ELECTRO-MAGNETISM. 



57 



is difficult to appreciate singly Ihe effect : 

 such as the intensity of the electric dis- 

 charge, the length of the straight wire, 

 its diameter, the thickness of the needles, 

 and their degree of coercive force. In 

 general, when the wires are very slender, 

 and the coercive force of the needles 

 feeble, the periodical alternations above 

 noticed arc less numerous ; and it 

 even frequently happens, with these 

 conditions, that the magnetization is 

 every where positive, and that the only 

 differences observable at different stages 

 of distance, are those of greater or less 

 intensity. 



(1G6.) When the discharge from the 

 electric battery is transmitted through 

 a wire coiled into the form of a helix 

 around glass or wooden tubes, a similar 

 diversity is met with in the effects pro- 

 duced on different needles successively 

 placed in the interior of the tubes, and 

 in different situations relative to the 

 axis. By varying the intensity of the 

 charge of the battery, or the length or 

 thickness of the needles, the nature of 

 the result is changed. The maximum 

 of magnetic intensity which may be pro- 

 duced by a given wire, depends on the 

 ratio between its thickness and its 

 length ; so that the degree of magne- 

 tization amounting to saturation, bears 

 a relation to the value of this ratio. The 

 degree of magnetic power that a needle 

 receives fronTthe influence of an electric 

 discharge, and even the direction of .its 

 magnetization, depend also on the na- 

 ture and the dimensions of the bodies 

 that are in contact with it, or that sur- 

 round it. 



(167.) The magnetizing influence of a 

 helix through which an electric dis- 

 charge is passed, is completely inter- 

 cepted by a cylinder of copper, of suffi- 

 cient thickness, inclosing the needle, 

 and introduced within the helix. When 

 the interposed cylinder is of less thick- 

 ness, some magnetic effect becomes 

 perceptible ; and when the thickness of 

 the copper cylinder is still farther re- 

 duced, the needle is rendered even more 

 strongly magnetic than when exposed to 

 the action of the helix without any inter- 

 posed substance. Tin, iron, and silver, 

 placed round the needle, produce a si- 

 milar modification of the electro-mag- 

 netic action of the helix ; when inter- 

 posed in very thin plates, they increase 

 this action ; when of a certain thickness, 

 they entirely intercept it. Cylinders 

 composed of metallic filings do not 

 produce this effect ; whereas we again 



meet with the intercepting property, if 

 the interposed substance is composed 

 of concentric layers consisting alter- 

 nately of metallic and of non-metallic 

 bodies. It would thence appear that 

 solutions of continuity in a direction per- 

 pendicular to the axis of the needle, or 

 to the axis of the helix, have a consi- 

 derable influence on the magnetizing 

 effects of the latler upon the former. 

 An influence of a similar kind has been 

 observed from metallic plates of different 

 thickness, placed in contact with a needle 

 properly disposed with regard to a 

 straight conducting wire receiving the 

 discharge of an electric battery ; being 

 found, according to their size or position, 

 to modify the intensity and even the 

 direction of the magnetism acquired by 

 the needle. 



(168.) All these phenomena appear 

 to depend on the suddenness of the 

 action exerted by the electric shock, 

 either directly on the particles it meets 

 with in its course, or on objects that are 

 situated at a distance in the surrounding 

 space. But the direction of the mag- 

 netization, as to its being of the positive 

 or negative kind, depends essentially on 

 the intensity of the discharge ; so that 

 discharges of different intensities deve- 

 lope in the metal a set of opposite 

 states analogous to the polarities of con- 

 trary signs, acquired at different dis- 

 tances from a conducting wire, or by 

 different intensities of electricity. 



CHAPTER XI. 

 Mutual Actions of Electric Currents. 



$ 1. Action of Parallel Rectilineal 

 Currents. 



(169.) THE discovery of Oersted, and 

 all the consequences we have developed 

 from the fundamental law that appears 

 to regulate the reciprocal action be- 

 tween electric currents and magnetic 

 bodies, belong to that division of the 

 subject to which the term Electro-Mag- 

 netism is more properly applied: for 

 they refer to the relation subsisting be- 

 tween the two agencies of electricity 

 and of magnetism, which we have been 

 accustomed to consider as distinct from 

 one another. But electric currents are 

 also found to have a mutual action on 

 one another: and this general fact, 

 which was ascertained by Ampere, soon 

 after the discovery of Oersted, esta- 



