f 



HISTORY AND PROGRESS. 21 



IY. TELEGRAPHS BY ELECTRO-MAGNETISM AND MAGNETO- 

 ELECTRICITY. 



27. The power of lightning to weaken the magnetism of 

 the compass needle, and even sometimes to reverse its 

 polarity for a long time, suggested the suspicion of a near 

 relation between electricity and magnetism. The definite 

 discovery was, however, not made until, in 1820, Professor 

 Oersted, of Copenhagen, found that a magnetic needle, sus- 

 pended in the neighbourhood of a wire in which a current 

 of electricity was passing, was deflected from its position 

 of rest. Ampere made experiments, and found the law by 

 which this influence was governed, and which he briefly 

 expressed as follows : 



" Imagine a human figure in the direction of a conductor 

 through which a positive current is flowing upwards, the 

 figure will have the north pole on the left hand if its face 

 be turned towards the needle." 



Thus, if a positive current pass along the upper wire in 

 the annexed figure (Fig. 8) from a towards b, the magnetic 

 needle N s, suspended in its neighbourhood, will be deflected, 

 and take up a position 

 indicated by the dotted 

 line n s, at nearly right , 



angles to the wire. When 

 the current is reversed 



the poles will be deflected a/ s ^^ 1' 



in the other direction, "&*& 8 - 



the pole n being where s is, in the figure. It will become 

 evident on regarding the figure also that, if a similar current 

 pass in the lower wire from b' to a', it must produce the 

 same magnetic direction upon the needle as a current in the 

 upper wire would from a to b. In the case of the upper 

 wire the observer's head is supposed to be near b and his 

 feet near a, the current passing upwards in the direction of 

 the arrow, and the north pole is found on his left hand 

 when he faces the needle. In the case of the lower wire, 

 however, the direction of the current is reversed, and the 



