138 EULOGY ON AMPERE. 



west; the deviation would be toward the east when, the conditions 

 being the same, the wire is helow. It is necessary to remark here that 

 the wire preserves absobitely none of that deviating power the moment 

 it ceases to be a couducting 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 rota- 

 tion. 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 exiseriment 

 of M. Oersted is directly opposed so 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 ])lanets around 

 the sun. Thus a physical theory which had been abandoned for more 

 than two centuries was recalled by the discovery of CErsted. 



AVe have already mentioned the important remark of the celebrated 

 Danish physicist, that the deviations of the needle of a horizontal 

 comxiass 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 wu'e. Feeble batteries, on the contrary, produce only 

 scarcely sensible movements. What is the part played by that myste- 

 rious power, seeming to reside in the arctic regions of the globe, to at- 

 tract magnetic bodies in a certain way, and repel others f What part 

 does it pei^form in lessening the deviations when the battery has little 

 j)0wer ? 



Ampere 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 experi- 

 ment of CErsted ; but how get rid of the attraction of the earth ; how 



