372 NINETEENTH CENTURY. FT. IIL 



Lines of Magnetic Force between two Electric 

 Wires. The next discovery which Ampere made was a 

 very important one. It was already well known that two 

 magnetic needles will either attract or repel each other 

 according to the position of their poles. Thus, if the north 

 pole of one needle is held towards the south pole of another, 

 they are attracted strongly together, but if the two north 

 poles are brought near together, the movable needle is re- 

 pelled. Now Ampere argued that if an electric current 

 always exerts a magnetic action around itself, then two 

 electric wires side by side will act magnetically upon each 

 other, or, in other words, will attract or repel each other as 

 if real magnets were lying between them. And this he 

 proved to be true. He put two wires side by side in such 

 a position that they could move freely, and when he sent 

 an electric current in the same direction through each of 

 them they moved towards each other ; while, if he sen* the 

 currents one way through one wire and the other way 

 through the other, they drew apart ; exactly in the same 

 way as magnets attract or repel each other, according to 

 the direction in which they lie. 



This may be difficult to understand without more expla- 

 nation, but you can remember that Ampere proved that 

 electric currents exert a magnetic force at right angles to 

 themselves in the air without needing any bar of steel to help 

 them. 



Electro -magnets made by means of an Electric 

 Current. It now occurred to Ampere that if electric cur- 

 rents give rise to magnetic force he ought to be able to 

 magnetise a steel bar by passing an electric current round it. 

 So he wound a copper wire (covered with silk to prevent the 

 electricity running into the steel) round a steel bar, and, 

 fastening the two ends of wire to a voltaic battery, he passed 



