50 POWER DISTRIBUTION FOR ELECTRIC RAILROADS.
considerations of track stability and durability point to the
use of the continuous rail which minimizes at the same time
the electrical difficulties.
It must be remembered that in long distance lines such
as are found in interurban and similar work, the use of
continuous rails is liable to cause trouble from insufficient
resistance to expansion, as such roads generally lack the
strong lateral support of the paving and are often exposed
to more violent changes of temperature. On the other
hand, in the case of such roads trouble from electrolytic
action is usually relatively small or entirely absent, so that
bonding is sufficient. Also, as will be explained later, in
these roads for heavy service and rather high speed there
may sometimes be good reason for using two trolleys, quite
aside from all questions of ground return.
Of course, when the alternating current motor is thor:
oughly developed for railway service all danger of elec:
trolysis will be past, whatever the character of the return
circuit, but there will still exist every reason for making
the rail return as perfect as possible from motives of economy
alone. For when bad bonding can increase the total re-
sistance of the track circuit ten or a dozen times, as has
happened many times, the waste of energy due to the in-
creased drop in the circuit becomes somewhat burdensome.
For example, take a single track of ninety pound rail
10,000 ft. long. With continuous rails the resistance per
thousand feet would be 515 of an ohm and for the whole
distance .033. With 200 amperes flowing, the drop would
be 6.6 volts and the loss of energy more than one kilowatt.
Now suppose each bond contact with its half of the bond
wire to have a resistance of .0cor ohm. On each line of
rail there would be 660 of these so that the total bond re-
sistance of the track would be .33 ohm and the drop due
to this bond resistance with a current of 200 amperes
would be 66 volts. The corresponding loss of energy
would be 13.2 k. w. more than enough to operate an
extra car. At thecost of power generally found this waste
would represent in the vicinity of 1000 per year net loss, a