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.