236 THE ELECTRIC TELEGRAPH. 



magnitude is termed the unit of comparison. Jacobi, of 

 St. Petersburg, compared the volumes of gas developed in 

 his voltameter with an unit of volume developed in an unit 

 of time by an unit of current at a certain temperature and 

 tension. ' His expression was : 



" The unit of current is that current which in one minute, at 

 a temperature of C., and under 760 mm pressure, develops 

 one cubic centimetre of explosive gas." 



The value of the deflections of magnetic needles suspended 

 in multipliers of wire, inserted in the circuit of a battery and 

 voltameter have been compared with the volumes of gas 

 developed in certain times, and the laws of their deflection 

 thus ascertained. 



Ampere observed that when a positive current moved in 

 a wire from south to north, over and parallel to a magnetic 

 needle, the latter was deflected, with a tendency to place 

 itself at right angles to the wire, the north pole pointing 

 westward. Subsequently, Biot and Savart have occupied 

 themselves with the task of establishing the relation between 

 the deflection of the needle and the distance of the galvanic 

 current moving in a straight conductor of infinite length, 

 and have found that : 



" The total effect of an infinitely long and straight con- 

 ductor upon any magnetic element is in inverse proportion 

 to the distance of the element from the nearest point of the 

 wire." 



For the effect of a circular current upon a magnetic element, 

 "Weber has given a mathematical development, from which 

 he proves that when the distance of the magnetic element 

 from the centre point of the current circle is x, its diameter y, 

 the intensity of the current g, and the magnetic intensity of 

 the element which is deflected /*, the force J with which 

 the deflection takes place is expressed by 



which, translated into words, is, that a magnetic element in 

 the axis of a circular current is attracted or repelled from the 



