pouring very hot cast iron into a mould placed around the ends of the
rails. These moulds are of iron; and to prevent their sticking to
the joint when it is cast, they are painted inside with a mixture of
linseed oil and graphite. Iron is usually poured so hot that, before it
cools, the base of the rail in the center of the molten joint becomes
partially melted, thus causing a true union of the steel rail and
cast-iron joint. This makes the joint solid mechanically and a good
electrical conductor. To supply melted cast iron during the process of
cast-welding joints on the street, a small portable cupola on wheels is
employed. Fig. 79 gives an idea of the process of making cast-welded
joints.
[Illustration: Fig. 79. Process of Cast-Welding Joint.]
=Electrically Welded Joints.= An electrically welded joint is made
by welding steel blocks to the rail ends. A steel block is placed on
each side of the joint, and current of very large volume is passed
through from one block to the other. This current is so large that
the electrical resistance between the rail and steel block causes
that point to become molten. Current is then shut off, and the joint
allowed to cool. There is in this case a true weld between the steel
blocks and the rails and joint. An electric welding outfit being
expensive to maintain and operate, this process is used only where a
large amount of welding can be done at once. Current is taken from the
trolley wire. A rotary converter set takes 500-volt direct current
from the trolley wire, and converts it into alternating current. This
alternating current is taken to a static transformer which reduces
the voltage and gives a current of great quantity at low voltage, the
latter current being passed through the blocks and rails in the welding
process. A massive pair of clamps is used to hold the blocks against
the rails, and to conduct the current to and from the joint while it is
being welded. These clamps are water-cooled by having water circulated
through them so that they will not become overheated at the point of
contact with the steel blocks.
=Thermit Welding.= A process of welding rail joints which was developed
after the cast-welding and electric-welding processes, is known as the
_Goldschmidt process_, which makes use of a material called “thermit”
for supplying heat to make the weld. A mould is placed around the joint
and the thermit is put in this mould and ignited. The heat produced by
the thermit is so intense as to reduce the iron in the thermit mixture
and make a welded joint. The thermit consists of a mixture of finely
powdered aluminum and iron oxide. When this is ignited, the aluminum
oxidizes, that is, absorbs oxygen so rapidly that an intense heat is
the result. In the process of oxidation, the aluminum takes the oxygen
from the oxide of iron, leaving molten metallic iron, which metallic
iron makes the weld by union with the molten rail ends. This process
has the advantage over other welding processes, of not requiring
an elaborate apparatus and a large crew of men to operate it; and
consequently it can be used where but a few joints are to be welded.
[Illustration: Fig. 80. Channel Pin Bond.]
=Bonding and Return Circuits.= When the track rails are used as the
conductors, as is usually the case, it is necessary to see that the
electrical conductivity of the rail joints does not offer too high a
resistance to the passage of the current. For this reason, when bolted
or angle-bar joints are used, the rails are bonded together by means
of copper bonds. It was soon found after electric roads were in use a
short time, that unless the rail ends were so bonded, the resistance
of the joints was so great as to cause great loss of power in the
track. First, small iron bonds were used; but these bonds were so
insufficient that large copper-wire bonds soon began to be used; and
at the present time, on large roads, bonds of heavy copper cable are
common. The resistance of a steel rail, such as used in city streets,
is about eleven times that of copper. In order to secure as great
carrying capacity at the rail joint as is afforded by the unbroken
rail, it is therefore necessary to install bonds having a total
cross-section ¹⁄₁₁ that of the rail. Where welded joints are used,
bonding is unnecessary, except at crossings and switches where bolted
joints are employed. Where track is welded, however, cross bonds should
be put in at frequent intervals from one rail to another, and, if the
track is double, from one track to the other, so that if one of the
track rails breaks at a joint there will be a path around the break for
the current.
[Illustration: Fig. 81. Chicago Rail Bond.]
[Illustration: Fig. 82. Rail Bond.]
A great many schemes have been devised to insure good contact between
the copper bond and the rail, as the terminal is the weak point in any
bond. One of the earliest and most efficient of small bonds was made
by the use of channel pins, Fig. 80. This bond consisted of a piece
of copper wire having its ends placed in the holes in the rail ends.
Alongside this wire, a channel pin was driven in. The objection to the
channel pin was the small area of contact between the copper bond and
rail.
Next after the channel pin came the Chicago type of bond, Fig. 81,
which is a piece of heavy copper wire with thimbles forged on the
ends. These thimbles were placed in accurately fitted holes in the
rail ends, and a wedge-shaped steel pin was driven into the thimbles
to expand them tightly into the hole in the rail. Several other bonds
using modifications of this principle are in use.
A type of bond in very common use consists of solid copper rivet-shaped
terminals, Fig. 82. Between these terminals is a piece of flexible
stranded copper cable, made flat to go under the angle bars. In one
type the terminal lugs are cast around the ends of the cables, and in
another type the cables are forged at their ends into solid rivet-like
terminals. These terminal rivets were first applied as any other
rivets, with the use of a riveting hammer. Because of the difficulty of
thoroughly expanding such large rivets into the holes made for them in
the rails, it has become customary to compress these rivets either with
a screw press or a portable hydraulic press, which brings such great
pressure to bear on the opposite ends of the rivet that it is forced
to expand itself so as to fill the hole in the rail completely. This
expansion is made possible by the ductile character of the copper. This
great ductility characteristic of copper, however, has been the source
of one of the difficulties in connection with rail bonding, because the
soft copper terminal has a tendency to work loose in the hole made for
it in the rail. It is practically impossible to maintain good bonding
where the rail joints are so loose as to allow considerable motion
between the rail ends.
Several types of bonds have been introduced, in which the contact
between the rail and bond is made by an extra piece or thimble.
Another method of expanding bond terminals into the holes made to
receive them, is that employed in the General Electric Company’s
bond. In it a soft pin in the center of the terminal is expanded by
compression of the terminal so that it forces the copper surrounding
it outward. The copper terminal, in expanding to fill the hole, is
therefore backed by the steel center pin.
All types of bonds must be installed with great care if they are to be
efficient. Unless the bond terminal thoroughly fills the hole and is
tightly expanded into it, moisture will creep into the space between
the copper and the iron, and the copper will become coated with a
non-conducting scale which destroys the conductivity of the contact.
The _plastic_ rail bond, so called because it depends for the contact
between the rail and the bond upon a plastic, putty-like alloy of
mercury and some other metal, is applied in a number of different ways.
One form consists of a strip of copper held by a spring against the
rail ends under the fish-plate. The rail ends at the point of contact
with this strip of copper are amalgamated and made bright by the use
of a mercury compound similar to the plastic alloy. These points of
contact are then daubed with plastic alloy, and the copper bond plate
applied. It is not necessary, with any form of plastic bond, that the
mechanical contact be unyielding, as the amalgamated surfaces with the
aid of the plastic alloy between them, maintain a good conductivity
in spite of any slight motion. The plastic alloy can be applied in a
number of other ways, one of which is to drill a hole forming a small
cup in the rail base in adjacent rail ends, fill these cups with
plastic alloy, and bridge the space between them with a short copper
bond having its ends projecting down into the cups.
=Resistance of the Track.= The resistance of the return circuit is
usually much higher than it should be owing to the bad contact of the
bonds. The resistance of rails varies greatly with the proportions of
carbon, manganese and phosphorus. The following figures, however, may
be regarded as the average.
Weight per Yard. Resistance Single Rail per Mile.
50 .0253 ohms
60 .0211 ”
70 .0180 ”
80 .0159 ”
90 .014 ”
A track laid with continuous rails as in the case of welded joints,
would have one-half the resistance given since there are two rails to
be considered.
Tests of new unbonded track constructed with rails 60 feet long show
that the joints cause an increase of .25 ohms or more per mile.
Several roads in testing bonds consider a bond good when the bond and
one foot of the rail over it have a resistance equal to five feet of
the solid rail.
=Supplementary Return Feeders.= On some large roads it is necessary to
run additional return feeders from the power house to various points on
the system, to supplement the conductivity of the rails. Otherwise the
track rails near the power house would have to carry all the current,
and in some cases there are not enough such lines of track passing
the power house to do this properly. Sometimes these feeders are laid
underground in troughs; sometimes they are laid bare in the ground, and
sometimes on overhead pole lines. When laid in the ground, frequently
old rails are used instead of copper or aluminum cables. The old
rails are, of course, thoroughly bonded together with bonds giving a
conductivity nearly equal to that of the unbroken rail.
FEEDER SYSTEMS.
[Illustration: Fig. 83.]
There are two general schemes of direct current feeding in common use.
One of these is shown in Fig. 83. Here the trolley wire is continuous
and is fed into at different points. The long feeders supplying the
more distant portion of the road are larger than those supplying the
trolley near by, so as to maintain as nearly as is feasible the same
potential the entire length of the line. With such a system of feeding,
in order to maintain absolutely the same voltage at all points, it
would be necessary to have just one trolley feeder and that feeding
into the extreme end of the line farthest from the power station and
further to make the resistance per 1,000 ft. of trolley and feeder the
same as the resistance per 1,000 ft. of the track return circuit. The
plan shown in Fig. 83 evidently does not fully carry out these rather
impracticable requirements but is in the nature of a compromise, giving
a higher potential near the power station than at distant points but
nevertheless much more even potential than if the heaviest feeders
were feeding into the trolley near the power house.
The other plan, shown in Fig. 84, divides the trolley wire into
sections and feeds each section through a separate feeder which is
calculated of such size as to maintain the same voltage on all the
sections with the ordinary load.
In calculating a feeder system a certain probable load is assumed at
certain points along the line. This load will manifestly depend on the
size and number of cars in operation, grades and many local conditions.
[Illustration: Fig. 84.
Drop in rail
section 3.1 Volts 2.1 Volts 1.05 Volts
Total drop in
rail 3.1 ” 5.2 ” 6.25 ”
Drop in trolley 20.5 ” 20.5 ” 20.5 ”
Drop in feeder 36.4 ” 34.3 ” 33.25 ”
Resistance
feeder .728 Ohms .686 Ohms .665 Ohms
Feet per ohm 7253 23000 39700
Size of wire No. 1 250,000 C. M. 420,000 C. M.]
The following example will show the method pursued. The figures
resulting from the calculations are placed immediately below the
sections to which they refer in Fig. 84. The rails are assumed to be 70
pound to the yard. These have a resistance of about .018 ohms per mile.
Adding one-sixth for additional resistance of bonds gives .021 and
since the track is composed of two rails the resistance of the track
will be one-half of this or .0105 ohms per mile.
The maximum drop in any section occurs when the car is farthest from
the power house. Each car is assumed to take 50 amperes and the feeders
are to be so designed as to allow a 10 per cent or 60 volts drop.
The current in the two miles of track nearest the power house is 150
amperes, in the next section 100 amperes, and in the last section 50
amperes. The drop in each section is as shown. The drop in the trolley
which is 00 wire is, in each section, 20.5 volts. Subtracting from 60
volts the drop in the return circuit and trolley, gives the allowable
drop in the feeder.
The resistance of each feeder can be calculated, since the current in
each one is 50 amperes. The first feeder is one mile long, the second
3 miles and the third 5 miles, and with these figures the feet per ohm
can be computed. The size of wire may be obtained by reference to a
table of copper wire resistances.
BLOCK SIGNALS FOR ELECTRIC RAILWAYS.
The simplest block signal used by electric roads is a hand-operated one
constructed on the principle shown in the diagram Fig. 85. A double
throw switch is placed at each terminal of the section of track that is
to be protected.
[Illustration: Fig. 85.]
The switches have no central position, the knife blade always making
contact with one or the other of the terminals shown. If the lamps are
lighted, throwing either one of the switches will put them out. If they
are not burning, they will be lighted by throwing either one of the
switches.
A motorman on reaching a section of track finding the lamps not burning
throws the switch. Lamps now burn in each switch box and show that the
section is in use. On arriving at the other terminal of the block the
switch is thrown, extinguishing the lights and showing that the block
is clear.
Automatic signal systems have been devised on the same principle, in
which magnets, operated by contacts made by the passage of the trolley
wheel, cause the lamps to be lighted and extinguished automatically.
ELECTROLYSIS.
Much has been said about the possibilities of electrolysis of
underground metal by the action of the return current of electric
railways, when such railways are operated with grounded circuits, as
they usually are. If electric current is passed through a liquid from
one metal electrode to another, electrolysis will take place; that is,
metal will be deposited on the negative pole, and the positive pole or
electrode will be dissolved by becoming oxidized from the action of the
oxygen collecting at that pole.
[Illustration: Fig. 86. Showing Electrolytic Action.]
In an electric-railway return circuit, there is necessarily a
difference of potential between the rails at outlying parts of the
system and the rails and other buried pieces of metal located near
the power house. Just what this total difference of potential is,
depends on the loss of voltage in the return circuit. Thus, suppose
there is 25 volts drop in the return circuit between a certain point
on the system and the power station. There is, therefore, a pressure
of 25 volts tending to force the current through the moist earth
from the rails at distant portions of the line, to the rails, water
pipes, and other connected metallic structures located in the earth
near the power station. The amount of current that will thus flow to
earth in preference to remaining in the rails, depends on the relative
resistance of the rails, the earth, and the other paths offered to the
current to return to the power house.
To take a very simple case, let us suppose a single-track road, Fig.
86, with a power house at one end, and a parallel line of water pipe
on the same street passing the power house. If the positive terminals
of the generators are connected to the trolley wire, the current
passes, as indicated by the arrows, out over the trolley wire through
the cars and to the rails. When it has reached the rails it has the
choice of two paths back to the power house. One is through the rails
and bonding; the other is through the moist earth to the line of water
pipe and back to the power house, leaving the pipe for the rails, at
the power house. Should the bonding of the rails be very defective,
considerable current might pass through the earth to the water pipe.
Remembering now the principles of electrolysis, we see that the
oxidizing action of this flow of current from the rails to the water
pipes at the distant portion of the road will tend to destroy the
rails, but will not harm the water pipe at that point, as it will tend
to deposit metal upon it. When, however, the current arrives at the
power house, it must in some way leave this water pipe to get back to
the rails, and so to the negative terminals of the generators.
Here we see that there is a chance for electrolysis of the water pipe,
because at this point the water pipe forms the positive electrode,
which is the one likely to be oxidized and destroyed. This very
simple case is taken merely for illustration. In actual practice the
conditions are never so simple as this, for there are various pipes
located in the ground running in various directions, which complicate
the case very much; but we can see from this simple example that the
principal place electrolysis of water pipe is to be feared is at points
where a large volume of current is leaving the water pipe to take to
some other conductor.
As an indication of how much current is likely to be leaving the
water pipes at various points, it is customary to measure the voltage
between the water pipes and the electric railway track and rails.
When this voltage is high, it does not necessarily mean that a large
volume of current is leaving the water pipes at the point where these
pipes are several volts positive with reference to the rails; but such
voltage readings indicate that, if there is a path of sufficiently
low resistance through the earth, and if the moisture in the earth is
sufficiently impregnated with salts or acids, there will be trouble
from an electrolytic action due to a large flow of current. There is
obviously no method of measuring exactly the amount of current leaving
a water pipe at any given point, since the pipe is buried in the earth.
Voltmeter readings between pipes and rails simply serve to give an
indication as to where there is likely to be trouble from electrolysis.
The danger to underground pipes and other metallic structures from
electrolysis has been much overestimated by some people, as the trouble
can be overcome by proper care and attention to the return circuit.
Trouble from electrolysis, however, is sure to occur unless such care
is given.
=Prevention of Electrolysis.= Remedies for electrolysis may be
classified under two heads—general and specific. The general remedy
is obviously to make the resistance of the circuit through the rails
and supplementary return feeders so low that there will be but little
tendency for the current to seek other conductors, such as water and
gas pipes and the lead covering of underground cables. This remedy
consists in heavy bonding, in ample connections, around switches and
special work where the bonding is especially liable to injury, and
in additional return conductors at points near the power house to
supplement the conductivity of the rails.
It is important that all rail bonds be tested at intervals of six
months to one year in order that defective bonds may be located and
renewed, as a few defective bonds can greatly lower the efficiency of
an otherwise low-resistance circuit.
The specific remedy for electrolysis which may be applied to reduce
electrolytic action at certain specific points, consists in connecting
the water pipe at the point where electrolysis is taking place, with
the rail or other conductor to which the current is flowing. Thus, for
example, if it is found that a large amount of current is leaving a
water pipe and flowing to the rails or to the negative return feeders
at the power house, the electrolytic action at this point can obviously
be stopped by connecting the water pipe with the rails by means of
a low-resistance copper wire or cable, thereby short-circuiting the
points between which electrolytic action is taking place. There are
certain cases in which it is advisable to adopt such a specific remedy.
It should be remembered, however, that a low-resistance connection
of this kind, while it reduces electrolysis at points near the power
house, is an added inducement to the current to take to the water pipes
at points distant from the power house, because of the decrease in
resistance of the water-pipe path to the power house resulting from the
introduction of the connection between the water pipe and the negative
return feeder at the power house. With the water pipes connected to the
return feeders in the vicinity of the power house, the current which
flows from the rails to the water pipes at points distant from the
power house will obviously cause electrolysis of the rails but not of
the water pipes, since the current is passing from the earth to the
pipe, and the pipe is negative to the earth. In this case the principal
danger is that the high resistance of the joints between the lengths
of water pipe will cause current to flow through the earth around each
joint, as indicated on some of the joints, Fig. 86, and will cause
electrolytic action at each joint. It is evident, however, that the
conditions of the track circuit and bonding must be very bad if current
would flow over a line of water pipe, with its high-resistance joints,
in sufficient volume to cause electrolysis, in preference to the
rail-return circuit, especially since ordinarily the resistance offered
to the flow of current over the water pipes back to the power house
must include the resistance of the earth between the tracks and water
pipes.
It is usually considered inadvisable to connect tracks and water pipes
at points distant from the power house, because of the danger of
electrolysis at water-pipe joints, as just explained.
Methods of testing rail bonds in the track will be explained under the
head of “Tests.”
POWER SUPPLY AND DISTRIBUTION.
=Direct-Current Feeding.= As already explained, the majority of
electric railways are operated on a 500-volt constant-potential
direct-current system with a ground return. A constant potential of 450
to 550 volts is maintained between the trolley wire and track. Where
the trolley wire is not sufficient, additional feeders are run from the
power house and connected to the trolley wire, the number of feeders
depending on the distance from the power house and the traffic.