70 Royal Society. 



as defined as, and exactly equal in amount to, those outside of it ; 

 that these are continuations of the former ; and that every line of 

 magnetic force, whatever distance it may extend to from a magnet, 

 (and in principle that is infinite,) is a closed curve, which in some 

 part of its course passes through the magnet in conformity with 

 what is called its polarity. 



A current being thus induced in a closed wire, when it travels 

 across magnetic lines of force, an inquiry is next made into the 

 effect of altering the mass or diameter of the wire, and another form 

 of apparatus is employed, in which loops of wire are made to inter- 

 sect a given amount of lines ; each loop consisting of a given length 

 of wire, but either of wires of different diameter, or of one or more 

 wires of the same diameter. The conclusion arrived at is, that the 

 current or amount of electricity evolved is not simply as the space 

 occupied by the breadth of the wire correspondent to the direction 

 of the line of force, which has relation to the polarity of the power ; 

 nor by that width or dimension of it which includes the number or 

 amount of lines of force intersected, and, which corresponding to 

 the direction of the motion has relation to the equatorial condition 

 of the lines ; but is jointly as the two, or as the mass of the wire. 



The moving wire was next surrounded by different media, as air, al- 

 cohol, water, oil of turpentine, &c, but the result was the same in all. 



Wires of different metals were used, and results in accordance 

 with those obtained and described in the Second Series of these 

 Researches were obtained : the conclusion is, that the current excited 

 appears to be directly as the conducting power of the substance 

 employed. It has no particular reference to the magnetic character 

 of the body; for iron' comes between tin and platinum, presenting no 

 other distinction than that due to conducting power, and differing 

 far less from these metals than they do from metals not magnetic. 



Magnetic polarity then comes under consideration. The author 

 understands by this phrase, the opposite and antithetical actions 

 which are manifest at the opposite ends, or the opposite sides, of a 

 limited portion of a line of force. He is of opinion that these 

 qualities, or conditions, are not shown with certainty in every case, 

 by attractions and repulsions ; thus a solution of sulphate of iron 

 will be attracted by a magnetic pole if surrounded by a solution 

 weaker than itself, as shown in former researches on diamagnetic 

 and paramagnetic action ; but if surrounded by a solution stronger 

 than itself it will be repelled. Yet the direction of the lines of force 

 passing through it and the surrounding media cannot be reversed in 

 these two cases, and therefore the polarity remains the same. The 

 moving wire however shows, in similar cases, the true polarity or 

 direction of the forces ; and for an application cf its principles, in 

 this respect, to the metals, an apparatus is described by which discs 

 of different metals can be revolved between the poles of a horse-shoe 

 magnet and the electric currents evolved in them carried off to the 

 galvanometer. Now, whether the discs be of paramagnetic or dia- 

 magnetic metals, whether of iron, or bismuth, or copper, or tin, or 

 lead, the direction of the current produced shows, that the lines of 



