US1902163A - Switch control mechanism for automatic telephone systems - Google Patents

Description

March 21, 1933. Q L GOODRUM 1,902,163

SWITCH CONTROL MECHANISM FOR AUTOMATIC TELEPHONE SYSTEMS Filed March 51, 1932 3 Sheets-Sheet 1 H ls ATTORNEYS u 6% I2I.VENTOR M) Q BY 4 March 21, 1933. Q GQODRUM 1,902,163

SWITCH CONTROL MEHANISM FOR AUTOMATIC TELEPHONE SYSTEMS Filed March 51, 1932 3 Sheets-Sheet 2 by INVENTOR MA A m ATTORNEYS March 21, 1933.

c. L. GOODRUM 1,902,163

SWITCH CONTROL MECHANISM FOR AUTOMATIC TELEPHONE SYSTEMS Filed March 31, 1932 3 Sheets-Shet 3 INVENTOR d544 .aima

H IS ATTORNEYS.

Patented Mar. 21, 1933 UNITED STATES PATENT m nor:-

cnnnnrzs L. eoonnnivr, or ATHENS, GEORGIA, Assrenon or ONE-HALF T Louise."

PACENT, OF NEW YORK, N. Y.

SWITCH CONTROL MECHANISM FOR AUTOMATIC TELEPHONE SYSTEMS Application filed March 31, 1932. semino. 602,189.

mechanism for automatic telephone systems,

which is capable ofoperation on low-line 3 voltages such as 22 volts as compared with the 48 volts now used, and one which is less afiected by line losses of current usually encountered in'practice.

A further objectof my invention is to supil ply a circuit arrangement which permits greater tolerance in the operating characteristics of the various sets of contacts which are called into play in controlling the operation of a switch. By means of my invention the present-day practices of establishing a very careful proportioning of the time-ofdwell or holding of the making contacts,

which close an operating circuit with reference to the period in which acooperating set of breaking contacts are opened, is avoided.

This is a feature which lessens the original cost of assembly of the apparatus and the installation of the system and materially re duces the subsequent cost in the maintenance of the. telephone lines. Further, my inven tion provides a switch mechanism which is less dependent than the present systems upon accuracy in the regularity with which the operating impulses are transmitted by the subscribers dialing device. I r

To these and other ends my invention consists in further improvements, all as will be fully described in the following specification, the novel features being pointed out in the appended claims. V.

Fig. ,1 is a diagrammatic illustration of a switch showing, the ,manner in which a connection is established between two subscribers stations and the means employed for effecting the step-by-step movement of the switch wipers and their release upon the termination'of acallr r Fig. 2 is a view showing the] essential mechanical elements. of a switch mechanism 5' Fig. 3is a sectional view thereoftaken on line 3w30c of Fig.2; V

Fig. l'is a diagrammatic viewfof a'sub scribers impulse transmitter, and V Figs. 5 and 6 are schematic illustrations of certain modifications of the circuit layout of Fig. 1.

Simllar reference characters in the several figures-indicate 'slmilar parts. a

It is well recognized in engineering practice employed in the manufacture, installation and'mai'ntenance of the present two-wire a-utomatic telephone systems that certain standards in line voltages and relative timing ad-Q ustmentsof certain contacts controlling the,

operating, circuits must be maintained in orderthat unfailing accuracy in the stepping movement of a switcher a series of switches employed for establishing the connection of a calling partys line with a desired calledpartys line may be effected. In the operation of the present-day systems, in which the switches are controlled over the two wires" comprising the subscribcrs telephone line, certain switch-control circuits are established at the central otlice by means of a so-called impulse relay comprising an armature which vibrates in response to lnterruptlons created on a telephone line by the subscribers d alingequipment. In its normal or released position this armature engages a Contact closing a circuit through the motor magnet of the switch and when said armature is at tract'ed engages a forward contact to close a circu t through an associate slow-to-release,

relay. 7 The latter is energized upon the clos ing of the subscribers line and is maintained energized during the transmission of cur'- rent impulses to maintain the continuity of the motor magnet circuit until after the completion of the dialing operation.

1 In order to insure the positive operation of the contacts on the quick" and slow relays just mentioned particularly, and to an equal extent insure the proper operation of the re-.

lease and switch steppingmagnets, it has beendeemed essential to use a comparatively high line voltagea voltage which is jfair greater thanthatrequired for c'ommunica tion purposes on. the telephone lines. In

addition to this, it has been found necessary to construct the'subscribers dialing device or impulse transmitting mechanism to: opcrate within very accurate limits so that practically no variation in their operation is a1 lowed over a group of impulses. In order to insure the transmission of the required voltage to the motor magnet circuit and the associate slow control relay a very careful timing of the armature of the quick operat ing or pulse relay is required. It will be sufficient for illustrative purposes in this case to state that the duration of the time in which the armature is allowed to remain attracted in the make position (during which the telephone line is closed by the dialing device contacts.) is one-third of the unit time, and the period in which said armature is allowed to rest in engagement with its back contact (the line circuit being then open at the dialing contacts) is the remaining twothirds of this unit. Great accuracy in timing the driving mechanism by which the dialing contacts arejopened'and closed must also be maintained so that in the transmission of the entire series'of ten digits the variation in successive operations shall not be more than one-tenth of a second. The reason for the allowance of the greater'portion of the unit time when the dialing contacts are closed and thearmature of the control relay is attracted is to allow for thejmovement of the armature and then permit current saturatlon of the associate slow relay and motor magnet to be accomplished.

f In order to obviate many of the difiiculties resulting from the limitations imposed by reason of these adjustments and current requirements, I have devised certain improvements which will become apparent from a description of my new system as a whole.

To facilitate the description of my improvide circuits I will first refer to certain details of a switch mechanism suitable for use in connection with them. In Figs. 2 and 3 I have shown schematically certain necessarily associate parts of a switch construction comprising an ar'cuate terminal bank in which insulated terminals are arranged in vertical sets, such as indicated by 1, 2 and 3 (Fig. 3) the first two comprising terminals for an incoming subscribers line wires and the third being for a busy test circuit for such line. In practice it will be understood that where the system is extended to a point where more than'one switch'is required for interconnecting a large group of subscribers serves to restore the switch wipers to their tacts. When the switch is in its normal po sition springs 8 and 9 are closed against their cooperating contacts 86; and 9a and spring 10 is disengaged from its contact 10a.

The circuit condition controlled by each of these several contacts is reversed as soon as the switchis advanced one step. i

In Fig. 4 I have shown diagrammatically the subscribers dialing device or transmitter. The latter comprises a rotatable member 13 adapted to be drivenin one direction by the tension of a spring 14. An oscillatory setting member in the form of a ratchet wheel 13a it is restored to its original position causes the actuator 13 to be driven through the pawl 13b. 'Thedialing device also embodies a pair of contacts through which the telephone line is normally-closed and comprising a spring 18 supported in a holder 19, and carrying on its lower side a stud 18?) which when acted upon by the member 13 will disengage the contact 18a.

The subscribers line A, B when in use extends through the normally closed contacts-18, 18a as shown in Fig. 1 and includes the subscribers telephone apparatus comprising the transmitter T, receiver R and the usual switch hook contacts H.

The subscribers circuit extends through relay contacts to be further described and terminates in the switch wipers 1a and 2a. In order that this line may be called by other subscribers, terminals are allocated to it on the switch bank. For the purposes of description I have also illustrated in Fig. 1 a called partys line A and B which represents one ofseveral which termin ate on different sets of switch terminals 1, '2 and 3. If the system requires two or more switches (a sec- 0nd switch being indicated at E) the corresponding terminals of the various lines are connected in multiple'as will'be understood.

In a telephone systemjdesigned to accommodate, a considerable number of subscribers, itis desirable to use aproportional number of switches which are. very much less in number than are the lines which are served,

and to provide for the automatic selection of one of the group of lines on which a call may be initiated. This is accomplished by a finder switch the function of which is to connect the calling line to an idle trunk leading to a connector switch over which the desired connection can be effected bythe transmission of the required number of impulses repli-ZO nals A B of a line finder switch F to which the conductor A and B are joined by wires A. and B. The wipers f and f of the finders switch are terminals of a trunk, comprising the conductors A B leading to the connector switch control mechanism at which point they lead through the main control relay A and its associate control relay'C. The effect of the operation of the linefinder switch F is to extend the calling line and to place the control relays of the connector switch in direct connection with the two sides of the subscribers circuit and also to extend said circuit beyond this point, by conductors A, B through relays which are ac tuated when the called party answers so that a talking connection is established between the calling and called partys stations through the advanced position of the connector switch wipers.

The function of the dialing contacts of the impulse transmitter is to provide means for intermittently opening the circuit of the subscribers line conductors inone-of which I provide a main relay A and in the other an associate control relay C. ,The former is in circuit with battery Xwhich is theusual central office storage battery having a' grounded connection at leading fromits positive pole and the latter relay is connected to the opposite or grounded'pole of the battery as indicated ate. A common battery is employed for supplyingall of the circuits which are referred to hereinbut in order to simplify the illustration separate batteries are shown with appropriate ground connections which indicate a common return for the several circuits. i

The main and associate central relays may be of a similar construction but as themain control relay is on the wet'or battery side ofthe line it will when energized operate to close a pair of contacts a and the associate control relay on the dry side of the line, serves to open a. pair of contacts 0 The contacts a apply a groundedconnection a through conductor 20 thei'zoil oi a slow to release relay D to battery X9 and its ground. On this relay are a pair of normally open-contacts (Z one of which isgrounaed at (Z and the other being'joined to the armature of the pair of contacts 0 bywire 20 Relay is rendered sluggish in the release of its 'contacts d by providing its core with a jacket ofcopper or other non-magnetic metal the purpose being to enable it to maintainiits contacts 0? closed "during the vibrations of thearmature of the pairs of contacts a and '6 movement of the motor magnet of the switch in response to impulses transmitted by the The contacts 0 control-the stepping action of the subscribers dialing device and their operation must be positive. There is no-requirement for the contacts a ,to vibrate but as the circuitthrough relay A is opened simultaneously with that of relay C, I re-. duce the spring tension on thearmature of contacts a tosuch a'weak point of adjustment that when a series of impulses traverse the subscribers circuit it will either hold over in its attracted position or possibly flutter only slightly without opening the circuit for a sufficient lengthof time to rob theslow relay of current. This adjustment is such that at no time during the transmission of a series of impulses will said armature a break the energizing circuit of relay D for a length of time sufficient to allow it to become clemagnetized. This is considered an important feature as the contacts a are making. or norma ly open contacts, while those of contacts c v are breakingyor normally closed contacts, andby placing them upon separate relays Y of similar electrical characteristics a balance on the two sides of the line can be maintained and there is no need of considering any differential operation or relative timing of the movement of the ,armatures of said pairs of contacts. Thus it will be seenthat Tam able to avoid the first cost incurred in building. a dialing device which controls accurately withinclose limits the open and closed periods on the subscribers line @and togreatly reduce the second cost involvedjin the maintenance of these dialing devices and the central ofiice apparatus in service. It will also be appreciated that in avoiding the necessity oflacc'urately timing the relative movement of-the contacts on the two control re ays their operation will not be so greatly aiiectedby losses in curent due to leakage from one or theother of the line conductors due to themore common causes experienced in telephone exchange operations.

For'advancing the switch I employ a motor magnet M having an armature m which is" attracted against the tension of a spring m and carries a pawl or finger m which engages with the-teeth of the ratchet wheel 5 toadvance the switch wipers one step upon each'impulse of current transmitted to themagnet.

In order to lock the-switch in its operated position, andv subsequently permit it to be restored to'its normal position, I arrange the release magnet, L and its circuit in such a mannerthat' it is -capable of performing both of these functions. Therelease magnet hasa pivoted armature Z carrying a detent or i dog liwhich is'normallyheld out of engage ment with the ratchet wheel 5when inthe re- Itracted position bya pin-Z Upon the .at-

traction of the armature offthis magnet the pawl is relieved and allowedto moveupward ly under the infiuence'o-f a :springzli. The relay magnet L. is connected :at one terminal to battery X and at its opposite entremity to the conductor 21 leading to contact 8a of the off-normalspring 8. From spring 8 a conductor 22 leads to conductor 20*. Hence it will be seen that when the main control relay A and the slow relay D which it controls is initially energized by closure of the sub scribers line the magnet L is energized so that the pawl Z engages'the first ratchet tooth and as the ratchet wheel 5 is subsequently advanced it will be held in the operated position. In order to hold magnet L energized after the off-normal contacts 8 and 8a are broken a branch circuit" is provided, as indicated by the wires 23 and 24 which extend to a pair of contacts L which are closed upon the energization of magnet L and thereafter serve to bridge the first mentioned contacts 8 and 8a in this circuit; In practice I prefer to make the release magnet slow to deenergize (in the manner described with reference to the relay 0) as this contributes somewhat to the assembly, testing and maintenance of the apparatus but it will however be understood that this magnet need not'be retarded in its action.

The motor magnet is intermittently energized by the closing of the contacts. 0 on the associate control relay C, but as this relay remains energized over the closed subscribers circuit after the transmission of the last impulse from the dialing device, it is necessary to provide in the circuit of the motor magnet means for maintaining its continuity during the period the impulses are being transmitted, said means being so arranged that itis actuated upon the first'impulse transmitter and disconnected automatically upon the completion of the group of impulses. To this end I provide in the motor magnet circuit a separate control relay 0. This is a slow to release relay, the core of which is provided with acopper sheath or equipped with a copper slug which will serve to retain the magnetism therein and delay the opening of its contactsfafter the circuit of the relay is opened. One'terminal of this relay is connected by conductor 25 to the offnormal contact 9 from which a circuit ex tends through spring 9 by way of wire 26 to the back terminal of the impulse contacts 0 The other terminal or relay 0 is connected by conductor 27-to motor magnet M which receives current impulses from battery X A pair of contacts 0' on relay 0 are closed when this relay is energized and after the first step of the switch bridge the openingin the circuit created-by the opening of the off-normal contacts 9 and 9 Consequently, upon each closure of' the associate control relay contacts 0 an impulse of current traverses the'motor magnet M and its associate relayO'through the bridging contacts 0. Theimpeded movement, or sluggish release operation of the relay 0 causes it to hold over during the rapid vibration ofcontacts 0 and it only releases its sustaining circuit contacts 0. when the contacts 0 are open permanently.

In order that the calling subscribers line may be rendered busy and access thereto prevented after a subscriber has set up a connection with a desired called party I provide means for grounding the third or associate line'terminal 3 of the switch through the closed terminals d of the slow relay D. This busy circuit is rendered operative upon the cessation of movement of the motor magnet by providing on the motor control relay 0 a. pair of contacts 0? which are held open whilethis relay is energized and when released serve to extend the circuit through the then closed pair of off-normal contacts 10 and 10a. The latter is'connected by conductor 28 to the back contact'of the pair of contacts 0 the armature of whichis connectedby wire 29 to conductor 24- grounded at 01. Leading from spring 10 is a conductor 30 connected to back contact 31 which is engaged by an armature 32 from which extends conductor 33 terminating in the switch wiper 3 7 When the switch is in its normal position contacts 10 and 10 are open so that subscribers line A, B is free for selectionby any other callingyparty, but after thisline l1as;completed a call a grounded condition of switch wiper 3 is eifected so that an attempt- 7 ed selection of the correspondingset of termiof the busy test relay P which is fed by bat= tery X the function of said relay is to prevent the passing of ringing current as the switch wipers step over successive sets of line terminals.- There are two windingson the busy test relay indicated by p and go both of which are connected to the battery X and additional contacts p and :0 the latter cooperating with the armature 32 and serving to keep'relay P energizedvwhen the switch wiper 3a is advanced into engagement with a grounded or busy terminal of a called line.

The ringing current for signaling the called partys line is controlled by the ringingrelay Qsupplied with current from the battery X and its circuit terminates in-the back contact 9 of armature'g which in its forward movement engages the forward contact p of the busy test relay P. The ringing current apparatus may be an alternating current ringing machine N and a source of direct current N the circuit for which termimoans nates Y in armatures g g on the relay Q. When the latter is energized said armatures engage with forward contacts which in turn are connected'to the called partys line by the conductors 36 and 37. A transformer N has itstwo windings included in the A. C. and D. C. circuits just mentioned so that the A. C. ringing current is dispatched when the armatures 9 g are attracted. In order that this ringing current may be applied intermittently there is placed in the A. C. circu1t a commutator n. Before tracing this ringing circuit reference should be made to additional instrumentalities by which the calling partys circuit is cut through in such a way that when the called party answers the ringing interruptions are automatically discontinued. The cut through relay S is supplied with current from a battery X through the resistances andby means of the conductor 38 (whichis a continuation of conductor 30, the completed circuit may, be traced, to the ground dthrough the closed slow .to release relay contacts d. However, in order to prevent-this circuit from causing the magnet to be energized before the called party answers said relay is short circuited through one of its own contacts and a pair of. contacts on the ringing trip relay T'. The cut through relay S is provided with an armature 8 connected to the switch wiper 1 bywire 39 and a similar armature s connected to wiper 2 by wire 40. Armature s normally engages its back contact 8 connected by a wire 41 with one terminal of the winding of the trip relay T, the opposite extremity of which is connected to one of the ringing conductors 37. The second armature s normally engages the back contact 8 which is the terminal of the other ringing conductor 36.

A third armature s5 is mounted on the relay Sv engaging a back contact 8 which is also connected to the conductor 38. On the relay T is an armature t engaging theback contact t. The armature t is connected by the conductor 43 to battery X through resistances and thearmature s and back contact t are joined by the conductor 44. By tracing the circuit from grounded conductor 38 through contact 8 armatures 8 con ductor 44, contact t, armature 1;, wire 43, resistance s to battery X and ground it will be seen that while the ringing trip relay T is deenergized ringing current will be transmitted to signal the called partys line because a short circuit exists to render the cut through relay S inoperative. When the called party answers the relay T is energized by lowering the'line resistance and the attraction of'the armature 6 opens the short circuit and permits the relay S to operate to attract its series of armatures. The pulling up of the armatures s and s connects the conductor 39 and 40, leading from the'switch wipers, to forward contacts 5 and s from which lead conductors 45 and 46 toarmatures u and u on the called partys answering relay U. The forward contacts of thesearmatures are terminals for the calling line A,

B. ;The relay U is provided with two Windings a and it each of which is connected at one end to the conductors 45 and 46 and at their opposite extremities to the two poles of the battery X Consequently these arma-f tures are attracted simultaneously with the armatures of the cut through. relay S and the calling partys line is extended throughthe switch wipers'to the called line. Upon the called party" answering hisline is closed at the switch hook contacts I-I and the current 'which then" flows over it from battery X holds the armatures u, u and 8', 8 closed to permit conversation'to be carried on between the two telephone stations.

1 If the called partys line is in use the switch wiper-3 comes to rest on a busy or grounded terminal :with the result that a circuit is established through the battery X the winding pand the armature 32 before thelatter is released from the posi- 1 tion in which it 'was attracted when the winding p is energized by the closing ofv the test contacts m upon each step of the motor magnet. magnet energized by the ground picked up by switch wiper 3 opens the circuit of the ringing relay Qby holdingthe armature g attracted. The busy test relay P under these circumstances applies a busygtone to' thecalling partys line from a specially designed commutator V in an induction circuit, which is closed by the armature 1) on relay P to dispatch the busy signal over a conductor 49 to the callin subscribers line.

The main control relay and its associate relay C may be similarly constructed so'far astheir electricalconstants and mechanical features are concerned'it being only necessary in practice to adjust the relative spring themselves to various circuit adaptations as The retaining of the busy test"* shown in Figs. 4 and 5 will be seen from the I illustration. In Fig. 4 I have. shown the main control relay A 11 and its associate control relay C 10 as provided with'double windings employed for line balancing purposes.

Fig. 5 is submitted as anillustration of the manner .in whichthe main and associate control relays A and C 100 may be incorpore-ted in a single relay structure. Here the windings are included in the two sides of the .subscribers line and arranged upon a -common core. Opposite the endv of the core which serves as an associate control relay I provide the impulse contacts C 200 controlling the motor magnet circuit.

In this illustration I have also shown schematically another way for arranging the main control relay contacts so that it is immaterial whether or not the controlling armature flutters. In this case the armature of the main control relay A110 is indicated by ecarrying at its lower end a pin g which at its free end abuts a switch lever h. Armature e is retracted by a spring 6 and the lever h is moved forwardly by the action of a separate spring h into engagement with a forward grounded contact 2'. These two elements servingthe function described for the main control relay contacts a The free end of pin 9 in its retracted position engages the face'of the switch lever h and as the spring 72/ is of less tension than that of spring 0 the latter will normally cause said lever to move out of engagement with the contact 2'. The distance through whichthe armature c is moved is sufficiently great-er than the distance between lever h and its contact 7: so that a considerable movement is permitted the armature e if it tends to flutter without causing pin 9 to cause thedisengagement of the contacts h and 2'.

What I claim is 1. In a telephone system, the combination with a switch mechanism comprising a motor magnet for advancing it and a locking magnet for holding it in its adjusted position, a pair of subscribers line conductors, means for transmittingcurrent impulses thereover,

of main and associate control relays in said conductors, each having sets of contacts positioned to operate in reverse directions when said relays are energized, one of said sets of contacts-being given a normal spring tension and the other a relatively weak spring tension and a slow to release relay arranged to be with a pair of subscribers line conductors each having a control relay provided with contacts, said relays being simultaneously energized when said line is closed and operating to open and close their respective pairs of contacts, one of said pairs or contacts having a given normal adjustment and the other a relatively weak spring tensionand a slow to release relay controlled by the latter set of contacts, of a switch inechanlsm comprismg a locking magnet and a motor magnet, a

. circuit for the former controlled by the slow 8. An automatic. telephone system comprising a pair of line conductorsleading from a subscrlbers substatlon apparatus consistmg of an impulse device and a telephone apparatus having switch hook contacts, a pair of relays in said conductors simultaneously energized upon closure of the telephone circuit eachhaving a pair of contacts one of which is given a normal adjustment to vibrate positively'in unison with the current interruptions created by the impulse device and the other pair given a relatively weak adjustment tending to cause them to hold over during such interruptions, a slow to release relay controlled by the last mentioned contacts, a step-by-step switch mechanism comprising a slow to release locking magnet and a motor magnet, a circuit for the former closed by the slow relay and a circuit for the motor magnet arranged to receive current impulses to having a subscribers sub-station telephone apparatus, and an impulse device, main and assoclate controlrelays 1n said conductors,

the main relay having a pair'of normally,

open weakly adjusted pair of contactsand I the associate relay having a normally closed pair of contacts having a spring tension sufficient to positively respond in unison with current interruptions created by the impulse device, a switch mechanism embodying a slow to release locking magnet and a motor magnet for advancing .the switch step-bystep, a circuit for the locking magnet controlled by said slow relay and a circuit for the motor magnet controlled bythe contacts on the associate relay.

5. An automatic telephone system consisting of a subscribers telephone apparatus, an impulse transmitter and a pair of line conductors, similar relays in the two sides of the line each having afpair of contacts and a source of battery current for said line, a main relay on the battery or wet side of the line having contacts which are normally open and adjusted with a weak spring tension and a relay on the grounded or dry side'of the line having contacts which are normally closed and adjusted with a suflicient. spring tension to positively respond in unison with the current interruptions created by the impulse device, a step-by-step switch having a motor magnet for advancing it provided with a circuit controlled by the normally closed pair of contacts, a slow to release relay energized by the closing of the normally open pair of contacts, a magnet for locking the switch in its adjusted position having a circuit controlled by said slow relay.

6. An automatic telephone system consisting of a subscribers telephone apparatus,

an impulse device and a pair of line conduc tors, a source of battery current for said line, a main relay in the battery side of the line having a pair of normally open contacts, an associate relay in the dry side ofthe line having a pair of normally closed contacts, a slow to release relay energized simultaneously with the main relay and operating to condition a circuit through the contacts of theassociate relay, a switch having a motor magnet provided with a circuit leading through the associate relay contacts and adapted to be energized in unison with current interruptions created by the impulse device to a'cl vance the switch, a locking magnet for holding the switch in its adjusted position having 7 a circuit controlled by said slow relay and a source of current for operating said magnets.

7. An automatic telephone system consisting of a subscribers telephone apparatus, an impulse transmitter and a pair of line conductors, a source of battery current for said line, a main relay in the battery side of the line having a pair of normally open spring contacts having a relatively weak adjustment, an associate relay in the dry side of the line having a pair of normally open spring contacts adjusted to positively close in'unison with current interruptions created by the impulse device, a slow to release relay energized simultaneously with the main relay, contacts on the slow relay which are closed when it is energized to condition a circuit for the associate relay contacts when they are closed, a switch having a motor magnet provided with a circuit leading through the associate relay contacts and adapted to be energized upon their release to advance the switch, a locking magnet for holding the switch in its adjusted; position and a circuit therefor maintained I through the slow relay contacts.

In testimony whereof, I have signed this specification. o

CHARLES L. GOODRUM.

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