724 COMMUNICATION BETWEEN TWO STATIONS. 



are sunk to some depth in the ground, and each of them 

 is connected by a wire with one end of the line; the 

 current then passes in one direction along the wire, and 

 returns in the opposite direction through the earth. 

 The earth is a worse conductor than the metals and 

 even than common water; but a comparatively bad 

 conductor which is very thick transmits a current more 

 readily than a much better conductor which is very 

 thin ; for example, of two wires of equal thickness, one 

 of copper and the other of iron, the copper wire con- 

 ducts much better than the iron wire ; but if the thick- 



FIG. 365. 



ness of the iron wire is many times as great as that of 

 the copper wire, it conducts electricity much better 

 than the copper wire. In the same way the earth, 

 being of almost infinitely greater thickness than the 

 metallic wire, is far superior to it as a conductor. B l 

 and jB 2 are the two batteries, Ji and T 2 the two keys, 

 Si and& 2 represent the electro-magnets of the indicators, 

 the other parts of which are left out for the sake of 

 simplicity. The electro-magnets of the two stations are 

 connected by the line ll : which is suspended by insulat- 



