32 POWER DISTRIBUTION FOR ELECTRIC RAILROADS.
The seriousness of the joint question may be seen by
a moment’s reflection upon the data already given. ‘There
are about thirty-three joints per thousand feet of rail. This
means sixty-six contacts per thousand feet between rail and
bond, in addition to the resistance of the bond wire itself.
Now, the resistance of a sixty pound rail per thousand feet
is, as we have seen, only 14y ohm, in decimals o.or. If
there should be even one-ten-thousandth of an ohm resist-
ance in each joint between bond and rail, the total resistance
would rise to 0.016 ohm per thousand feet. Add to this, the
actual resistance of, say, sixty feet of bonding wire No. o,
and the total foots up to 0.022 ohm, more than doubling
the original resistance. If the joints were here and there
quite imperfect, as generally happens, the rail resistance
might easily be increased far more.
One would be thought lacking in common sense who
needlessly doubled the resistance of an overhead circuit,
but in the rail circuit far more atrocious blunders are only
too common. A few years ago it was frequent enough to
find bond wire simply driven through a hole in the web of
the rail and headed on the outside. Fortunately, the need
of care here 1s now better realized and in the last few years
the name of the rail bond is legion. Most of the contacts
are modified rivets, not infrequently supplied with some
sort of wedging device to ensure a tight contact. They
are, most of them, good enough if properly applied, but a
careless workman can easily destroy the usefulness of even
the best bonds. The bond contact proper is often quite
distinct from the bond wire and is generally given a greater
cross section than the latter, to ensure an ample contact
with the rail. Figs. 22 to 25, inclusive, show some of the
best current forms of bonds. Fig. 25, the ‘‘plastic”
bond, is composed of a layer of a species of amalgam re-
tained by an outer wall of cork and squeezed into intimate
contact with rail and channel plate. It gives a singularly
low resistance contact.
As to the real resistance of a bonding contact, experi-
ments, as might be expected, vary enormously. The re-
