An Electric Traction System.
Year: 1896

page_text: Date of Application, 3rd Sept., 1896-Accepted, 17th Oct., 1896
COMPLETE SPECIFICATION.
An Electric Traction System.
We SILVANUS PHILLIPS THOMPSON of Morland, Chislett Road, West
Hampstead, Doctor of Science, and MILES WALKER of 23 Southcote Road,
London, N., Electrical Engineer do hereby declare the nature of this invention and
in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement :-
Our invention relates to a system of electric traction in which the electric
current is supplied to the cars or locomotives from studs or sectional rails laid in
the road, each of such studs or sectional rails being in turn connected to a main
conductor or feeder main by means of an automatic switch when the car is above
or near it; and disconnected from the main conductor or feeder after the car has
passed on.
Our improved system has been devised to avoid various defects found in systems
of traction with surface contact and sectional conductors, such as the systems
described in the Specifications of Ayrton & Perry Nos. 783 of 1881 and 2395
of 1883, and in the Specification of Hopkinson No. 4718 of 1882. Our system
embodies the following improvements :-
Firstly. While employing the main motor current to operate the switches during
the ordinary running of the car, we make it impossible for the switch to be actuated
or kept on after the passage of the car, by leakage from the stud or sectional rail;
and this we accomplish without the employment of a second system of studs or
sectional rails independently connected to the coils of the automatic switch.
Hence we gain in security without sacrificing simplicity.
Secondly. We employ an automatic switch which can at the will of the car-
driver be actuated by the motor current before or after the car reaches the stud
or sectional rail to which that switch belongs; the switch being constructed so as
to open automatically after the car has passed that stud or sectional rail.
Thirdly. By the employment of a guard-plate, connected as hereinafter described,
we not only eliminate the risks arising out of possible leakage of the current
during bad weather from the studs or sectional rails while they are alive, but also
are enabled to dispense with any second set of devices for disconnecting the studs
or surface contacts after the car has passed.
Fourthly. By placing the coils of the automatic switches as the only parts of
the circuit between the stud or sectional rail and the neighbouring guard-plate,
we are able at starting to actuate the switches by a comparatively small auxiliary
battery on board the car.
Fifthly. By putting between the main and the stud which supplies current to
the car, in series with one another, (1) a coil on the automatic switch thro' which
for the moment current.is being supplied to the moving car, and (2) a coil on the
automatic switch at the point next ahead of the car, we ensure the prompt and
certain operation of the switches in due succession.
Sixthly. In cases where-as for single-track tramways-it is desired that the
cars shall run in either direction, we connect three coils in series; one being on
the automatic switch through which for the moment the current is being supplied,
the other two being on the switches immediately ahead in either direction.
Seventhly. In cases where while the.main motor current is taken.through studs
or sectional rails, the automatic switches are operated by an auxiliary current from
accumulators or other local source of current on board the car, we cause this

page_text: auxiliary current to operate not only the switch pertaining to the stud or sectional
rail over which for the moment the car may be passing but also the switch pertaining
to the stud or sectional rail next ahead. This we accomplish by the same device
of connecting in series a coil on the magnet of the one switch and a coil on the
magnet of the 'other.
Eighthly. We also employ simple safety devices which, in case any switch
should by unforeseen circumstances remain closed after the car has passed, will
give an alarm, or will cut the connection with the main cable or feeder."
We are aware of other systems having some of these advantages, but we know
of none in which they can all be combined.
The general scope of our improved system will be understood by the example
diagrammatically represented in Fig. I of the annexed drawings, which relate to
the typical case of a car provided with a skate to take up current from studs laid
in the track at intervals of say fifteen or eighteen feet apart; the return circuit
from the motor on board the car being by the rails, which should be suitably
bonded in the usual way to form a continuous return conductor.
The main insulated conductor or feeder main is marked F. C, is one of the
working-contacts of the automatic switch: it is mounted on insulating supports
and permanently connected to F. G is a guard-plate which wholly or partially
surrounds the contact-stud S, but is insulated from it, and is itself sufficiently
insulated from the surrounding ground or track., To the guard-plate is permanently
connected C, the second of the working-contacts of the automatic switch. The
-operation of the nutomatic switch, when actuated is to establish contact between
C and C. The mechanism for effecting this may be of various kinds, as is well-
known to all electrical engineers. No claim is made in the present specification to
any particular form of automatic switch, but the drawing is intended in general
to indicate that when a current passes around either or both of the magnet coils
. marked M, and M, their electromagnetic action will move a working armature,
lever, or rocker R and thereby raise a contact block so as to bring C, into electrical
communication with C. Any other suitable electromagnetic mechanism which
will effect the same end may be used. In Fig. I switch No. 1 is represented as
open, while switches Nos. 2 and 3 are closed. On each automatic switch are two
magnet coils, M, being the "actuating" coil and M, the "sustaining" coil.
The guard-plate G is connected to one end of the sustaining coil M₂, the other
end of that coil being connected to the actuating coil M, of the next switch, and
a connexion from the other end of M, is brought back to the stud S.
It will be seen that the current to pass from G to S must pass through two coils
in series, namely M, of one switch and M, of the next switch. K is a skate carried
beneath the car, and depressed so as to touch in succession the studs S, to receive
current from them. It is made long enough to bridge two studs. From the
skate the current passes through the car motor (not shown) with the usual controller,
from which it returns. to the generating station in the usual way by bonded rails
or by return feeders from the rails. The method of starting the car is not
indicated in Fig. I; but is described later.
The operation of the system is as follows:-
In Fig. I the car is supposed already to have passed over the first switch, during
which time switch No. 2 was actuated and closed so that current flowed from the
main F to the guard-plate G, thence through the sustaining coil M, to keep the
switch closed after the car left the first stud, then through the actuating coil M₁ of
the switch next ahead, then back to S and so to the skate K and through the motor
of the car. Hence switch No. 3 is actuated in readiness while the car is still passing
over No. 2.
The car moves on in the direction of the arrow until the front of the skate K
comes into contact with stud No. 3, when some of the current flows through it and
actuates switch No. 4 and so forth. As soon as the skate K leaves the stud of
No. 2 the current in M, is broken, and the lever of the No. 2 switch falls and opens
the contact between C, and C, so that neither the stud S nor the guard-plate G

page_text: of No. 2 are left alive. The presence of the guard-plate prevents the switch from
being actuated or sustained by any stray current leaking through mud or water on
the road-way; for any such current will merely flow from the main to the guard
plate to earth and will not traverse the magnet coils; and no appreciable current
will leak from G to S by reason of the small difference of potential between
them.
Hence it is important that the two coils that are in series namely the actuating
coil of one switch and the sustaining coil of the next, should be connected between G
and S, not between C, and G, nor between F and C₁.
It will be evident that the magnet coils do not operate except at times when the
two following conditions are fulfilled, (1) the lever R of one of the switches must
cause contact between C, and C,; (2) the stud S must communicate with a
conductor which without touching the guard-plate, will carry to the bonded rails a
certain current, namely the minimum current, (about 2 to 3 amperes) needed to
sustain the switch.
These two conditions are fulfilled when the car is standing or moving over one
of the studs; but cannot be fulfilled after the car has passed, by any vehicle passing
over the stud.
If the main current is at any time broken or falls below the minimum value all
the switches will be open and none of the studs alive. Hence for starting it is
needful to pass an auxiliary minimum current through the switch under the car.
Such current is on such occasions preferably supplied from a small auxiliary battery
of accumulators on board the car. This current is conveyed by an auxiliary
skate K, shown in Fig. II, from the guard-plate G G to the stud S, and operates
the two switches without passing either through the motor or through the contacts
of the automatic switch. For this purpose the guard-plate has a projection on one
side such that the auxiliary skate K, can conveniently make contact with it. This
is shown in Fig. II which is a section of the guard-plate, stud, and skates taken
at right angles to the direction of the track.
In cases where the use of contact-rails cut into sections in contact with brushes
on the cars is preferred to the use of studs, and skates ou the cars, the system here
described is equally applicable. The only necessary alteration being the substitu-
tion of a rail enveloped on all sides except the top by a metal covering or guard-
plate from which it is insulated; such metal covering to be itself sufficiently insulated
from the road, and the whole cut into sections each of which would be connected
to the switches as shown in Fig. 1. Two sets of brushes would then be employed
one rubbing on the contact rail collecting the motor current which would work the
switches in the ordinary course, and the other rubbing on the guard-plate, this
latter being only needed for the purpose of starting
The term guard-plate wherever used in this specification is intended to mean and
include any conductor surrounding or partly surrounding another conductor capable
of being connected to the source of electric supply operating so as to prevent
leakage from that other conductor in the manner hereinbefore described.
It will be seen that a system of switches and interconnections as shown in Fig. I
could be worked continuously by the accumulators or other source of electric
supply carried on the car, and by a slight modification of the connections (as for
instance by taking the main current from the projection of the guard-plate Fig. II,
and using the stud S merely for the accumulator current), there would be no
occasion to pass the main current through the coils at all. The system would then
belong to that class of electric traction systems in which the switches are worked
not by the main current but by an independent source of electric supply, and it
would be superior to other systema of that class, in as much as, by having the
inter connections with the actuating coil of the switch immediately in front, that
switch would be allowed a small interval of time to close before it was called upon
to deliver current a matter of great importance where high speeds are to be attained.
In the case shown in Fig. I if it should be necessary the car can be run backwards
the switches being actuated by the accumulators.

page_text: For single track working where it is desired to run with equal facility in
either direction the magnet of each switch is furnished with coils M₁, M., and M
as shown in Fig. III. Of these M, is a sustaining coil as before M₁ is an
actuating coil connected to the next switch in one direction, and M, is an
actuating coil connected to the next switch in the other direction. In this case
there are therefore three coils on three separate switches connected between each
stud and its guard-plate.
In the case illustrated in Fig. I where it is desired to give the driver of the
car power to either actuate a switch before the car reaches its corresponding
stud or to cause such switch not to come into action until after the car has
covered its corresponding stud, we divide the skate into two or more portions
either insulated from each other or connected by means of resistances and by means
of a switch, or contrivance for lowering or raising the said portions, enable the
driver to draw the current for the motor from either one or more portions. As an
example we will assume that a car is 32 feet in length overall. The skate K may
be divided into any number of portions for convenience of support or otherwise;
but for our purpose, it is only necessary to consider aggregations of such portions
which virtually form two skates each 16 feet in length, one at the fore end of the
car, and the other at the rear end. If the driver draws current from the rear
skate only, then if the studs are 15 feet apart a stud in advance will not be made
alive until it is covered by the car. If current is taken from both of the skates,
and particularly if the skates being joined by a resistance the motor is connected
to the skate in the fore end of the car the switch in advance will be operated while
it is still 15 feet in advance of the car and thus greater time will be allowed for it
to close. The driver will be able to regulate the distance in advance at which the
switches are operated by controlling the number of portions of the skate lowered
in contact with the studs or connected to the motor.
Having now particularly described and ascertained the nature of our said
invention, and in what manner the same is to be performed we declare that what
we claim is :-
1. In an electric traction system the use of a guard-plate, as herein before
defined, placed around or about a stud or other conductor so connected to the
source of electric supply that the current which would otherwise tend to leak from
such stud or conductor leaks instead from the guard-plate and thereby does not
cause trouble either in actuating or holding on any switch, or in any other similar
manner.
2. In an electric traction system the combination of a guard-plate connected to
the source of electric supply, substantially as shown in Fig. I, and a stud or other
conductor permanently so connected with the said guard-plate through the coils of
an automatic switch or switches that no very great difference of electric potential
exists between them.
3. The combination of a guard-plate, stud, or sectional rail, and interconnections
with electromagnetic switches and source of electric supply, substantially as
described.
4. In a surface-contact electric traction system which employs the main current
on its way from the insulated cable or feeder to the car motor to operate automatic
switches, the use of two main current coils in series, one coil being on the magnet
of the switch supplying current to the car, and the other on the switch immediately
ahead.
5. In a surface-contact electric traction system the use of three permanently
interconnected coils; one coil being on the magnet of the switch supplying current
to the car, one on the switch immediately ahead in one direction, the third on the
switch immediately ahead in the other direction.
6. In a surface-contact electric traction system, the arrangement of circuits as
between the feeder or main conductor and the car motor such that in each such
said circuit the motor current passes from the main first through the working-

page_text: contact of a switch and afterwards through the coils of two or more switches in.
series on its way to the car-motor.
7. In a surface-contact electric traction system having the arrangement claimed
in the preceding claim of a circuit passing first through the working-contact of a
switch and afterwards through the coils of two or more switches, the combination
of the said arrangement with a guard-plate as hereinbefore defined, the said guard-
plate being connected to the said circuit at a point between the said working-contact.
and the said coil or coils.
8. In a surface-contact electric traction system which employs the current from.
accumulators or other source on board the car to work automatic switches the use
of two coils in series, one coil being on the magnet of the switch supplying current
to the car motor and the other on a switch which supplies current to a stud or
sectional rail ahead of the car, that is to say a stud or sectional rail with which the
car is not in contact.
9. In a surface-contact electric traction system the use of a skate divided into.
two or more portions capable of being insulated from each other or connected, by
means of resistances or otherwise, to one another or to the car motor, substantially
for the purposes above described.
Dated this Second day of September, in the year 1896
SILVANUS P. THOMPSON.
MILES WALKER.