116 Nature's Atractee. 



Now if an equal current is simultaneously 

 passed through each of the wires of the coil in 

 opposite directions the effect on the iron will 

 be nothing, because one current is trying to 

 develop a certain kind of polarity at each pole 

 of the magnet, while the current in the other 

 wire is trying to develop an opposite kind in 

 each pole. There is an equal struggle between 

 the two opposing forces, and the result is no 

 magnetism. This assumes that the two cur- 

 rents are exactly the same strength. 



If we break the current in one of the coils 

 we immediately have magnetism in the iron; 

 or if we destroy the balance of the two cur- 

 rents by making one stronger than the other 

 we shall have magnetism of a strength that 

 measures the difference between the two. 



Without specifically describing here the en* 

 tire mechanism since this is not a text-book 

 or a treatise we may say that a duplex tele- 

 graph-line is fitted with these differentially 

 wound electromagnets at every station. When 

 Station A (Fig. 3) is connected to the line 

 by the positive pole of its battery, Station B 

 will have its negative pole to line and its posi- 

 tive to earth. When A depresses his key to 

 send a message, half the current passes by one 

 set of coils around his differential magnet 

 through a short resistance-coil to the earth, 

 and the other half by the contrary coil around 

 the magnet to the line, and so to Station B. 



