on various Phcenomena of Indtiction. 267 



mena, whether unpublished or already known, by endeavour- 

 ing to sketch out this general theory which the science does 

 not possess. 



§ I. Description of the Apparatus. 



3. I constructed a triple helix of large dimensions by roll- 

 ing at one operation, on a great wooden bobbin, three copper 

 wires covered with silk and perfectly annealed. Each of these 

 wires is 23™*6 long and 0™*003 in diameter. They are so ar- 

 ranged that in all the circumvolutions the middle wire pre- 

 serves its position with regard to the two others. It is this 

 which, for brevity, 1 shall designate the induced wire, reserving 

 the name of inductor wires for its two neighbours indifferently. 

 Their diameter is sufficient to ensure that they are never heated 

 by the currents to which they are subjected. An aperture is 

 formed in the bobbin, destined in certain cases to receive a 

 cylinder of soft iron of O*"*!? long by 0°^'05 in diameter. 



4. A small helix was also formed of three copper wires, not 

 more than O'^'OOOS in diameter. Two amongst them are equal, 

 and each of them makes 500 turns; the third, a little thicker, 

 only makes 75 revolutions on the rectangular frame of wood 

 which forms the interior of the apparatus : a parallelopiped of 

 soft iron may be introduced into this frame. This helix differs 

 from the first, in the circumstance that the wires in their su- 

 perposed layers are always in the same direction and do not 

 cross each other. 



5. The measuring instruments which were used are the fol- 

 lowing : — 



a. A multiplying rheometer of 3000 turns, to which an al- 

 most astatic system of very light needles gives extreme sensi- 

 bility J I shall call it the hi/dro-electric rheometer. 



b. A second rheometer, likewise very delicate, and which I 

 shall call the thermo-electric rheometer, because its wire is 

 thicker and only forms 75 revolutions. 



c. A metallic thermometer of Breguet, the helix of which, 

 composed of silver, of gold, and of platinum, presents 46 free 

 spiral turns. It is arranged so that it may be placed in a vol- 

 taic circuit. The instrument appreciates 0°'0714 cent.* 



d. A needle called astatic, the graduated circle of which is 

 Qm.^02 in diameter: metallic wires of different dimensions 

 might in this instrument be stretched in the plane of the mag- 

 netic meridian, and they would thus become parallel to the 

 axis of the needle. 



* For a complete description of this instrument, see Memoires de la So- 

 ciete de Physique et d'Histoire Naturetle de Geneve, vol. ix. p. 123, or Ar- 

 chives de PElectricite, vol. i. p. 77. 



