THE RETURN CIRCUIT. 47 
may be electrical differences at the joints of the pipes quite 
sufficient to cause local corrosion in serious amount. Joints 
in water pipe are better mechanically than electrically and 
the currents flowing through them may, as we have seen, 
be rather heavy. Take for example Fig. 35. Suppose 
that owing to oxidized and dirty surface of contact the 
joint A has a resistance of .co5 ohm and that a current of 
one hundred amperes is flowing through it in the direction 
indicated by the arrow. The fall of potential through the 
joint would then be .5 volt, lines of current flow would be 
set up as shown by the dotted lines and a ring of corrosion 
B C would be set up on the positive side of the joint. Half 
a volt is quite enough to do the work, and though the 
action might be slow it would be sure. 
Therefore all conduction by pipes ought to be avoided 
as far as possible unless they are electrically continuous. 
Evenif they are, protection et 
by supplementary wires is fii\ 
somewhat risky since while 
it may relieve trouble in 
the conductors so connect- 
ed it may enhance the 
  
eV 
1 
danger to neighboring pipes H/) /l 
not thus protected. s o 
Liberal use of supple- H1e. 15, 
mentary wires has great 
use as an emergency measure, applied to systems already 
existing, but here, as generally, an ounce of prevention 
is worth a pound of cure. The proper return circuit of 
the railway should be made so good that the stray currents 
shall be quite negligible, and all methods of palliating their 
evil effects should be considered secondary in importance 
and to be shunned rather than courted. Itmust not be un- 
derstood that these methods are condemned, for they may 
be of much use, but they should be employed only to deal 
with the residual currents after they have been reduced to 
the lowest practicable terms by means of improving the 
track circuit.