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Documents et études > ARAGO, EULOGY ON AMPÈRE., 1872.
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west; the deviation would be toward the east when, the conditions being the same, the wire is below. 
It is necessary to remark here that the wire preserves absolutely none of that deviating power the 
moment it ceases to be a conducting wire, or to join the two poles of the battery. It would indicate 
a total want of scientific perception not to understand how extraordinary and important are the 
results I have just announced; not to observe with surprise an imponderable fluid imparting for the 
moment to the slender wire along which it passes, properties so powerful.  
These properties, studied in their specific characters, are not less wonderful.
Even a child knows it would be useless to try to turn a horizontal lever around a pivot on which 
its center rests by pushing or pulling it lengthways – I mean, following the line leading to the 
center of rotation. The force must necessarily be transverse. The perpendicular to the length of the 
lever is, no matter in what direction, that which requires the least force to create a given 
movement. The experiment of M. Oersted is directly opposed to these elementary rules of mechanics. 
Please then to remember, when the forces developed by the passage of the electrical current in each 
point of the conducting wire are found to correspond vertically with the axis of the needle itself, 
either above or below, the deviation is at its maximum. The needle remains at rest, on the contrary, 
when the wire is presented to it in a direction nearly perpendicular. 
Such is the strangeness of these facts that, in order to explain them, various physicists have had 
recourse to a continued flow of electrical matter circulating round the conducting wire at right 
angles to it, and producing the deviations of the needle by way of impulse. This was nothing less, 
on a small scale, than the famous vortices contrived by Descartes to account for the general 
movement of the planets around the sun. Thus a physical theory which had been abandoned for more 
than two centuries was recalled by the discovery of Oersted. 
We have already mentioned the important remark of the celebrated Danish physicist, that the 
deviations of the needle of a horizontal compass approach nearer and nearer 90 degrees in proportion 
to the increase of the power of the battery during the connection of the two poles by the wire. 
Feeble batteries, on the contrary, produce only scarcely sensible movements. What is the part played 
by that mysterious power, seeming to reside in the arctic regions of the globe, to attract magnetic 
bodies in a certain way, and repel others? What part does it perform in lessening the deviations 
when the battery has little power? 
Ampère perceived the importance of this question at the very first glance; he saw it was not a 
mere nice and subtle refinement without bearing; he understood that the solution of the problem 
would stamp with characteristic features the forces brought into play by the experiment of Oersted; 
but how get rid of the attraction of the earth; how 
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