US2340809A - Electrical calculating equipment - Google Patents

Description

Feb. 1, 1944. w. HATTON ET AL ELECTRICAL CALCULATING EQUIPMENT Filed April 5, 1940 4 Sheets-Sheet l.

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Feb. 1, 1944.

72: Fflre? W. HATTON ET AL ELECTRI CAL CALGULAT ING EQUIPMENT Filed April 5, 1940 CALCULATOP A pz 4 Sheets-Sheet 4 m A RNEYS.

Patented Feb. 1, 1944 ELECTRICAL CALCULATING EQUIPMENT William Hatton and Ladislas Kozma, Antwerp, Belgium, assignors to International Standard Electric Corporation, New York, N. Y.

Application April 5, 1940, Serial No. 328L150 In Great Britain April 4, 1939 12 Claims.

This invention relates to electric systems, such as those for the control of a plurality of calculating machines.

An object of the invention is to provide for the working of equipments to be checked durin slack periods, and this without the necessity of substantially increasing the number of equipments in the'system.

In accordance with this invention an electric system provided with a plurality of similar equipments is characterised in that, when the number of equipments in use is less than a certain value, further equipments are brought into use in pairs, which work in parallelism so as to check their working, but when the number of equipments in use is more than said value, further equipments are brought into use singly.

In accordance with a further feature of the invention an electrical system provided with a .plurality of similar equipments is characterised in that the equipments are brought into use in pairs, which work in parallelism so as to check their working, but that when all the equipments are in use a further call upon the system is satisfled by the release 01 one equipment from a pair already in use.-

By similar equipments is meant those that, although not necessarily of identical construction and operation, will, in response to identical signals or inputs, give identical results or outputs.

In order that the invention may be clearly understood reference will now be made to the accompanying drawings in which:

Figs. 1, 2, 2A and a, Show in detail the use of the invention in connection with a bank of calculating machines all accessible from a number of operator's stations; and

Fig. 4 is a key diagram for Figs. 1, 2, 2A and 3.

In connection with the system to be described, it is assumed that a large house, such as a bank or department store, is equipped with a plurality of calculating machines located at one point, and makes all these centrally located machines severally available to a larger number of operators scattered throughout the building. Each calculating machine includes a line-finder to which every operators set is banked. When a call comes in from an operator, the line finders of all those calculating machines that are idle at that time begin to function, and the first to find the calling line puts its calculating machine into communication with that operators set so that it can take the job. The advantage of such a system in respect of economy in the number of calculating machines to be provided is obvious. The present invention is concerned with giving to an operator who calls during a time when the calculating machines are slack, the benefit of a check upon the work of the machine that may be allotted to him forthat job, by pairing off two machines at random from among those that are idle, so that the pair of identical machines, working in parallel, will give notice to the operator if their two results contradict one another. Ii. the trafllc increases and reaches a certain value. the checking for all further calls is suppressed. Further, if, when a new call enters, no calculating machine is available, the system provides for one of the machines in a pair to be released to take up the new job. The number of already existing pairs to be severed may simply be equivalent to the increase in the demands being made upon the system; on the other hand the pairing may be completely broken down throughout the entire system.

In Figs. 1 to 3 of the drawings, which are intended to be read as a whole being placed as indicated in Fig. 4, Fig. 1 shows an individual operator's set 08 and the line circuit LC; Figs. 2 and 2A illustrate so much of two calculating machines I and X as is necessary .for an understanding of the invention, and in particular show the line finders and associated selector portions of such calculators; while Fig. 3 shows the pairing suppressing circuit PS.

When an operator has thrown the starting key SK and has been connected through to the calculating machines, one machine during busy times and two during slack, he manipulates the various operating keys OK each representing in code a certain numerical value between 0 and 9 or a certain function such as add or subtract, and thus sends in the problem to the allotted calculator(s) over the lines A.

After the necessary calculations have been made the answer is returned over the lines B to some suitable indicator such as a printer P. The present invention may be applied to any type of calculating machine which is operable by electric signals over two or more wires such as A, and which also transmits its results as electric signals over two or more wires such as. B. The preferred type of calculator is one which is capable of performing all four basic calculations: addition, subtraction, multiplication, and division, one such calculator being described in British Patents 525,330 and 525,396 both published August 27, 1940, as well as in the U. S.

not herein disclosed, since they have nothing to do with the invention per so, as here claimed. Only so much is shown of each calculator as is necessary to understand how they are connected with the system both for receiving and sending back impulses to the controlling circuits.

One main feature of the present invention is that, instead of providing separate reserve calculators or special routine test equipment, for the purpose of testing the calculator taken into use, one of the regular calculators is paired with the calculator first taken into use, when and only when such second calculator is free and not in use, so as to duplicate all calculations and thus to indicate whether or not the calculator taken into use functions correctly, or efiectively so. Each operation is thus simultaneously performed twice and if the two operations correspond precisely to one another, one obtains an eifective indication of the correctness of the operation. The chances of two of the regular calculators being subject to the same fault at the same time are negligible.

Operation with paired checking To originate a call, the operator at one of the operator's sets B.has to throw the starting key SK. While there has been shown only a single operators set, it is to be understood that there can be a very large number of such sets, all connected to a multiple extending along th line finders of a plurality of calculators, as elsewhere explained in detail.

. Common start relay F8 (Fig. 3) operates from battery through 200 ohm resistor RI (Fig. 1). right contacts of SE, 800 ohmvresistor R2. and common start wire WI (Figs. 1, 2, 3). rs in turn operates 8T which is provided with separate front contacts for each selector circuit by which latter expression is meant the circuit which, throughthe operation of a line finder, will cause the selection of a particular calculator or calculators for the use of a given operator's set, e. g. the set shown in Fig. 1 (only three sets of such contacts being shown). The operation of ST energizes the clutch magnets LFM associated with the line finders LP of all idle calculators. e. of calculator I in Fig. 2 and of calculator x in Fig. 2A, each via the back contacts of its associated Ti and TI relays and the respective front contacts of ST. Thus the line finders LF of all the calculating machines that are at that moment idle start to rotate. Line finders employed in this organization may be of any suitable type as well known in the automatic telephone art, for example, such line finders as are disclosed in U. 8. Patent No. 1,882,981, issued to Scheiber et a1. Test potential is connected via the 200 ohm resistor, RI and left hand contacts of SK to wire 1013 which extends the corresponding 1 terminal, i. e. the third 1 terminal (which is the only one shown in Figs. 2 and 2A) ofevery free line finder LF. The connections from every operator's set are multipled over all the line finders in the usual way, so that the ten wires, wal whl, wmt, wit, which are individual to operator's set number I are connected in each finder to ten of the terminals of the third row. Only the third row of terminals is shown in Figs. 2. 2A.

The operator's sending set is connected with the multiple banks of the finder switches in the same manner in which a telephone subscriber's station is connected with the terminal banks of line finders in automatic telephone exchange systems. As shown in Figs. 1, 2 and 2A, all the wires leading to an op'erators sending set are multipled to corresponding sets of terminals in a plurality of line finder banks. Assuming that the set of terminals shown in Figs. 2 and 2A are the first set that the line finder wipers engage during their rotation, the second set of terminals of the two line finders illustrated will be multipled to a second operator's sending set. Since each operator's sending set must be accessible to more than two calculators, the sending set will be multipled not only to .the terms] banks of two line finders, but to the terminal banks of many additional line finders, preferably to all the line finders in the system.

Suppose that the first among the line finders to reach its third position and thus pick up the calling potential on its contact a is that forming.

part of the calculating machine I (Fig. 2); relay Ti in this machine will operate from such test potential on this :i terminal, brush 1 of LF, high resistanceleft hand winding of TI to ground, and will stop the driving magnet LF'M by opening its above traced circuit which passes through the back contacts of TI The operation of TI also initiates what is generally known as a "double test. More particularly the closure of the front contacts of TI grounds the 1% ohm winding-of Di which is in series with the 3 ohm right hand winding of Ti, so that this series combination (having a total resistance of 4% ohms) is now connected to the lead from brush 1 and thence to test Wire 1013 in parallel with the high resistance left winding of TI. Since the battery potential on wire :01! is coming through 200 ohm resistor RI, the low resistance load now connected thereto drops the potential of this test wire 1013 to a small fraction of its previous value so that any other TI relays which may have their high resistance windwas connected thereto in the next instant will be unable to operate. Even if by chance another TI relay became connected to start wire 1013 at the very same instant as that of calculatorI and also cut in its low resistance winding exactly simultaneously, no accidental double connection will result. For the combined current of two 4% ohm loads will reduce the voltage so low that neither Ti relay-can hold even with their low resistance windings. Thus in such case both will release and then both will reoperate, repeating this action until one gets ahead of the other and connects in its own low resistance winding at an instant when the other has only its high resistance winding connected. The 1 /2 ohm DI relays require even more current than the 3 ohm windings of the Ti relays. Thus if a repeated operation and release of two Tl relays occurs the associated DI relays will not be operated dur- 7 ing such chattering operation.

When finally one of the Ti relays (say that of calculator I) operates and stays operated three results are thereby accomplished. First the reduction in the potential of test wire wit busies this test wire with respect to the selector circuits of all other calculators, i. e. prevents the test relay Tl of any other calculator from subsequently operating, as above explained. Second this reduction of potential of rod! also reduces the energization of common start relay FS since the voltage for FS comes also from the same resistor Rl to which wjl is connected. FS however will probably not actually release at this time because it almost immediately receives a fresh energization as described below. Third the operation of TI causes operation of Di in series with the low resistance winding of TI. n

The operation of Dl operates relay Q over left outer front contacts of Di. Also it connects test potential via 200 ohm resistor RB, right hand back contacts of F, right hand inner front contacts of DI, to wire wki, which is multipled to the third in terminals of all line finders LF. At the same time it applies this same potential via RB, right back contacts of *F, right outer front contacts of DI, 800 ohm resistor RA to common start wire WI, thus giving start relay FS the fresh energization above mentioned; Hence the line finders LF of all the other still available calculating machines continue to rotate until one of them finds the k test potential. Suppose that it is the line finder of machine X which does this, then its relay T2 operates from this test potential, brush k of LF, high resistance. left hand by the calculator is accordingly thought not to be pertinent. The manner in which the signals operate printer P of Fig. i will be hereinafter described in greater detail. The operator of sender OS will be unaware of the connection or disconnection of a second calculator, since the attacnment signals will be received, whether one or both calculators of a pair have been connected to his sender.

0n receiving this indication the operator at the set OS manipulates the operating keys OK to send in his calculation problem over the four lines A. The code signals are received simultaneously by both calculating machines I and X over the brushes 0., b, c and (1, leading to the input relays Al A4, which are now connected in parallel in the two machines, over'wires wa8, wb3, 1003, 10113.

- The solution of the problem comes back to the apparatus P in the operators set 08 over the lines B, which include the relays Bl to B4, and the brushes e, f, g and It. If there should be a discrepancy between the solutions given by the two calculating machines, then one of the relays CI to C4 in one of the units will operate. For

example if the two relays Bl in calculators I and X did not agree in position, that in I being operwinding of T2, to ground and stops its driving magnet LFM by opening its above traced circuit at the back contact of T2. After successful double testing, in the same manner previously described for the TI relay in the circuit of calculator I, relay D2 in this machine operates like relay DI in the calculator I. Simultaneously the starting relay FS is shunted out again, but this time it releases, since no fresh potential is connected to re-energise it. The release of FS releases ST which opens the circuits of clutch magnets LFM of all other line finders, so that now the other still rotating line finders stop. if, of course, no other call is originated at the same time.

Relay D2 in machine X operates the ready-totake relay RT not only in the machine X but also in the machine I. In calculator x (Fig. 2A) the circuit extends directly from ground, via right make contacts of D2, to relay RT of calculator X. For the RT relay of calculator I the circuit extends from ground, via the right make contacts of D2 (Fig: 2A), right contacts of Q. brush i of LF, thence over wire wl3 to brush I of LF (Fig. 2), right contacts of Q to relay RT (Fig. 2). RT applies ground over its outer right front contacts to wire SCW which places the calculation I circuits in condition to receive codes, if these circuits are-"of a type requiring this. As well knownin the art, calculators of this type are frequently arranged to send back to the operator an attachment signal, in order that the operator may know when a calculator has been properly connected to his sender, thus indicating to him that he may now begin to depress his various sending keys. Such attachment signals are usually in the nature of a few impulses, e. g. three dots, which signals will be sent out by a sending calculator, if only one such calculator has been connected to the sender OS of Fig. 1,

but will be sent out-jointly by the first and sec- 0nd calculators, in case that a pair of calculators is' attached to the single sender OS. These attachment signals will be transmitted to the sender over the conductorsWel, Wf3, W93, WM and Wm3. Such attachment signals do not constitute an essential portion of the present invention and the manner in which they are produced ated while that in X was not operated, relay CI in calculator X would be energized from battery through front contacts of Bl (Fig. 2) fifth set of contacts from left in Q relay (Fig. 2), brush e of LF (Fig. 2), wire we3, brush e of LF' (Fig. 2A), fifth contact from left of Q relay (Fig. 2A), back contacts of Bi (Fig. 2A), to winding of Cl (Fig.

2A) Such operation of CI in machine X grounds wire wii over front contacts of Cl, second set of contacts from right in Q relay (Fig. 2A), and brush i of LF. (Fig. 2A).

Similarly, any other of the relays CIC4 in either machine would, it operated, ground the wire wi3. Any such ground on wi! will operate the error relays E in both calculators (over brush i of LF and second contacts from right in Q relay of each calculator) The E relay is connected in any suitable man nor to prevent the printing in OS of the disagreeing signals now stored in relays Bl-Bl. The particular connection illustrated is suitable if the printer P in operators set 08. is of the general class whose character selection is prepared by energisation of a number of magnets (e. g. SI,

S2, etc.) but whose printing is not effected until a mechanism by which the type wheel is positioned under the control of the five selector magnets may be of any of the types well known in prim,- ing telegraphy. U. S. Patent No. 2,127,145 to R. D. Salmon, discloses one such mechanism in which the type wheel is positioned under the control of five crank pins 22 each of which can take up one of two positions diametrically opposed with respect to a common axis, Although each pin 22 in the disclosure cited is controlled by a revolvable disc, it will be clear to one skilled in the art that it could equally be controlled by an electro-magnet having an armature with a revolution of the type wheel, whereas, upon the operation of a second, third, fourth and fifth electro-magnet the rack is driven through a distance corresponding respectively to /4, r: and part of a revolution.

The printer is equipped with mechanical delay means whereby the printer magnet PI is prevented from energizing and causing the printing of a character while the type wheel is in motion, i. e. until the desired character has been selected.

It will be noted that the energizing circuit of magnet Pl includes a front contact of one of the selector magnets SI to S4 as well as the back contacts and armatures of the E relays. It is necessary, therefore, that one of the selector magnets be energized and that the E relay be in the normal or de-energized condition. If the E relay is energized, indicating the improper operation a calculator, the printer magnet cannot becom energized. Assuming the printer P is of such typegas above described, then relay E disables the printing by removing battery from wire wm3 so that print magnet Pl cannot operate.

For other types of printers the method of disabling printing would be modified.

The E relay also closes at its right front contacts a circuit which releases th calculator or blocks it in any known manner, e. g. by having its right hand armature and contact soconnected as to blow the main fuse of the power supply feeding the calculator, so that all the printing operations determined by transmissions from this calculator will then cease, this stoppage of the printer thus serving to notify the operator who is manipulating sender OS, that the calculators to which his sender has been assigned, are not functioning properly.

The restricting relay RS (Fig. 3) operates when a new call comes in at a time when a certain predetermined number of calculating machines are in use, that is when a certain number of relays RT are operated. Each operated RT relay applies battery over RD via its outer left front contacts to point J (Fig. 3). This point is also connected to ground through resistor RE via ST left middle front contact each time ST is operated by a new call. Thus if four RT relays are operated, then when ST is operated by a new call, the four RD resistors in parallel will form one arm of a voltage divider of which RE forms the second arm, the point J thus having a. potential dependent on the number of RT relays operated. RS is connected between point J and a fixed potential point K between resistors QR and OR. A rectifier RC is included in series with RS so that RS can only operate if J is more neguntil the balance of the bridge is upset by changes in the external arm thereof, which arm comprises (connected in parallel) as many of the resistors RD, each one pertaining to an individual calculater, as may happen by the operation of the RT relays of such calculators, to be placed in this external arm. If now this external arm have its resistance lowered by the placing therein of an RD resistance in addition to those already forming such arm, the balance of the voltage divider or Wheatstone bridge is upset, thus allowing current to flow through rectifier RC in such manner as to bring about the functioning of relay RS. While the conditions for establishing a Wheatstone bridge according to this description will be apparent to those skilled in the art, it may briefly be stated that if the resistance of RE be taken equal to the resistance of QR, and if the resistance of OR be made equal to the resistance presented by the external arm of the bridge, when 60 to 70 per cent of the total RD resistors are provided by the total calculators of the system, then the balance of the brid e will be such that relay RS will receiv no current, except when more than 60 to 70 per cent of the calculators become occupied by various senders. If RS is operated, it operates RZ which has a set of contacts connected separately to each calculator, and connects ground to the wires connected to the back contacts of all the RT relays in the calculators, with the result that when a single line finder finds the calling line involved in the new call, the operation of relay Dl will bring about the following sequence of operations.

When the relay DI becomes energized, it will close an energizing circuit for the relay F exative than K in potential. The value of the opposing resistance OR can be chosen so that the current will flow in the sense of the rectifier RC, when 70% of the calculators other than I are already occupied. The arrangement just described can be considered in the light of a Wheatstone bridge. Looked at from another viewpoint, it can be considered to be a WheatstOne bridge so balanced that the current which can fiow over a single RD resistor (Figs. 2 and 2A) will be unable to establish any current fiow through the rectifier RC, in the direction in which this rectifier is capable of transmitting current. The function of the particular unit comprising resistors RE. OR, QR and the rectifier RC as well as the battery connected to resistance OR, is to establish a certain stable potential across rectifier RC, so that current cannot flow therethrough tending over the left hand inner front contact and armature of DI, the left hand inner back contact and armature of RT and the left hand outer front contact and armature of the relay R2. The relay F becomes energized and locks up to ground at the left hand and outer front contact of armature DI. In its right hand and inner armature of the back contact, the relay prevents connection of battery to the line finder brush 7c and thus to the conductor W703 over which a second calculator would otherwise become attached, as described in the first case which we have discussed.

Normally, as described in the first case, the.

Breaking of pairs When all calculating machines are in use. some singly and some in pairs, all the RT relays are operated, and the pair-breaking relay PBl releases since it is normally energized over outer right back contacts of one or more RT relays. If now a new call enters, relay PBZ operates from ground, via PBI back contacts, ST left outer front contact; and the marker RM starts to rotate from ground, via PB3 back contacts, PB2 left front contacts, SL2 right back contacts, interrupter contacts of RM to the winding of RM in order to find a calculator which is assisted by a second one. Such a calculator is characterised by having its relay DI operated and its relay F non-operated. At the moment brush RMI of RM reaches the contact representing such a cal- 1 culator in a contact bank rm, PBS operates in series with F from ground via right winding of P133, right front contacts of P32, brush RMI and contact of rm, left inner front contact of RT, left inner front contact of DI, winding of F relay to battery. The right winding of P33 has sumcient resistance so that relay F does not now operate. The operation of P33 opens th above traced circuit of RM stopping this. PB3 energises the slow relay SLI over left front contacts of PB3. SLI energises SL2 over its front contacts. This latter introduces the low resistance winding of P33, by grounding such winding over SL2 left front contacts in order to operate F. When this operates, it removes the holding potential of the assisting calculator so that this releases and attaches itself to the calling station. The starting circuit is released, and therefore RM, after the release of relays SLI and SL2 will not move further. Ii, however, there are still unsatisfied calls, RM will advance to the contact rm of the next paired calculator and will liberate another circuit in the manner explained above.

What we claim is:

1. An electric calculating system comprising a plurality of stations each having means for calling and means for transmitting signals representing a problem, a plurality -01 remotely controllable electric calculator equipments each adapted to produce signals representing a solution responsive to the reception of said signals representing a problem, switching means for interconnecting any one of said stations variably with one or two random ones of said calculator equipments, a circuit common to all of said calculator equipments, relay means in said circuit operable only when a predetermined number of calculator equipments are in use, means for setting said switching means into operation responsive to the actuation of the calling means of one of said stations, and means variably controlling said switching means so as to cause the connection of one or two of said calculator equipments to said one station under the control of said relay means.

2. .An electric calculating system comprising a plurality of stations each havingmeans for calling and means for transmitting signals representing a problem, a plurality of remotely controllable electric calculator equipments each adapted to produce signals representing a solution responsive to the reception of said signals representing a problem, switching means for interconnecting any one of said stations variably with one or two of said calculator equipments, test means comprising a relay responsive to the condition that all the calculating equipments are simultaneously in use, and means controlled by said relay and responsive to an additional call received from central station, each device having a plurality of transmitting contacts, a plurality of substations each comprising a transmitter, a receiver responsive to signals from the transmitting contacts of a calculating device, a calling circuit for initiating the connection of the associated substation transmitter and receiver with at least one or said calculating devices, a line finding switch individual to each calculating device having a set of con tacts connected thereto, a plurality of groups of contacts on each switch, circuits connecting each substation to an individual contact group, other circuits interconnecting corresponding contacts of said groups or the respective switches, means responsive to the operation 01' the calling circuit at a substation for setting all line finding switches of idle calculating devices in motion, means for arresting a line finding switch and thereby connecting an idle calculating device to the calling substation, and means effective thereafter when more than a predetermined percentage of the calculating devices are idle to maintain the remaining line finding switches of the idle calculating devices in motion, means for arresting the line finding switch of another of the idle calculating devices in position to connect the asscciated calculating device in parallel with the calculating device first connected to the calling substation, and means for stopping the line switches of the idle calculators after a sub-station has been connected to a pair of calculating devices.

5. A calculating system according to claim 4 in which additional means responsive to the initiation of a call from an additional calling sub-station are provided for disconnecting one of the pair of calculating devices connected to a substation, said last means being effective when more than a predetermined percentage of calculators are in use.

6. A calculating system according to claim 4 in which means are provided for disconnecting one of the pair 'of calculating devices connected to a substation effective when more than a predebreaking relay held in ineflective position by another calling station for disconnecting one calfrom all of the pairs of calculator equipments which are connected to the calling station.

4. A calculator system including a central station, a plurality of calculating devices at said said circuit so long as any calculating device is not being used.

'7. A calculator system according to claim 4,

in which said calculating devices are each pro-' vided with means for comparing the operation of the transmitting contacts of the respective calculating devices which are con ected in parallel to one another, while said paral el connected calculating devices are operating to 'efiect transmission oi signals to the sub-station receiver operatively associated with said calculating devices.

8. In a calculator system having a plurality of calculating devices, each or which has a pmrality of contacts connected to output circuits for transmission of signals representing successive digits of a calculated result, means for connecting a pair of said calculating devices to a single calculator control means so that each calculating device operates to solve the same problem in the same manner, a plurality or circuits interconnecting corresponding output circuits from the respective said contacts of said pair of calculating devices with each other, a relay adapted to respond to a difference in operation of said contacts of v the respective calculating devices and means to' operate said relay under control of one or more of said interconnecting circuits.

9. In a calculating system having a plurality of calculating devices of which two are connected in parallel and also connected to a common sender of calculating signals so as to cause identi- 1 cal operation of both said parallel connected calculating devices and parallel solution of the same problem thereby, there being a plurality of output circuits for each calculating device, a plurality of armatures and pairs of cooperating stationary contacts controlled by each of said devices for variably energizing said output circuits for the transmission of signals representing digits of calculations performed by the particular calculating device, means for detectinga difference in the operation of said pair of calculating devices com- '1 prising circuits interconnecting the output circults of the respective calculating devices, relay main tie-energized, and with a variation from the normal condition. one or more of said relay means will be energized over one or more of said output interconnecting circuits to indicate a ditierence in the operation of the devices.

10. In a calculator device, a plurality of problem data sending and receiving stations, 9, plurality of normally idle substantially identical calculators, less in number than said stations, switching means controlled by a calling signal from any station for connecting said station to any two idle calculators in parallel, means for sending successive signals representing digits of a computed result to a station simultaneously from both calculators connected therewith, and relay means controlled by said sending means and responsive to a significant diilerence between the signals sent from the two calculators.

11. In a calculator device, a pluralityoi problem data sending and receiving stations, a plurality of normally idle substantially identical calculators, less in number than said stations, switching means controlled by a calling signal from any calling station for connecting said station to an idle calculator, or to two idle calculators in parallel, it two are available, means for sending successive signals representing digits of a computed result to a station simultaneously from each calculator connected therewith, and relay means controlled'by said sending means and responsive to a significant diiiference between the signals sent from two calculators connected in parallel.

- 12. The calculating device according to claim 11, and'means for disconnecting from a station one of the two calculators connected therewith and so rendering it idle it no other calculator is idle when another station transmits a calling signal to said switching means.

I WILLIAM HA'ITON.

LADIBLAS KOZMA.

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