Magnets on Mobile Conductors of Currents. 507 



IV. Systems of Conductors of which the Electric Arc forms 

 a Portion, — In 1821, Davy* observed that the electric arc was 

 affected by a magnet, being apparently drawn towards, or 

 repelled from between, the poles of a magnet, according to 

 the direction of the current and the position of the poles of 

 the magnet. 



In 1844 Casselmannt conducted a research on the influ- 

 ence of the earth's magnetism upon the arc when placed in 

 various positions. When the arc was horizontal, and so placed 

 that the current through it flowed from north to south, the 

 arc was deflected towards the eastern side; when from 



south to north, the arc was deflected towards the west ; 

 west to east, „ „ „ north ; 



east to west, „ „ „ south. 



Later, Walker J found that when the arc is made to pass 

 from a pole of carbon to one of iron, the iron being at the 

 same time powerfully magnetized, the arc rotates round the 

 iron pole, the rotation taking effect in a sense opposed to that 

 assumed by Ampere for the hypothetical molecular currents. 



Quite recently two lamps have been constructed in which 

 this phenomenon is turned to account. M. Fernet has in 

 this way sought to increase the effective illumination obtain- 

 able in a given direction with a pair of carbon poles arranged 

 as usual, one above the other in a regulator, a powerful mag- 

 net being placed behind to compel the arc to maintain a posi- 

 tion in front of the carbon poles between which it is produced. 



M. Jamin § has constructed an electric candle in which the 

 arc is maintained at the extreme end of the two parallel pencils 

 of carbon by the influence of an external conductor of rectan- 

 gular form traversed in a parallel direction by the same current. 



V. Systems of Conductors of which the Luminous Discharge 

 through a Rarefied Medium forms a Part. — Pliicker || first 

 drew attention to the remarkable effects produced by a mag- 

 net on the luminous discharges in the Geissler tubes. De 

 la Rive's U beautiful apparatus for demonstrating the actual 

 rotation of the discharge about the pole of a powerful mag- 

 net is also too well knwon to require further comment. More 

 recently Mr. Crookes** has devised a number of striking ex- 

 periments, which illustrate in greater detail the laws of elec- 

 trodynamics which determine the movements of currents. 



* Phil. Trans. 1821, ii.p. 427. t Pogg. Ann. 1844. 



X Transactions of the Electrical Society, 1837-40. 



§ Journal de Physique, 1879. 



|| Pogg. Ann. ciii. 1856, p. 88 &c 



11 Pogg. Ann. civ. p. 129. ** Proc. Ptoy. Soc. 1879. 



2M2 



