5 66 



HISTORY OF SCIENCE. 



explains magnetic attractions and repulsions, and shows how it happens 

 that each fragment into which a magnetized bar may be broken is itself 

 a complete magnet. 



Ampere announced his discovery of the mutual action of electrical 

 currents to the Academic des Sciences in September, 1820, a week 

 after that body had received the report of (Ersted's discovery. Within 

 a few days after the reading of Ampere's paper, another announcement, 

 also the prolific germ of a new series of important discoveries, was 

 made by FRANCOIS ARAGO (1786 1853). It was his observation that 

 a copper wire, while traversed by the current from a powerful battery, 

 attracts iron filings, which cling to it on all sides so long as the current 

 passes, but drop off when it is interrupted. The effect is produced 



FIG. 281. 



with particles of iron only, and not with particles of brass, copper, etc. 

 This experiment thus confirmed that of (Ersted in showing the con- 

 nection between electric and magnetic actions. Arago succeeded also 

 in making a sewing-needle permanently magnetic by placing it across 

 the wire conveying the current. Ampere at once perceived that the 

 most powerful effect would, if his theory were correct, be obtained by 



placing the needle within a helical 

 coil of the wire. Arago and Am- 

 pere, pursuing these experiments, 

 found that a bar of soft iron placed 

 in such a coil (Fig. 281) became 

 more powerfully magnetic during 

 the time the current was parsing 

 than did steel; but on the cessa- 

 tion of the current, the iron re- 

 verted to its former condition. 

 The poles of the permanent and 

 temporary magnets thus formed 

 were found to be in all cases those 

 which Ampere's law requires, that 

 FIG. 282. is, the north pole was always on 



the left of current (page 562). The 



different cases are represented in Fig. 282, where N is the north and s 

 the south poles of the bar, surrounded by current passing from 4- to . 

 In experiments of this kind the conducting-wires are covered by some 

 insulating substances, silk, for example, in order that the current may 

 be compelled to traverse their length. This discovery not only ex- 



