THE RETURN CIRCUIT. 35 
is, all other so-called conductors are very bad compared even 
with a relatively poor conductor like iron. For example, 
carbon in the form of graphite or gas coke, is usually consid_ 
ered a very fair conductor, yet it has several hundred times: 
the resistance of iron, while nitric acid and dilute sulphuric 
acid, the best conductors among electrolytes, have many 
thousand times the resistance of iron. ‘The acid last men- 
tioned has a specific resistance of about 0.4 ohm for a 
cubic centimeter, while the resistance of a cubic centimeter 
of iron is only 0.00001 chm. Water, even when dirty as it 
is found in the streets, would show a specific resistance of 
1000 ohms or more. FEarth, rock and other miscellaneous 
components of the ground are even worse, so that it is at 
once fairly evident that it would take an enormous con- 
ducting mass even of water to approximate the conductiv- 
ity of a line of rails. 
Even in theory the mass of earth really available for 
conducting purposes is somewhat limited, for if a current 
be passed between two earth plates, the current density de- 
creases very rapidly as the lines of flow depart from the 
direct path between the plates. It has long ago been 
shown, too, that when such a current is established be- 
tween, let us say, a pair of metallic balls sunk in the earth, 
the resistance of the circuit does not vary much with the 
distance apart of the terminals, but depends greatly on the 
surface of the ground connections. Numerous experiments, 
too, have shown that the earth is so heterogeneous, so 
broken up into strata of varying conductivity, that the 
current flow takes place mainly along special lines, the 
general mass taking very little part in the action. If, for 
example, a ledge of rock is in the line between earth plates, 
save for possible crevices filled with water, it is practically 
a non-conductor. 
At various times and places the value of a true earth 
return for railway and similar work has been thoroughly 
tried and has uniformly been found to be practically 7. 
In two cases the ground plates were sunk in considerable 
rivers which formed return circuits for lines in each case