US2923833A - Selection system - Google Patents

Selection system Download PDF

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US2923833A
US2923833A US504072A US50407255A US2923833A US 2923833 A US2923833 A US 2923833A US 504072 A US504072 A US 504072A US 50407255 A US50407255 A US 50407255A US 2923833 A US2923833 A US 2923833A
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core
cores
pulses
coils
current
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Jr Joseph D Lawrence
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Sperry Corp
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Sperry Rand Corp
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    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03KPULSE TECHNIQUE
    • H03K17/00Electronic switching or gating, i.e. not by contact-making and –breaking
    • H03K17/51Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used
    • H03K17/80Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used using non-linear magnetic devices; using non-linear dielectric devices
    • H03K17/81Switching arrangements with several input- or output-terminals, e.g. multiplexers, distributors

Description

J. D. LAWRENCE, JR 2,923,833

SELECTION SYSTEM Feb. 2, 1960 2 Sheets-Sheet 1 Filed April 26, 1955 FIG. 2.

FIG. I.

Firs! Address Dlglt B Secorgd Address Dlqlf Bing Pulse- Wlre N b. Requlutmq Current 1 Current Regulating Core:

PP-l

IN VEN TOR.

JOSEPH D. LAWRENCE, JR.

AGENT Feb. 2, 1960 J. D.- LAWRENCE, JR

SELECTION SYSTEM Filed April 26, 1955 2 Sheets-Sheet 2 NrI Firs} Address Second Address '0" Diqi! Bios Pulse H Regulating Current N 30 r Til Current 32 Requlofinq Q 29 Fig.5.

IN V EN TOR.

AGENT UniteclSta-tcs Patent SELECTION SYSTEM Joseph D. Lawrence, Jr., Philadelphia, Pa., assignor, by mesne assignments, to Sperry Rand Corporation, New York, N.Y., a corporation of Delaware Application April 26, 1955, Serial No. 504,072 I 27 Claims. (Cl. 307-88) This invention relates to selection systems and more particularly to such systems in which particular outputs are selected according to which of several binary signals are fed to the input. The present invention is particularly applicable to selection systems having magnetic amplifiers or transformers as components thereof.

Selection systems of the general character hereinafter described have long been known. The prior art systems, however, are purely switching systems which select a given output according to a particular binary number fed to the input. The present invention not only performs this result but has gain at the output as well, that is, the power appearing at the output is greater than that required at the input. Moreover, in the prior art there were certain disadvantages in that when a large number of cores were involved, the impedance to the input currents varied widely and consequently the device was unreliable. The present invention provides for regulation of these currents to improve the reliability.

One object of this invention is to provide a magnetic selection system having gain.

Another object of the invention is to provide a magnetic selection system with improved reliability.

An additional object of the invention is to provide a magnetic selection system in which there is greater output than has heretofore been practical with a given input.

In carrying out the aforesaid objects, a plurality of magnetic amplifiers are provided, each with a plurality of input;;. coils. The input coils are so connected with the inputs that particular cores are selected according to known principles. By virtue of the fact that magnetic amplifiers are employed in connection with each of said cores, the apparatus may be arranged to have gain. Moreover, a current regulating winding is provided in series with each input to prevent unreliable operation of the system. Other features of the invention will appear in conjunction with the detailed description which follows.

In the drawings:

9 Figure 1 is a hysteresis loop for the core materials used for cores 12, 13, 14, and 15 of Figure 2.

Figure 2 is a schematic diagram of a simplified form of the invention.

Figure 3 is a timing diagram for the pulse generators of Figure 2.

Figure 4 is a hysteresis loop used in explaining the operation of cores 30, 31, and 32 of Figure 2.

Figure 5 is a schematic diagram of a modified form of the system.

" In Figure 2, there are two pulse generators PP-l and PP-2 which produce square wave alternating currents timed as shown'in Figure 3. There are two input switches 10 and 11 which respectively represent the first and second address digits. There are four output cores 12, 13, 14, and 15 which may be selected depending on the closure of switches 10 and 11. Switch 10 is in series with coils 16, 17, 18, and 19 respectively, on the cores 12, 13,114, and-15. Switch 11 is respectively in series with coils 20, 21, 22, and 23 respectively, .on the cores 2,923,833 PatentedFeb. 2, 1960 ice 12, 13, 14, and 15. A bias pulse wire 24 feeds coils 25 and 26 on cores 12 and 15. The windings 1619 inclusive, 20-23 inclusive, 25 and 26 produce magnetizing forces in the several cores in directions depending on the directions in which the coils are wound. Those coils marked F produce a negative magnetizing force of substantially the magnitude shown by the arrow -F of Figure 1, while those coils accompanied by the legend +F produce positive magnetizing forces of approximately the magnitude shown by the arrow ]F of Figure 1. If it be assumed at the start of the apparatus that all of the cores are at negative remanence Br, it is clear that for any particular binary signals fed into the system by switches 10 and 11, only one of the four cores will be flipped to plus remanence +Br. For example, if both switches 10 and 11 remain open, the only core which has a positive magnetizing force thereon is core 15. This magnetizing force arises by virtue of pulses flowing from wire 24 through coil 26. Since no currents are flowing through switches 10 and 11, coils 16 to 19 inclusive, and 20 to 23 inclusive, have no currents flowing therethrough. Coils 25 and 26 have currents flowing therethrough but coil 25 produces a negative magnetizing force, hence the only core subjected to a positive magnetizing force is core 15. It will then be flipped to positive remanence |Br (see Figure 1). If it next be assumed that all cores are returned to negative remanence Br and then switch 10 is closed while switch 11 remains open, the only core which will be flipped will be core 13. Core 12 will not be flipped, for although it has current flowing through coil 16 producing a positive magnetizing force, this is neutralized by the fact that the current flow through coil 25 produces a negative magnetizing force. Coil 20 is idle. Core 14 will not be flipped since only coil 18 is energized and it produces a negative magnetizing force. Core 15 will remain at negative remanence, since coil 23 is idle and the magnetizing forces of coils 19 and 26 oppose each other. The current from switch 10 flowing through coil 17 will, however, flip core 13 to positive remanence.

By similar reasoning, if switch 11 is closed and switch 10 remains open, the only core receiving a positive magnetizing force will be core 14.

If switches 10 and 11 are concurrently closed, the only core receiving a positive magnetizing force will be core 12. In this case the two coils 17 and 21 on core 13 will have opposing magnetizing forces. The coils 18 and 22 on core 14 also have opposite magnetizing forces. Core 15 would have a net magnetizing force of F since all three of its coils would be energized, hence it would remain at negative remanence.

Figure 3 shows that the positive excursions of source PP-l occur in the spaces between the positive excursions of source PP-2. Hence, after each positive excursion of source PP-1, one of the four cores 12 to 15 inclusive, will be driven to positive remanence as hereinabove described. During the next time interval, a positive excursion from source PP-2 will return the previously flipped core to negative remanence and insure that all of the other cores stand at negative remanence. Each positive excursion of source PP-2 is a resetting pulsethat flows through coils 35 to 38 inclusive which apply negative magnetizing forces to the cores. Rectifiers 39 to 42 inclusive, which are respectively in series with said four coils, enable the device to have gain. In other words, if terminals A, B, C, and D are regarded as four output terminals, it is possible to produce greater power at these outputs than is fed into the input circuits 10 and 11. Moreover, currents at the outputs A to D inclusive will indicate which of the four cores was flipped. The core which was previously flipped to positive remahence will cause the coil thereon to have high impedance to the next pulse from source PP-2 and hence the current to the output (A to D inclusive) associated with that core will be rather small compared to the other output currents. In the system described above, the amplifiers are series magnetic amplifiers, thus, during the positive portion of PP-Z all but one will have an output. If parallel magnetic amplifiers are used as shown in Figure 5, only one amplifier will have an output during the positive portion of PP-2. The power windings 35 to 38 inclusive are connected in series. The output coils 43 to 46 inclusive provide outputs at E to H inclusive. The selected core is driven from negative remanence -Br to positive remanence +Br by the positive pulse of source PP-l. This would not normally produce an output a E to H inclusive, since the rectifiers 47 to 50 inclusive are in the wrong direction to permit such. However, the next positive excursion of source PP-2 will provide a resetting -pulse that will drive the selected core from positive rem'ane'n'ce back to negative remanence and induce a-potential in the coil (43 to 46 as the case maybe) associated with the selected core but not in any of the other coils. Hence a signal will only appear at one'of the outputs E to H inclusive. Except as described in this paragraph, the system of Figure 5 operates the same as the one of Figure 2. a

Systems somewhat similar to the ones just described have been previously described but they are magnetic switching systems only. In other words, they perform a switching function only without having the propertyof gain. By virtue of the rectifiers 39 to 42 inclusive and '47 to 50 inclusive, in combination with the circuits shown, it is possible to produce gain so that thesignals arriving at the several outputs 'A "to H inclusive have greater power than those-fed through switches and 11.

The reason't hat the diodes 39 to 42 and 47 to 50 enable the apparatus to have gain'is as follows. Taking Figure 5, for example, if the diodes were not present, the coil 46 would be shunted directly across the load. Hence, any pulse passing through the primary winding of the core could not flip the core unless it had enough power to overcome the eflect of the short-circuited coil 46. However, when thediode 50 is present, it effectively isolates the load from the coil 46 during the period "of time when pulses'through'the primary are flipping the core. Therefore, a pulse 'in' the primaryof the-core 15 may flip the core without also energizin'g the load. It follows that only a small amount of power is necessary to flip the core. Thus, the output signal resulting when source PP-2 resets the core has a large amount of power as compared to the input power which is required when the diode 50 is present; yet would be about equal to that which is required'at the input when the diode 50' is not present. A similar explanation applies to Figure 2.

The previous disclosures have carefully explained'how the aforesaid systems may be expanded to have a large number of inputs and a very large number of cores, any one of which may be selected by closing particular input switches such as 10 and 11. In the expanded form of this system, some of thebias windings, such as and 26, have two, three or four times'the turns of other-bias windings. Since the principleupon which the present system may be expanded is well known, it 'will notbe elaborated upon. However, when so expanded,'a'difficulty arisesin that the input circuits have varying impedances depending on the circumstances and as a result it is sometimes found that the current variations in'one or more of the input circuits are so serious that the system gives a false indication. This may be overcome by .employinga current regulator in series with each input.

One convenient form of current regulator is' shown-in Figure} wherein there are coils'27, 28, and 29 respectively, in series with the inputs 10,11, and'24're'spectively. These coils are respectively wound on cores "30 I to 32 inclusive.

The battery 33 and resistor 34 cause a flow of current through coils 34a, 34b, and 340 which produce magnetizing forces in the cores 30, 31 and 32 in a negative direction so large as to drive the core very far into saturation region. Figure 4 illustrates this operation and shows that the bias current I produced by the battery 33 to gether with the number of turnsN on coil 34a produces a negative magnetizing force driving the core into a highly negatively saturated region a 50 of the hysteresis loop. During its positive excursion, source PP-lproduces more voltage than is dropped across the amplifier set windings. Therefore, the current output of PP-l will increase until limited by back voltages developed in regulator coils 27, 28, and 29. These coils will not develop appreciable voltage until magnetizing force N I (which derives from current produced by PP-l) reaches the knee of the hysteresis loop, point 51. The magnetizing force of coil 27 is'represented by'a vector N I of Figure 4, where N is the number of turnson coil 27 and I is the peak pulse current in coil 27 As the pulse current rises to the value I the coil 27 has low back voltage since the core is being driven in a positive direction along the hysteresis loop from point 50 to point 51 in which region thereisver'y little-'fl'ux change. However, as the current I tends to rise above a limiting value so that the vector N l -becom'es longer, tending to-drive the core more positively than-point 51 on'the hysteresis loop of Figure 4, the back voltage developed incoil 27 begins to rise rapidly because of the large flux change occurring to the right of point'51. In fact, the coil '27 would exhibita very-high back voltage if the current in coil 27 tended to'drivethe-core from'point 51 to point 52 on the hysteresis loop. I

The current in'coil' 27 may be represented by vector- I of Figure 4 andthis cur-rent is limited to a value between "A' and 'B -of-Figure '4. Eventhough the impedanc'es of one or more of the coils 16 to 26 should change considerably, the over-all current in each circuit will still beheld substantially constant by reason of the current regulator.

I claim to have invented:

1. In a-selectio'n system for magnetic cores, a "plurality of magnetic cores, a plurality of series circuits each of which has a coil on each of said cores, a pulse generator for producing a train of regularly spaced pulses and feeding-said pulses" to said series circuits, an address input for'each series'circuit and having diflerent operating states for selectively controlling flow of said regularly-spaced pulses from said'gene'rator through the series circuit to select one of said cores to be flipped, a bias series circuit connected to said pulse generator and having coils on someo'f the cores energized by each of said pulses, the winding directions and numbersof turns on any given core being suchthat the core has a m'a'gnetizingforce therein in one direction when the address inputs are in respective :states calling for flipping of the given core, a power winding on each core, and means for passing pulses through the power windings during the vspacesbetween pulses of said generator for magnetizing the core'in the other direction.

2. In a selection system as defined in claim .i1,-.'at least some of the series circuits having current regulating means therein to control the amplitudes ofthe pulses thereth'rough.

3. In a selection system as defined in claim 1,-each series circuit having means therein for holding the maximum amplitudes of the pulses therethrough constant.

'4. -In a-selectionsystemfor magnetic cores, aplurality of magnetic cores, a plurality of series circuits cathof which hasa coil on each ofsaid cores, a pulse generator for .prod ucing-a train of regularly spaced pulses and feeding the'pulses to each series circuit, an address input for'each series "circuit and having different operating states forselectively controlling flow'of said regularly spaced pulses from said generator through the series circuit to select one of said cores to be flipped, a bias series circuit connected to said pulse generator and having coils on some of the cores energized by each of said pulses,

'current regulating means in each series circuit for holding the peak pulse current in each circuit substantially constant, the winding directions and number of turns on any given core being such that that core has a magnetizing force therein in one direction when the address inputs are in respective states calling for flipping of that given core, and means for applying pulses of magnetizing forces to the core in a second direction opposite to said one direction during the spaces between pulses of said generator.

5. In a selection system for magnetic cores, a plurality of magnetic cores, a plurality of selectors, a plurality of series circuits controlled by said selectors, each series circuit having coils on a plurality of cores and each core having at least two coils thereon whereby the regions of the hysteresis loops on which the cores operate may be controlled by operating said selectors, a pulse generator connected to said series circuits so that its pulses are controlled by said selectors, and current regulating means in at least some of the series circuits for holding the peak pulse current in said circuits substantially constant.

6. In a selection system for magnetic cores, a plurality of magnetic cores each having at least two coils thereon, means connecting said coils into groups of circuits with each circuit being separate from the others, means tend ing to pass regularly spaced pulses through each of said circuits, selecting means in said circuits for controlling the flow of pulses therethrough, a bias series circuit fed by said pulse generator and having coils on some of the cores, and current regulating means in each of said circuits for holding the peak pulse current of the circuit substantially constant.

7. In a selection system for magnetic cores, a plurality of magnetic cores, a plurality of series circuits each of which has a coil on each of said cores, a pulse generator for producing a train of regularly spaced pulses and feeding the pulses to said series circuits, an address input for each series circuit and having different operating states for selectively controlling flow of said regularly spaced pulses from saidgenerator through the series circuit to select one of said cores to be flipped, a bias series circuit connected to said pulse generator and having coils on some ofthe cores energized by each of said pulses, the winding directions and numbers of turns on any given core being such that the core has a magnetizing force therein in one direction when the address inputs are in respective states calling for flipping of that given core, a power winding on each core, means for passing pulses through the power windings in a direction opposite said one direction during the spacing between pulses of said generator, and an output winding on each core.

8. In a selection system as defined in claim 7, means having a rectifier in series with each output winding.

9. A selection system as defined in claim 1 in which the last named means includes a unilateral conducting element disposed in series with the respective power windings wound on each core.

A selection system as defined in claim 4 in which the last named means includes each of the following associated with each core, a coil on the core, a rectifier, and an output, said coil, rectifier and output being connected in series.

11. A selection system as defined in claim 4 having an output coil on each core and a rectifier in series with each output coil whereby the system has gain.

12.. A selection system as defined in claim 5 having output means associated with each core for sensing the cores and producing output pulses with greater power than was present in the series circuit having the greatest power.

13. A selection system as defined in claim 6 having output means associated with each core for sensing the cores and producing output indications with greater power than is fed into one of said circuits.

14. In a selection system for magnetic cores, a plurality of magnetic cores, a plurality of series circuits each of which has a coil on each of said cores, a pulse generator for producing a train of regularly spaced pulses and feeding said pulses to said series circuits, an address input for each series circuit and having different operating states for selectively controlling flow of said regularly spaced pulses from said generator through the series circuit to select one of said cores to be flipped, a bias series circuit connected to said pulse generator and having coils on some of the cores energized by each of said pulses, the winding directions and numbers of turns on any given, core being such that the core has a magnetizing force therein in one direction when the address inputs are in respective states calling for flipping of the given core, and means cooperating with said cores for providing a signal that indicates which core has been flipped and which applies a resetting magnetizing force to each core during the spaces between pulses of said train.

15. A selection system as defined in claim 14 in which the last-named means includes two separate coils on each core, one of which is an output coil, and also includes means for passing resetting pulses through the other of said two coils during the spaces between pulses of said traln.

16. A selection system as defined in claim 15 having a rectifier in series with the output coil, the rectifier being connected to allow a pulse to flow therethrough from the coil when the core is reset by a resetting pulse.

17. A selection system as defined in claim 14 in which the last-named means employs only one coil on each core and in which the last-named means feeds the resetting pulse to one end of each said coil, the other end' of each said coil constituting an output.

18. A selection system as defined in claim 17 in which there is a rectifier in the output wire of each said output coil, each rectifier being connected to allow a pulse to flow therethrough from the coil when the latter has low impedance and is fed by a resetting pulse.

19. In a selection system for magnetic cores, the combination of a plurality of saturable magnetic cores, a plurality of circuits having coils on said cores, means for selectively applying pulses to said circuits and separate current regulating means in said circuits for holding the peak pulse currents therein substantially constant, each of said current regulating means comprising a further magnetic core, a first winding on said further magnetic core, said first winding being in series with one of said circuits to be regulated, a second winding on said further magnetic core, and a source of regulating current coupled to said second winding thereby to regulate current flow through said first winding.

20. A selection system as in claim 19 wherein said further magnetic core is formed from a material exhibiting a substantially rectangular hysteresis loop.

21. In a selection system for magnetic cores, the combination of a plurality of saturable magnetic cores, a plurality of circuits having coils on said cores, means for selectively applying pulses to said circuits and separate current regulating means in said circuits for holding the peak pulse currents therein substantially constant, each of said current regulating means comprising a further magnetic core, a first winding on said further magnetic core, said first winding being in series with one of said circuits to be regulated, a Second winding on said further magnetic core, and a source of regulating current coupled to said second winding said source of regulating current being of intensity suflicient to bias said further core into a substantially saturated region of its hysteresis. loop, the direction of current flow through said first winding being such as to drive said further core out of said saturated region whereby current flow up to a certain maximum encounters relatively low impedance and current flow exceeding said maximum drives said core out of itssaturated region'thereby increasing the impedance of said first winding. I p t 22. A selection system as in claim 21 further including means to control flow of said regulating current through said second winding on said further core thereby establishing maximum flow of said current to be regulated.

'23. In a selection system for magnetic cores, the combination of a plurality of saturable magnetic cores, a plurality of circuits having coils on said cores in different combinations with each of said cores having coils from a plurality of said circuits, means for selectively applying pulses to said circuits in certain combinations to select one of said cores, and separate current regulating means in said circuits for holding the peak pulse currents therein substantially constant, each of said current regulating means comprising a further magnetic core, a first Winding on said further magnetic core, said first winding being in .series with one of said circuits to be regulated, a second winding onsaid. further magnetic core, and asource of regulating current coupled to said second winding said source of regulating current being of intensity sufficient to bias said further core into a substantially saturated region of its hysteresis loop, the direction of current flow through said first winding being such as to drive said further core out of said saturated region whereby current flow up to a certain maximum encounters relatively low impedance and current flow exceeding said maximum drives said core out of its saturated region thereby. increasing the impedance of said first Winding.

24. In a system for selecting magnetic coresrin accordance with binary signals, the combination of a plurality of magnetic cores, a plurality of input coils linked to each of said cores a plurality ofaddress input switching means each corresponding to a digit position, a single series circuit for and controlled byeach of said address input means, each of said series circuits including a .coil on each of a plurality of said cores, a further series circuit having coils on each of a plurality of said cores, and a pulse source connected for applying each of its pulses to said address series circuits via said address in put means and to said further series circuit directly, whereby said address input means are each adapted to control the driving or the absence of driving of the associated series circuit .in accordance with said binary signals, said further series circuit being driven during every pulse period.

25. A selection system as in claim '24, wherein each of said cores has a separate set winding thereon, a further pulse sourcefor supplying pulses to said set-winds ings, said pulses' from' said further source being out df phase with "respect to pulses from the first-named source, and'means coupling an output terminal to each of said set windings so that {pulses from said'further source appear at the "output T-terminals of all of said set windings except at the output terminal of'the set winding on a selected core.

'26. A selection system as in claim 24 wherein each of said cores has a separate set winding thereon and an output winding thereon, a further pulse source for supplying pulses to said set windings, said pulses from said further source'being out of phase with respect to pulses from the first-named source, and means coupling an output terminal to each of said output windings whereby pulses from said further source are eifective to revert-a selected core driven by a pulse from said first-named source during a preceding pulse period, and an output appears at the output terminal of the output winding on said selected core.

27. In combination, a plurality of saturable magnetic cores, a plurality of windings on each of said cores, a plurality of circuits respectively comprising certain of said windingsfrom different ones of said cores connected together in series,-means for selectively supplying pulses to various combinations of said circuits so that for each such combination of circuits to which pulses are supplied a diiferentone of said cores is driven to saturation, and separate current regulating means connected to said circuits for holding the peak pulse currents therein substantially constant, each of said current regulating-means comprising a further saturable magnetic core, a first winding on said further magnetic core, said first winding being in a series circuit with the associated one of said circuits to be regulated and said pulsesupplying means and having a sense of linkage to drive said further core in one direction in response to said pulses, a second winding 'on said further magnetic core, and a source of regulating current coupled to said second winding to bias said further core to saturation in the opposite direction thereby to regulate current flow I through said first winding.

References Cited in the file of this patent "UNITED STATES PATENTS 2,730,694 Williamson Jan. 10, 1956 2,734,183 Rajchman Feb. 7, 1956 2,853,693 Lindenblad Sept. 23, 1958-

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3085162A (en) * 1958-11-28 1963-04-09 Ass Elect Ind Woolwich Ltd Electrical selector circuit arrangements
US3086198A (en) * 1958-07-24 1963-04-16 Ibm Core code translator
US3098157A (en) * 1957-12-23 1963-07-16 Kodusai Denshin Denwa Kabushik Logical element
US3167660A (en) * 1960-02-02 1965-01-26 Giddings & Lewis Selective counting apparatus
US3233112A (en) * 1960-02-04 1966-02-01 Bell Telephone Labor Inc Preference circuit employing magnetic elements

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2730694A (en) * 1951-02-02 1956-01-10 Ferranti Ltd Amplitude recording system utilizing saturable core reactors
US2734183A (en) * 1952-12-22 1956-02-07 Magnetic switching devices
US2853693A (en) * 1950-12-28 1958-09-23 Rca Corp Switching devices

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2853693A (en) * 1950-12-28 1958-09-23 Rca Corp Switching devices
US2730694A (en) * 1951-02-02 1956-01-10 Ferranti Ltd Amplitude recording system utilizing saturable core reactors
US2734183A (en) * 1952-12-22 1956-02-07 Magnetic switching devices

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3098157A (en) * 1957-12-23 1963-07-16 Kodusai Denshin Denwa Kabushik Logical element
US3086198A (en) * 1958-07-24 1963-04-16 Ibm Core code translator
US3085162A (en) * 1958-11-28 1963-04-09 Ass Elect Ind Woolwich Ltd Electrical selector circuit arrangements
US3167660A (en) * 1960-02-02 1965-01-26 Giddings & Lewis Selective counting apparatus
US3233112A (en) * 1960-02-04 1966-02-01 Bell Telephone Labor Inc Preference circuit employing magnetic elements

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