86 Galvanism, from Galvani to Ohm. 



circle that the motions in opposite parts should have an opposite 1 

 direction." 



Oersted's discovery was described at the meeting of the 

 French Academy on September llth, 1820, by an academician 

 (Arago) who had just returned from abroad. Several investi- 

 gators in France repeated and extended his experiments ; and 

 the first precise analysis of the effect was published by two of 

 these, Jean-Baptiste Biot (b. 1774, d. 1862) and Felix Savart 

 (b. 1791, d. 1841), who, at a meeting of the Academy of Sciences 

 on October 30th, 1820, announced* that the action experienced 

 by a pole of austral or boreal magnetism, when placed at any 

 distance from a straight wire carrying a voltaic current, may be 



4 thus expressed : " Draw from the pole a perpiendicular to the 

 wire ; the force on the pole is at right angles to this line and ta 

 the wire, and its intensity is proportional to the reciprocal of 

 the distance." This result was soon further analysed, the 

 attractive force being divided into constituents, each of which 

 was supposed to be due to some particular element of the 

 current ; in its new form the law may be stated thus : the- 

 magnetic force due to an element ds of a circuit, in which a 

 current i is flowing, at a point whose vector distance from ds is r,, 

 is (in suitable units) 



i ids 



|ds,r|t or curl .+ 

 r 3 J r 



It was now recognized that a magnetic field may be produced 

 as readily by an electric current as by a magnet ; and, as Arago 

 soon showed, this, like any other magnetic field, is capable of 



* Annales de Chimie, xv (1820), p. 222 ; Journal de Phys., xli, p. 51. 



f If a and b denote two vectors, the vector whose components are (a y b z a z b y ^ 

 a z b* a*b z , a x b y a y b x ) is called the vector product of a and b, and is denoted by 

 [a, b]. Its direction is at right angles to those of a and b, and its magnitude is 

 represented by twice the area of the triangle formed by them. 



+ If a denotes any vector, the vector whose components are ^- z - -^, - - ^-* r 



3% 9 a * -, 



z-l - - is denoted by curl a. 



fo ty 



Annales de Chimie, xv (1820), p. 93. 



