Royal Institution of Great Britain. 129 







the other. This is obvious, since dissimilar poles are opposite to 

 each other, attraction must take place. If they be placed, as in the 

 second case, the moveable one will be repelled, and continue to 

 move parallel to itself, till it get without the sensible repulsive action 

 of the other. 



Viewing these compound bars as metallic slips conducting voltaic 

 electricity, it is obvious that attraction will take place when the cur- 

 rents move in the same direction, and repulsion when they move in 

 opposite directions. 



The same illustration will apply to a terminated current moveable 

 about one of its extremities, when acted on by a straight indefinite 

 current. Let AB be an indefinite straight conductor, and DC a 

 terminated conductor moveable about the point C. It is obvious that 

 in this position repulsion will take place, and the moveable con- 

 ductor will turn round C, till it arrive at a position at right angles 

 to the straight conductor. At this point the attraction of the other 

 half of the straight conductor will act till it be brought to a parallel 

 position. By viewing the tendency to move in every position, we 

 clearly see that the terminated conductor has a tendency to revolve 

 about the point C, and by a sufficiently powerful battery this revolu- 

 tion may be made to take place. 



ahn 



9t, ~~ **- 



A I I I I I I I I I I I I I I B 



0? 



By substituting a magnet for the conductor the same thing takes 

 place, as in the beautiful experiments of Mr. Faraday and M. Am- 

 pere. 



A portion of a circular conductor may be viewed as a straight 

 one, and all the phenomena of terrestrial magnetism may thus be 

 satisfactorily explained, by supposing currents of electricity circu- 

 lating about the earth from east to west. Such currents may be 

 made to act on terminated currents, and produce all the phenomena 

 of attraction, repulsion, and rotatory motion, as has been done by 

 M. Ampere. 



By means of an artificial globe, surrounded with coils of copper 

 wire, Mr. Barlow has exhibited all the effects of terrestrial magnet- 

 ism, on the direction and dip of the needle. Without entering into 

 the scientific details of the theory of terrestrial magnetism, Mr. 

 Ritchie showed, by a very simple experiment, that the old theory, 

 which viewed the earth as a huge magnet having two poles and a 

 middle line, was inconsistent with known facts. If a vessel of water 

 be placed on the middle of a large magnet, and a light magnetic 

 needle floated on the surface of the water, this needle will arrange 

 itself in the direction of the poles of the magnet. So far it agrees 



VOL. II. AUG. 1831. K 



