other. The variations of intensity are very slight. 
4. Rotation of magnets by currents, and of currents by magnets. The intensity 
_ of the current is weaker when the apparatus is in motion under the influence of the 
current ; it is stronger when it is forced to take a rotatory movement opposed to 
that which it would naturally take. These variations of intensity, which are very 
slight, arise from currents of awial induction, according to the denomination of 
M. Matteucci. 
_ The secondary cases, which I have also studied, are,— 
5. Attraction of an armature of soft iron by a permanent magnet. If the per- 
manent magnet be surrounded by a coil, of which the extremities are in communi- 
cation with a galvanometer, the latter shows, at the moment when the armature is 
attracted, a current of which the direction is the same as that which would be pro- 
duced by a momentary diminution of the magnetism of the magnet. When the 
armature is pulled away, the current is in the opposite direction, as if there were an 
augmentation of the magnetism. These facts may be considered as confirming the 
law enounced. 
_ 6. Magnetism of rotation. It is well known, that when a sphere of copper is 
made to turn rapidly between the poles of an electro-magnet, currents of induction 
are developed in the sphere which present a considerable resistance to the movement 
of rotation, The motion of the sphere of copper is always contrary to that which 
_ the forces emanating from the electro-magnet tend to impress upon it; but these’ 
_ only originate when the movement takes place. If the law were still applicable in 
: 1857. 
: 
