324 Prof. J. W. Mallet 07i the Weir/ht of a Wire 2)lciced 



were then made with a voltaic instrument ; but I was sur- 

 prised to find that the wire seemed to become lighter in both 

 directions, though not so much Avhen its motion was towards 

 the south as towards the north. On further trial it was found 

 to ascend on the contacts being made, whatever its position to 

 the magnetic meridian ; and 1 soon ascertained that it did not 

 depend on the earth's magnetism^ nor on any local magnetic 

 action of the conductors^ or surrounding bodies, on the wire. 

 After some examination I discovered the cause of this unex- 

 pected phenomenon. An amalgamated piece of the thin cop- 

 per wire was dipped into clean mercury having a stratum of 

 water or dilute acid over it ; this, however, was not necessary, 

 but it preserved the mercury clean and the wire cool. In this 

 position the cohesive attraction of the mercury raised a little 

 elevation of the metal round the wire of a certain magnitude, 

 w^hich tended to depress the wire by adding to its weight. 

 When the mercury and the wire were connected with the 

 poles of the voltaic apparatus, this elevation visibly diminished 

 in magnitude by an apparent alteration in the cohesive attrac- 

 tion of the mercury, and a part of the force which before 

 tended to depress the wire was thus removed. This alteration 

 took place equally, whatever the direction in which the current 

 was passing through the wire and the mercury ; and the effect 



ceased the moment the connexions were broken 



Whether the effect is an actual diminution of the attraction 

 of the particles of the mercury, or depends on some other 

 cause, remains as yet to be determined. But, in any case, its 

 influence is so powerful that it must always be estimated in 

 experiments made to determine the force and direction of an 

 electromagnetic wire acted on by a magnetic pole, if the 

 direction is otherwise than horizontal, and if they are observed 

 in the way described in this note. Thus, at the magnetic 

 equator, for instance, where the apparent alteration of weight 

 in an electromagnetic wire may be expected to be greatest, 

 the diminution of weight in its attemj^t to ascend would be 

 increased by this effect, and the apparently increased gravity 

 produced by its attempt to descend vfould be diminished or, 

 perhaps, entirely counteracted." 



The results of these experiments of Faraday upon the earth's 

 action on a movable horizontal current have been transferred 

 to all the text-books ; and, old as they now are, I know of no 

 essential addition to thern, though the mode of theoretically 

 viewing the relation in question has varied a little in different 

 hands. It appears that he did not succeed in directly demon- 

 strating the vertical component of the attraction or repulsion, 

 due to the earth, in the shape of apparent change of Aveight 



