PHYSICS OF NINETEENTH CENT. ELECTRICITY. 565 



Ampere arranged electric conductors to convey the current in the 

 same manner as he imagined the resultant of the molecular currents to 

 circulate in a magnet. The conductors thus constructed consisted 

 of a wire wound in the form of a helix, and suspended in such a manner 

 as to admit of free movement. Such a conductor, called a solenoid, is 

 represented in Fig. 279, and in Fig. 280 a mode of suspending it is 



i/a 



FIG. 278. 



FIG. 279. 



shown by which it is free to move horizontally. Solenoids behave 

 exactly like magnets ; thus, when a strong current circulates through 

 the apparatus represented 

 in Fig. 280, CD will, under 

 the influence of the earth's 

 magnetism, turn and settle 

 in the magnetic meridian; 

 and, if the current be as- 

 cending in the side of the 

 solenoid next the spectator, 

 it will be the end c that will 

 be directed towards the 

 north, c may, therefore, be 

 called the north pole of the 

 solenoid, and D the south 

 pole. If the pole of an- 

 other solenoid be brought 

 near c or D, there will be 

 attraction or repulsion ac- 

 cording as the poles are of 

 diffferent or of the same 

 names. 



This ingenious and ele- 

 gant theory presents the 

 most important phenomena 

 of magnetism, of electro- 

 magnetism, and also those 

 of magneto-electricity (a subject we have not yet arrived at) in the light 

 of a single simple notion. For instance, it explains by the observed 

 laws of electro-dynamics such facts as the induction of temporary 

 magnetism in soft iron by permanent magnets and by currents. It 



FIG. 280. 



