Improvements in Automatic Switches and Safe-guards for Electric Traction Systems.
Year: 1896

page_text: Date of Application, 3rd Sept., 1896
Complete Specification Left, 3rd June, 1897-Accepted, 3rd Sept., 1897
PROVISIONAL SPECIFICATION.
Improvements in Automatic Switches and Safe-guards for Electric
Traction Systems.
We, SILVANUS PHILLIPS THOMPSON of Morland, Chislett Road, West Hampstead
and MILES WALKER of 23, Southcote Road, London, N., Doctor of Science and
Electrical Engineer, respectively, do hereby declare the nature of this invention.
to be as follows:-
Our invention relates to improvements in automatic switches for use in electric
traction systems in which, the electric current is supplied to the car from studs or
sections of a rail laid in the road, such studs or sections being connected to the
inain conductor by means of automatic switches only while the car is above or
near them, and disconnected after the car has passed on. In cases where the
automatic switches are intended to be actuated by the main current supplied to the
car our improvements are directed to the following points.
In order to prevent any switch from being actuated by current leaking from the
stud and also to enable the switch to be actuated when starting the car without,
communicating with it electrically, we construct the magnetic parts of the switch
so that the magnetic circuit can only be completed by a mass of iron carried on the
car and we magnetically balance the said parts so that the switch cannot operate
until the magnetic circuit is. so completed. We further combine switches of this
kind with the series connections described in another Specification of Patent by us
bearing even date with this, being No. 19456 of 1896
Deted this 26th day of May 1807
W. P. THOMPSON & Co.;
Of 6 Lord Street, Liverpool, Patent Agents for the Applicants.
Improvements in Automatic Switches and Safe-guards for Electric
Traction Systems.
We SILVANUS PHILLIPS THOMPSON of "Morland" Chislett Road, West Hampstead.
in the County of London 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 improvements in automatic switches for use in those
systems 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

page_text: 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.
In the Specification of Patent of Messrs. Ayrton & Perry No. 2395 of 1883 it was
proposed to operate switches under the roadway by means of magnets carried on board
the car, and since that patent several inventors have employed devices of the sort with
variations in the arrangement of the parts. These systems may be referred to
generally as "magnetic pickup" systems. In the magnetic pickup systems hitherto
proposed, use has been made of an electro-magnet or permanent magnet carried on
the car and they have therefore been open to one or other of the following objections.
Either the poles of the magnet extend for a considerable distance under the cars
and are excited along their entire length involving the use of a bulky magnet for
which there is not sufficient room on cars as ordinarily constructed, or the magnet
pole is smaller in size and operates on its armature for such a short instant of time,
when the car is in motion, that it cannot be made to operate a thoroughly reliable
switch. Moreover the magnet windings being necessarily at some considerable
distance from the moving parts of the switch act with diminished force so that the
switches are not so reliable as in those systems in which the switches are operated
electrically by means of currents passing through coils on the switch itself.
In our system the switches are operated by the approach of a simple piece of iron
carried on the car, which closes a magnetic circuit energized by an electric current
passing through coils in the switch itself and the parts of the switch are so balanced
magnetically that it is impossible for the switch to operate until the magnetic
circuit is so completed. We are able in this way to obtain the certainty of netion.
of electrically operated switches and yet retain the freedom from danger enjoyed by
magnetic pick-up systems; for the switches will open when the car has passed
whatever current may be leaking from the road contact to earth. Moreover it is
possible on starting the car to pick up the armature of one of these switches by
means of a magnet on the car (the car being at rest) without, communicating
electrically with the switch but we prefer to dispense with the magnet on the car
altogether and pick up at starting in the manner hereinafter described.
The general mode of making our connections is shewn in Figure 1, which is
intended to represent diagraminatieally the connections between four consecutive
studs and the main cable F F. A connection is made from the cable FF to the
insulated switch contact C.. From C2, the other switch contact, a wire is carried to
the switch next ahead and round the coil M¹ of that switch back to coil M2 of the
first switch and from thence to the insulated road contact stud S.
The operation of this system of switches is as follows:-
Assume that a car is passing in the direction shewn by the arrow in Figure 1 and
that it has just passed stud No. 1 whose switch is shewn open. The skate is in
contact with stud No. 2 whose switch is shewn closed. Current is passing from the
main cable through the coil M1 of switch No. 3, through the coil Ms of switch No. 2
to the contact stud S, thence to the skate K of the car and so through the motors
of the car and to the return rail (not shewn in the figure) in the usual way. The
current in passing round coil M1 of switch No. 3 cannot completely close the contacts
of the switch, because the iron parts are balanced, in the manner hereinafter
described so that they will not make contact until the magnetic circuit has been
completed by a piece of iron on the car. This piece of iron is by preference the
skate itself, but we sometimes use an auxiliary piece of iron on parts of the car to
which it is not convenient to extend the skate. When the car advances so that the
skate K or the auxiliary piece of iron comes above switch No. 3, that switch closes
and supplies current to the skate at the same time exciting the coil M¹ of the
switch No. 4 and so forth. The size of the skate and its position relatively.to the
car are preferably arranged so that stud No. 3 is covered by the car when switch No. 2
is operated so that no piece of iron such as a horse's shoe can be over stud No. 3 at
the time when the coil M¹ of its switch is energized. When the car has passed
switch No. 2 it must necessarily drop open whatever current may be leaking from
the stud S, because such current is incapable of holding the switch closed unless the

page_text: magnetic circuit is completed in the. manner described. It is moreover impossible
for the switch to be operated by any piece of iron (such as a horse's shoe) unless the
coils are excited and the coils cannot be excited after the car has passed and the
switch opened. In order to start the car we use an auxiliary skate K2 Figure 2,
capable of being lowered upon a small auxiliary stud Z. Current from an accumulator
can then be passed from Z through M' and M2 to the stud S and the switch under
the skate operated, but to this arrangement we here make no claim. Some of the
methods of arranging the magnetic circuit so that it can only be closed or nearly
closed by masses of iron on the cars are shewn in. Figures 3, 4, 5 and 6, but we do
not confine ourselves to the methods there shewn."
One form of switch box. and stud combined is shewn in Figure 3 in sectional
elevation. The box consists of a hollow cast iron cylinder B B having a wide
flange GG in its upper end. This end is closed by a ring D D insulated from the
iron and a solid lid S fitting into the ring, both made of phosphor bronze or other
nonmagnetic material. The lid is rendered watertight by means of a rubber washer
in a groove in its seat and is screwed hard home against its seat. The lid is slightly
rounded on its surface and forms the stud or road contact. It may have a renewable
top preferably in the form of a ring to take the wear and prevent horses slipping
upon it. Upon the wide flange G and round the stud S is laid a layer of hard
insulating material such as asphalte or artificial stone. This is slightly sloped off
as shown in the figure so that when the box is laid on the road the stud lies a little
above the level of the roadway. Below the lid are fixed the two magnetising.
coils M' M' connected up as shown in Figure 1. Within these coils is an iron plunger P
rather shorter than the coils which slides in a brass tube in line with an indentation
in the stud and connected by means of.a.brass tube to a contact making device not
shown in the figure. If a piece of iron is placed over G G and S while an electric
current is passing round one of the coils, the plunger P will rise to the top of the
hole in the stud and will then complete the electric circuit as shown in switch No. 2
in Figure 1. If the iron is withdrawn, the plunger will fall a considerable distance
whatever current is passing through the coils and will so open the electric circuit.
In fact the current in the coils tends to drag the plunger down a certain 'distance,
instead of. tending to lift it and this effect may be still further increased by making
the lower end of the bobbin of iron and shaping it as shown in the figure. The
plunger will take up a position in which the upward pull on the upper end of the
plunger is balanced by the downward pull of the lower end and the force of gravity.,
When iron is placed over the top the upward pull is greatly increased. The
auxiliary stud Z (see Figures 1 and 2) is shown dotted in Figure 3.
i
Figure 4 shows another arrangement with winding. upon the. iron return circuit.
S is a stud of chilled steel capable of taking a high polish as the skate rubs over it
or covered with a renewable cap of phosphor-bronze to make a good electrical contact.
It is insulated from the iron lid with hard fibre washers saturated in soft pitch and
screwed home with great force. The iron piece R is wound with the coils M' and M².
At the lower end of R is a vertical hole lined with a brass bush in which works easily
the iron plunger P with a brass extension at its lower end. Around P are wound
other coils M and M² in series respectively with those wound on R. Above P is a
phosphor bronze cap H which may be used as a road contact instead of S if desired.
The length of P is so adjusted that whatever current is passed round the coils M
and M2 the switch lever L the end view of which is shown in Figure 4 will not
make contact unless there is a piece of iron close over H and S. In fact so far from
there being any tendency for the lever L to close contact when there is no iron
over H and S there is a strong magnetic drag tending to keep the switch open,"
which drag becomes greater as the current is increased. As soon as iron is placed
over H and S and a current of about 3 ampères passed through the coils, the switch
lever L closes the switch contacts. When the iron is withdrawn the switch opens
smartly. In systems of this kind it is not required to break the working current on
the switch contacts because the switch ahead is closed before they are opened, and
if the current is broken at all it is broken on the stud contacts. Nevertheless we

page_text: find that in practice it is desirable to have a long break and to have contacts which
will stand sparking because the switch has to break any current that may be leaking
from the stud.
In another form of switch magnet the coils are designed to move with the iron
core as shown in Figure 5, the whole being counterpoised on the switch lever as
shown in Figure 4.
Another method of carrying out our invention is shown diagrammatically in
Figure 6. Two masses of iron P P1 are laid in the lid of the switch box is the
roadway with a very poor magnetic circuit between them. From each of these a
thick iron core C extends into the switch box and is wound with the coils M¹ and M.
Across the lower ends of the cores C C and magnetically connecting them in the
thin bridge piece B B of soft iron. This bridge piece is made of such a cross
section that it can carry without saturation all the magnetic lines which will
leak through the air from P to P when a current equal to five or six ampères is
passing round the coils. Just below the bridge piece and around the poles of the
cores CC¹ are wound a few turns of wire N and N in series with the coils M' and M'
but wound in the opposite direction. The number of turns in N and N¹ is just so
great that the current passing in them produces a difference of magnetic potential
nearly equal to that required to force the said leakage lines through the bridge
piece B B. The effect of this is that when there are non-magnetic materials
between P and P very large currents can be passed through the coils without
producing sufficient pull to operate the armature A A, but if a mass of iron such
as K K shown in the figure is placed upon or near to the masses P P¹ so as to
complete the magnetic circuit, the bridge piece B B becomes super-saturated and
interferes very little with the pull of the thick cores C C upon the armature. The
method of connecting up and operating this form of switch are described above.
Another form is shown in Figure (7). In this case the pieces P P¹ have, in the
absence of the masses of iron K and K' no tendency to raise the armature A A
however strongly the coils on C C magnetise the limbs R R¹. On the contrary the
projections from the limbs R R' shown in the figure, have a tendency to pull the
armature down. But if masses of iron such as K and K' be placed on or near
to P and Pas shown, they complete a magnetic circuit which causes a very great
lifting pull upon A A.
Figure 8 shows the application of one form of our, switch above described to a
simple arrangement of circuits in which the magnetizing coil M is connected direct
in series with the switch C₁ between the main and earth, but in shunt with the
car motor.
The switch coils are in Figure 1 shown connected with one another in series with
the connections through the car motor; but it is obvious that they may be connected
in some cases in shunt thereto, or as shunts to one another. If arranged as shunts
to one another they may still be combined with the series connections described by
us in Specification of Patent No. 19456 of 1896 of even date with this. But no
claim is here made to any specific mode of combining these switch coils with the
circuit or its connections.
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. An electro-magnetic switch (for an electric traction system) the moving part of
which is operated when a mass of iron or other magnetic material carried on the
car by its mere proximity closes or partly closes a magnetic circuit.
2. An electro-magnet switch (for an electric traction system) having operating
magnets in which the parts are magnetically balanced in the manner shown in
Figures 3, 4, 5, 6 and 7 so that the moving part is not operated except by the
closing or partial closing of a magnetic circuit by a not specially magnetised mass
of magnetic material carried on the car substantially as described.

page_text: 3. The electro-magnetic switch operating in conjunction with the masses of iron
substantially as described with reference to Figures 1, 2, 3, 4, 5, 6, & 7.
4. In an electric traction system the combination of the following elements (a) a
main conductor F F', (b) switch contacts C¹ C, (c) magnetising coil or coils which
excite a magnetic circuit but not sufficiently to operate the switch, until (d) a mass
of iron carried on the car more nearly closes the magnetic circuit, rendering it
effective for operating the switch.
W. P. THOMPSON & Co.,
Of London, Liverpool, Manchester, and Birmingham,
Patent Agents for the Applicants.