}0 THE POLAR WIRES ACT AS CENTRES OF ATTRACTION. 



itive, and n negative, the extremity a of the suspended needle would be negative, aud 

 the extremity c positive by induction. The conjoined effort of the forces thus brought 

 to bear on the needle, acting on its opposite extremities in opposite directions, would 

 solicit it to move on its axis, the extremity a in the direction n' (fig. 7), and the ex- 

 tremity c in the direction p> ; the line of rest would be as expressed by the dots in the 

 figure, and slow oscillations should take place on either side of that line if the density 

 or other properties of the medium permitted. 



26. The experiment was thus tried, and, to prevent any derangement from hygro- 

 mctric twist of the silk, the needle was hung on a glass thread, of sufficient length to 

 reach above the surface of the water, and there attached to the silk ; on passing the 

 current of forty-five pairs of four inch plates, the needle immediately moved, and after 

 two or three oscillations, took its position of rest ; on being moved to the opposite side 

 of the polar wire, an opposite motion ensued until the same position was gained. Du- 

 ring this movement, gas was freely liberated from the extremities of the polar wires, 

 and also from both ends of the needle, it hindered considerably that freedom of mo- 

 tion which I had hoped for in observing the oscillations. The experiment was also 

 varied by terminating the polar wires with plates of platina, with a view of increasing 

 the effect ; the needle was also suspended in pyroligneous ether, and the attractive pow- 

 er of the same battery, newly charged, was very marked ; it was not so observable in 

 alcohol, and still less in muriatic acid ; in ammonia, though only one end of the needle 

 appeared to evolve gas, it was not so obedient to the attractive force. These cir- 

 cumstances indicate that the phenomena of motion, as here exhibited, have not their 

 origin in any magnetic action produced either by the disturbance of the earth or the 

 passage of the voltaic currents. Magnetic action, to be complete, requires that the bod- 

 ies along which currents are passing should be possessed of high conducting power ; 

 hence a thermal current, whose tension is almost extinct, is still capable of producing a 

 powerful effect on a suspended needle. A current capable of producing a given devia- 

 tion when moving along metallic wires, would meet with resistance in passing through 

 water ; and alcohol or ether would forbid its passage. It is, moreover, impossible to 

 produce any visible effect on the platina wire of this arrangement by the action of a 

 single pair, even possessing extensive surface, though the same pair, if cut into lesser 

 plates, and arranged for the production of a current of greater tension, immediately 

 causes the movement here described. Dr. Faraday has stated, in his recent researches 

 on this point, that there is not any proof that the poles of a battery do exercise any 

 power of attraction or repulsion (Baches Turner's Chem., p. 102 ; idem, 108) ; but that 

 opinion would appear to be inconsistent with the fac.t there must be an accumulation 

 of tension on an electrode, if the medium which separates it from its fellow is not so 

 good a conductor as the liquid filling the cells of the battery ; and experiment warrants 

 this conclusion. 



27. The principles here laid down also indicate the construction of a galvanometer 

 which I have recently fitted up. It is intended to exhibit, by the torsion of a fine fibre, 

 the force of attraction between the polar wires and the ends of the suspended needle. 

 The obstacle I have observed to the accuracy of the results furnished by it, is due to the 

 development of gas on the polar wires and on the needle. 



