THE LORIMER AUTOMATIC SYSTEM
The Lorimer automatic telephone system has not been commercially used in
this country but is in commercial operation in a few places in Canada.
It is interesting from several points of view. It was invented, built,
and installed by the Lorimer Brothers--Hoyt, George William, and
Egbert--of Brantford, Ontario. These young men without previous
telephonic training and, according to their statements, without ever
having seen the inside of a telephone office, conceived and developed
this system and put it in practical operation. With the struggles and
efforts of these young men in accomplishing this feat we have some
familiarity, and it impresses us as one of the most remarkable inventive
achievements that has come to our attention, regardless of whatever the
merits or demerits of the system may be.
The Lorimer system is interesting also from the fact that, in most
cases, it represents the mechanical rather than the electrical way of
doing things. The switches are power driven and electrically controlled
rather than electrically driven and electrically controlled, as in the
system of the Automatic Electric Company.
The subscriber's station apparatus consists of the usual receiver,
speech transmitter, call bell, and hook switch, and in addition a signal
transmitter arranged to be manipulated by the subscriber so as to
control the operation of the central-office apparatus in connecting with
any desired line in the system.
The central-office apparatus is designed throughout upon the principle
of switching by means of power-driven switches which are under the
control of the signal transmitters of the calling subscriber's station.
The switches employed in making a connection are all so arranged with
respect to constantly rotating shafts that the movable member of such
switches may be connected to the shafts by means of electromagnets
controlled directly or indirectly by relays, which, in turn, are brought
under the control of the signal transmitters.
The circuits are so designed in many instances that the changes
necessary for the different steps are brought about by the movement of
the switches themselves, thus permitting the use of circuits which are
rather simple. The switches employed are all of a rotary type; the
co-ordinate selection, which is accomplished in the Automatic Electric
Company's system by a vertical and rotary movement, being brought about
in this system by the independent rotation of two switches.
=Subscriber's Station Equipment.= A subscriber's desk-stand set, except
the call bell, is shown in Fig. 401, and a wall set complete in Fig.
402. In both of these illustrations may be seen the familiar
transmitter, receiver, and hook switch, and in the wall set, the call
bell. The portion of these telephone sets which is unfamiliar at present
is the part which is enclosed in the enlarged base of the desk stand and
the protruding device below the speech transmitter in the wall set--the
signal transmitter referred to earlier in the chapter. The small push
button and small plate through which the number may be seen directly
below the transmitter in Fig. 402, are for the purpose of registering
calls.
[Illustration: Fig. 401. Lorimer Automatic Desk Stand]
The signal transmitter is a device whose function is to record
mechanically the number of the subscriber's station with which
connection is desired, and to transmit that record to the central office
by a system of electrical impulses over the line conductors. Instead of
operating by its own initiative, the signal transmitter is adapted to
respond to central-office control in transmitting electrically the
number which has been recorded mechanically upon it.
The signal transmitter shown removed from the base of the desk stand at
the left in Fig. 403 comprises in part four sets of contact pins having
ten pins in each set, one set for each of the digits of a four-digit
number. There are also several additional contact pins for signaling and
auxiliary controlling purposes. All of these contact pins are arranged
upon the circumference of a circle and a movable brush mounted upon a
shaft at the center of the circle is adapted to be rotated by a clock
spring and to make contact with each of the pins successively. The call
is started, after the number desired has been set on the dial, by giving
the crank at the right of the signal transmitter a complete turn and
thus winding the spring. The shaft carrying the signal transmitter brush
carries also an escapement wheel, the pallet of which is directly
controlled by an electromagnet.
[Illustration: Fig. 402. Lorimer Automatic Wall Set]
The four dials with the numerals printed on them are attached to four
levers, respectively, and are moved by their levers opposite windows,
near the top of the casing. Through each of these windows a single
numeral may be seen on the corresponding one of the dials. The dials may
be adjusted so that the four numerals seen will read from left to right
to correspond to the number of the line with which connection is
desired.
The setting of the dials so that the number desired shows at the small
circular opening results in connecting the earth or a common return
conductor to one pin of each set of ten pins, the pin grounded in each
set depending upon the numerical value of the digit for which the dial
is set.
The circle of contact pins is set in an insulating disk, the signal
transmitting brush operates upon the pins on one side of the disk, and
electrical fingers attached to the dials operate upon the pins on the
other side of the disk. The escapement wheel is a single toothed disk
attached directly to the shaft which carries the signal brush and its
pallet is attached rigidly to the magnet armature.
[Illustration: Fig. 403. Desk Stand with Signal Transmitter Removed]
Once a call has been turned in, the entire subscriber's station
equipment is locked beyond power of the subscriber to tamper with it in
any way, rendering it impossible either to defeat the call which has
been started or to prevent the subscriber's station as a whole from
returning completely to normal position and thus restoring itself for
regular service. The key shown just below the signal transmitter in the
case of the desk stand, and at the right in the wall set, is for the
purpose of operating a relay at the central office which, in turn,
connects ringing current to the line of the subscriber with which
connection has been made, and thus actuates the call bell.
As the number set up at the signal transmitter remains in full view
until reset for some other number, it is easily checked by inspection
and also lessens the labor involved in making a second call for the same
line, which is frequently necessary when the line is found busy the
first time called.
=Central-Office Apparatus.= The subscriber's lines are divided into
groups of one hundred lines each at the central office, each group being
served by a single unit of central-office apparatus. In a
central-office unit there is "sectional apparatus" which appears but
once for the unit of one hundred lines; "divisional apparatus" which
appears a number of times for each unit, depending upon the traffic; and
"line apparatus" which appears one hundred times for each unit or once
for each line.
The sectional apparatus comprises devices whose duties are, first, to
detect a calling line, and second, to assign to the calling line a set
of idle divisional apparatus which serves to perform the necessary
switching functions and complete the connection.
The sets of divisional apparatus, or, as called in this system,
"divisions," are common to a section and are employed in a manner
similar to the connecting cords of a manual switchboard. The number of
these divisions provided for each section is, therefore, determined by
the number of simultaneous connections resulting from calls originating
in the section. It has been the custom in building this apparatus to
provide each section with seven divisions or connective elements.
The line apparatus comprises one relay, having a single winding, and two
pairs of contacts operated by its armature. This device is substantially
the well known cut-off relay almost universally employed in
common-battery systems. The fixed multiple contacts of the lines in the
switching banks of the connecting apparatus are considered as pertaining
to the various pieces of apparatus on which they are found rather than
to their respective lines. A good idea may be obtained of the
arrangement of the sectional and divisional apparatus by referring to
Fig. 404, which is one unit of a thousand-line equipment. The apparatus
in the vertical row at the extreme left of the illustration is the
sectional apparatus, while the remaining seven vertical rows of
apparatus are the divisions.
_The Section._ The sectional apparatus for each unit consists of three
separate devices called for convenience a _decimal indicator_, a
_division starter_, and a _decimal-register controller_. All of these
devices are normally motionless when idle. The energization of the
decimal indicator, in response to the inauguration of a call at a
subscriber's station, results immediately in an action of the division
starter which starts a division to connect with the line calling. It
results also in the starting of the decimal-register controller, the
remaining unit of sectional apparatus.
It is thus seen that upon the starting of a call by a subscriber, all
of the sectional apparatus belonging to his one hundred lines
immediately becomes active, the division starter acting to start a
division, the decimal indicator becoming energized to indicate the tens
group in which the call has appeared, and the decimal-register
controller becoming active to adjust the decimal register of the
division assigned by the division starter. The division starter having
assigned a division for the exclusive use of this particular call,
passes to a position from which it may start a similar idle division
when the next call is received. The decimal register controller makes
its half revolution for the call and comes to rest, awaiting a
subsequent call, and the decimal indicator continues energized but only
momentarily, since it is released by the action of the cut-off relay
when the call is taken in charge by the divisional connective devices.
Calls may follow each other rapidly, the connective devices being
entirely independent of each other after having been assigned to the
respective calling lines. As has been described, the decimal indicator
starts the division starter and the decimal-register controller in quick
succession. The division starter, shown at the extreme bottom of the
left-hand row of Fig. 404, is a cylinder switch of the same general type
as used throughout this system. In it the terminals of a switch in each
division appear as fixed contact points in a circle over which move the
brushes of the division starter.
The decimal-register controller has the duties of transmitting to the
divisional apparatus a series of current impulses corresponding in
number to the numerical value of the tens digit of the calling line.
This is effected by providing before a movable brush ten contacts from
which the brush may receive current. These contacts are normally not
connected to battery, so that the brush in passing over them does not
receive current from them; however, when the brush has reached the
contact corresponding in number to the tens digit of the calling line, a
relay associated with the decimal-register controller charges the
contacts with the potential of the main battery, and each of the
remaining contacts passed over by the brush sends a current impulse to a
device designed to indicate on the division selected for the call the
tens digit of the calling line.
_The Connective Division._ The connective division, seven of which are
shown in Fig. 404, is an assemblage of switches comprising, as a whole,
a set suitable for a complete connection from calling to called
subscriber. Each connective division in the unit illustrated is
completely equipped to care for a called number of three digits, _i.
e._, each division will connect its calling line with any one of one
thousand lines which may be called. By a system of interconnecting
between divisions, each division may be equipped with interconnecting
apparatus so as to make it possible to complete a call with any one of
ten thousand lines. Each connecting division of a ten-thousand-line
exchange comprises six major switches. Of the six major switches, one is
termed a _secondary connector_, another an _interconnector_, and the
four remaining are termed the _primary portion_ of the division.
[Illustration: Fig. 404. Unit of Switching Apparatus]
Before taking up the operation of the switches, the mechanical nature of
the switches themselves will be described. The switches are built with a
contact bank cylindrical in form and with internal movable brushes
traveling in a rotary manner in circular paths upon horizontal rows of
contacts fixed in the cylindrical banks. For driving these brushes a
constantly rotating main power-driven shaft is provided. Between each
shaft and the rotating brushes of each major switch is an electric
clutch, which, by the movement of an armature, causes the brushes of the
switch to partake of the motion of the shaft and by the return of the
armature to come again to rest. The motion of the brushes of the major
switches, or cylinder switches, as they are frequently called because of
their form, is constantly in the same direction. They have a normal
position upon a set of the cylinder contacts. They leave their normal
position and take any predetermined position as controlled by the
magnets of the clutch, and, having served the transient purpose, they
return to their normal position by traversing the remainder of their
complete revolution and stopping in their position of rest or idleness.
The mechanical construction of each of the cylinder switches is such
that it may disengage its clutch and bring its brushes to rest only with
the brushes in some one of a number of predetermined positions. The
locations of the brushes in these positions of rest, or "stop"
positions, as they are called, may differ with the different cylinder
switches, according to the nature of the duty required of the switch,
and the total number of stop positions also may vary. The primary and
secondary connectors, the interconnector selectors, and the
interconnectors each have eleven stop positions; the rotary switch has
eight stop positions; the signal-transmitter controller has but two.
In the six cylinder switches making up a connective division and
required for any conversation, in a ten-thousand-line exchange some of
the switches are set to positions which are determined by the control of
the calling subscriber and represent by their selective positions the
value of some digit of the calling or called subscriber's number. Others
are switches controlling the call in its progress and controlling the
switches responsive to the call. These latter switches take positions
independent of the numbers.
In addition to the major switches, there are upon each division four
minor switches termed _registers_. Each consists of an arc of fixed
contacts accompanied by a set of brushes which sweep over the contacts.
Instead of being driven by an electromagnet, the register brushes are
placed under tension of a spring which tends at all times to draw them
forward. They are then restrained by an escapement device similar to a
pallet escapement in a clock, the pallet being controlled by the
register's magnets. When a series of impulses are received by the
register magnets, the pallet is actuated a corresponding number of times
and the register brushes are permitted to move forward under tension of
their powerful propelling spring. Each register is associated with a
major switch, and the register brushes are engaged by a cam upon the
associated major switch, and are restored to normal position against the
tension of their propelling spring, the force of restoration being
obtained from the main shaft.
The electrical clutches which connect and disconnect the movable brushes
of the major switches from the main driving shaft are controlled in all
instances by circuits local to the central office. In some instances
these circuits include relay contacts and are controlled by a relay. In
other instances they are formed solely through switch contacts. In all
cases the control, when from a distance, is received upon relays
suitable for being controlled by the small currents which are adapted to
flow over long lines. In all instances the power for moving a brush is
derived from the main shaft and only the control of the movement is
derived from electromagnets, relays, or other electric sources. In many
instances the clutch circuit is closed through contacts of its own
switch and, therefore, may be closed only when its switch is in some
predetermined position. All of the switches are mechanically powerful
and designed particularly to sustain the wear of long-continued and
oft-repeated usage. This is true also of the moving parts which carry
the brushes and of the journals sustaining those parts.
_The Switches of the Connective Division._ The six major switches of the
connecting division are as follows:
The Primary Connector:--The function of this switch is to connect the
conductors of the calling line with the switching devices of the
connective division. Associated with this switch is a register termed
the _decimal register_. The one hundred lines of the section are
terminated in fixed multiple contacts in the cylinder switch of the
primary connector. The calling line is selected and connected with by
adjusting the decimal register to a position corresponding to the
calling line's tens digit and adjusting the brushes of the cylinder
switch to a position corresponding to the calling line's unit digit.
The Rotary Switch:--This is a master switch, or pilot switch,
consisting of a cylinder switch without register. Its duty is the
control of other switches and the completion of circuits formed in part
through other switches. It is the pilot switch and the switch of
initiative and control for the entire connective division.
Signal-Transmitter Controller:--The primary function of this switch is
the generation of signaling impulses of two classes. Impulses of the
first class pass over central-office circuits only and are effective
upon magnets of the divers major and minor switches; impulses of the
second class pass over a line conductor of the calling line and are
effective upon the signal transmitter at the subscriber's station. The
impulses sent out over the line to the subscriber's station cause the
brush to pass over the contacts and thereby indicate the numerical
values of the various digits set by the dials. This switch also enters
in an important manner into the circuits involved in the testing of the
called line for the busy condition. It is controlled by the rotary
switch.
Interconnector Selector:--In an exchange using four digits in the
numbers, the register of the interconnector selector is adjusted in each
call to a position corresponding to the numerical value of the thousands
digit of the called number. The cylinder switch then acts to select an
idle trunk. The switch is controlled by the rotary switch in connection
with the signal transmitter controller.
Interconnector:--This switch is similar to the interconnector selector
in design and in function. It is a cylinder switch with register. The
register is adjusted in each call to a position corresponding to the
numerical value of the hundreds digit of the number called and the
cylinder switch then operates to select an idle trunk. The switch is
controlled by the rotary switch in connection with the signal
transmitter controller.
Secondary Connector:--This switch contains in its cylinder bank of
contacts the multiple points of one hundred subscribers' lines and its
function is to connect the conductors of the called line to the
conductors of the connective division. This is accomplished by adjusting
the register to correspond to the value of the tens digit of the line
desired and by adjusting the cylinder brushes to correspond to the value
of the units digit of the line. The switch is controlled by the rotary
switch in connection with the signal-transmitter controller.
=Operation.= A brief description of the progress of a call from its
institution to the complete connection and subsequent disconnection
begins with the adjustment of the dial indicators of the telephone set
and the turning of the crank of the signal transmitter one revolution.
This act, performed by the calling subscriber, connects one of the line
conductors to earth. Immediately the decimal indicator associated with
the section in which the calling line terminates is energized and starts
the division starter. The division starter instantly starts the rotary
switch of an idle division. The rotary switch now starts the
decimal-register controller and connects to it the decimal register of
the primary connector of the division selected.
All of the above acts in the central office occur practically
simultaneously. The impulses generated by the controller are effective
upon the decimal register of the started division and, therefore, adjust
that register to a position corresponding to the tens value of the
calling line.
The rotary switch now disconnects the tens register and starts the
cylinder brushes of the primary connector which automatically stop when
they encounter the calling line. At this instant the cut-off relay of
the line is energized and the decimal indicator is released. The call
now is clear of all sectional apparatus and another call may come
through immediately, being assigned in charge of another idle division.
The total time in which any call is in charge of the sectional
apparatus, _i. e._, the total time from the grounding of the line
conductor at the sub-station until the line has been connected with by
the primary connector of some division of that section and the sectional
apparatus has been released by the operation of the cut-off relay,
approximates two-fifths of a second.
The next operation initiated by the rotary switch is the starting of the
signal-transmitter controller of the connective division, which, in
turn, adjusts the register of the interconnector selector to a position
corresponding to the thousands digit of the number of the called line as
indicated by the signal transmitter at the calling station. This selects
an interconnector serving the lines of the selected thousand.
This initial selection being completed the rotary switch readjusts the
circuits of the connective division in such manner that in the further
progress of the signal-transmitter controller, its impulses will be
effective upon the register of the selected interconnector. In this
manner, the register of the interconnector, which may be upon the same
connective division as the rotary switch handling the call, or which may
be the interconnector of some other division, as determined by the
number of the called subscriber, is adjusted to a position corresponding
to the second or hundreds digit of the number called. The cylinder
switch of the interconnector then selects and appropriates an idle trunk
extending to a secondary connector upon some connective division serving
the hundred selected.
The rotary switch again shifts the circuits of the connective division
in such manner that the signal-transmitter controller is effective upon
the secondary connector, both register and cylinder, and adjusts the
register and cylinder, respectively, with their brushes in contact with
the tens and units digits, respectively, of the number of the called
line.
The conductors of the called line now are connected through the
secondary connector, the interconnector, and the interconnector selector
to the rotary switch; the conductors of the calling line are connected
through the primary connector to the rotary switch; thus completely
connecting the lines except at the rotary switch. To effect the
connecting together of the two lines, both rotary switch and
signal-transmitter controller must pass forward into their next
positions, the connection when thus effected being made through
conductors containing a repeating coil and main battery connection for
supplying talking current to the two lines and containing also ringing
and supervisory relays.
The called line is tested to determine if busy during the short interval
in which the rotary switch takes a short step to connect the calling and
the called lines. In this step of the rotary switch the busy-test relay
is connected to the guard wire or busy-test wire of the called line, and
if that line be busy, the relay interferes with the control exercised by
the rotary switch upon the signal-transmitter controller, and the
controller is prevented from taking the step required to connect the
line. Thus, when a busy line is encountered, the final step of the
rotary switch is taken to set up the conversation conditions, but the
signal-transmitter controller does not take its final step; by this
failure of the signal-transmitter controller due to the action of the
busy-test relay, the calling line is not connected to the called line
but is connected to a busy-back tone generator instead.
Whether the line encountered be busy or idle, the connective division
remains in its condition as then adjusted until the subscriber hangs his
receiver upon the hook switch to obtain disconnection. The ringing of
the bell of the called station is done directly by the calling
subscriber in pressing the ringing key.
The disconnection is effected, when the receiver of the calling line is
hung up, by the supervisory relay in the central office, whose winding
is included in the line circuit, and whose contacts act directly to
start the rotary switch. In disconnecting, the rotary switch starts the
primary and the secondary connectors and thus instantly releases both
the calling and the called lines. Thereafter the rotary switch in
passing from position to position restores switch after switch of the
connective division to normal and finally itself returns to normal in
preparation for its assignment to service in answering a subsequent
call.