322 Prof. J. W. Mallet on the Weight of a Wire placed 



currents, which pass also from east to west on the upper sur- 

 face of the earth beneath. 



In one respect, however, the analogy bet\yeen the earth 

 and a large magnet seems at first sight to fail. If into the 

 neighbourhood of an ordinary magnet we bring a second 

 magnetic bar, or instead of it a spiral wire traversed by an 

 electric current, not only have we directive action, tending to 

 render the axes of the bars (or of bar and helix) parallel, but 

 there is also attraction^ tending to bring the two acting masses 

 together, whence we might expect to find that a magnet, or 

 spiral conductor conveying a current, would also be specially 

 attracted by our earth after taking up the naturally assumed 

 north and south position — or, in other words, that the weight 

 of such mass would, under these circumstances, seem to be 

 increased. Conversely, if two magnetic bars be placed 

 parallel to each other with similar poles adjacent, we have 

 rejndsion, the currents of Ampere on the nearer sides of the 

 bars being in opposite directions ; and so we might look for 

 repulsion by the earth, or an apparent decrease of weight, on 

 the part of an ordinary magnet placed parallel to the earth's 

 magnetic axis, but with reversed poles, or on the part of a 

 helix of wire similarly placed but traversed by a current 

 passing from west to east on the lower side of the spiral. 'No 

 such appearance of change of weight occurs. 



This, however, may be readily seen to arise from the neu- 

 tralizing effect of the action on one side of the magaet or 

 helix upon that exerted on the other, the difference of distance 

 of the two from the earth as a whole being practically nothing. 

 But if one side could be detached from the other, and action 

 upon the one alone observed, attraction and repulsion ought 

 to be producible. In the case of a magnetic bar this cannot 

 be accomplished, but with a helix it can, one portion of the 

 wire being rendered movable in relation to the rest, while 

 preserving continuity of the electric current. The helix may, 

 of course, for simplicity's sake, be reduced, as in Ampere's 

 well-known experiments, to a single circle, and this to a 

 square or rectangle the sides of which represent the vertical 

 and horizontal components of the current. A straight hori- 

 zontal wire, then, placed east and west and traversed by a 

 current from east to west, should be attracted by the earth — 

 and if the direction of the current vrere reversed, should be 

 repelled. 



This result was clearly foreseen by Faraday in the course 

 of his early researches on electromagnetic rotation. At the 

 close of his paper of October 1821 *, he says : — " Theoretically, 

 * Quarterly Journal of Science, xii. 74. 



