MAGNETIC FIELD OF A CURRENT. 421 



opposite. The force, moreover, is proportional to the magnetic 

 mass in question, for if we put near the current a small needle, 

 which at the same time is under the action of the Earth, the 

 direction which it takes is independent of its magnetic moment ; 

 the resultant of the two forces which arise from the terrestrial 

 field, and from the field created by the current, has thus itself a 

 fixed direction, and therefore the two forces maintain a constant 

 ratio. 



The action of the current also changes its sign, without 

 changing its magnitude, when the direction of the current is 

 simply reversed; thus, when the conducting wire is bent upon 

 itself, the two portions close to each other, which are traversed 

 by equal currents in opposite directions, have no action on the 

 pole of a magnetised needle. 



The existence of the field produced by the current may be 

 rendered evident by the ordinary method of magnetic images. 

 Thus, if iron filings are scattered on a sheet of paper traversed at 

 right angles in its centre by a rectilinear current, the filings are 

 seen to arrange themselves in concentric circles about the path 

 of the current. We conclude from this that the lines of force are 

 circumferences whose centre is the axis of the current. The force, 

 therefore, is perpendicular at each point to the plane passing through 

 this point and the current ; it is, moreover, turned to the left of the 

 observer in Ampere's rule. 



The successive equipotential surfaces obtained about a rectilinear 

 current are thus formed by a series of planes passing through the 

 axis of the wire, and making equal angles with each other. The same 

 is the case near any given current, so that the equipotential surfaces 

 are formed about each portion of the wire, making equal angles 

 with each other. 



444, ACTION OF A RECTILINEAR CURRENT ON A POLE. 

 EXPERIMENTS OF BIOT AND SAVART. Biot and Savart determined 

 experimentally the magnitude of the force at each point. They 

 examined the action of a vertical current on a small horizontal 

 magnetic needle placed at various distances on a right line passing 

 through the current, and perpendicular to the magnetic meridian. 

 In these conditions the resultant force is the sum of the horizontal 

 component H of the Earth's field, and of the force < of the 

 current. 



The needle is first caused to oscillate under the influence of the 

 Earth alone, then at distances a and a' from the wire under the 

 combined influence of the Earth and of the current. If #, N, and 



