US 3581301 A
Description (OCR text may contain errors)
United States Patent  lnventors Theo Stutz Bassersdorf; Werner Mueller, Aarau, both of, Switzerland  Appl. No. 677,801  Filed Oct. 24, 1967  Patented May 25, 1971  Assignee A. G. Contraves Zurich, Switzerland  Priority Oct. 24, 1966  Switzerland  15399/66 [5 4] DISPLAY ARRANGEMENT FOR SYMBOLS 12 Claims, 7 Drawing Figs.  1.1.5. Cl 340/324, 340/334, 340/336  Int. Cl G09f 9/30  Field of Search 340/324, 325, 339, 373, 378, 334, 336; 84/343345; 335/84, 85, 179, 229, 234,  References Cited UNITED STATES PATENTS RE24,943 2/ 1961 Scrobisch 340/373 3,022,400 2/1962 Von Ahlefeldt 335/84 3,140,431 7/1964 Schalkwiik 335/234 3,266,033 8/1966 Frohbach 340/378 2,920,408 1/1960 McGuire ABSTRACT: A display arrangement for displaying any desired symbol in an arbitrary :manner in any desired sequence. The display is in the form of a grid pattern in which the grid areas confined by the lines and columns of the grid are either of one visual surface or another visually contrasting surface. The surfaces are flipped from one to the other through the application of a magnetic actuating device. The actuating device supports the visually contrasting surfaces and includes two permanent magnetic discs mounted in a plane perpendicular to the axis of movement or rotation about which the device moves in flipping from one display surface to the other. The permanent magnets operate in conjunction with an electromagnet which generates a field directed through an airgap. When the direction of the magnetic field is in one sense, one of the two permanent magnetic discs is attracted into the airgap while the other is repelled. When the direction of the magnetic field is reversed, the action upon the permanent mag netic discs is correspondingly reversed and the actuating device is moved so as to display the appropriate visual surface. By energizing the electromagnet with current in the proper direction, a particular area within the grid pattern may be made of one or the other contrasting surface and as a result any desired symbol may be displayed.
PATENTEU W2 5 |97| SHEET 2 OF 4 PATENTED HAY2 5 I97:
SHEET 3 OF 4 1 DISPLAY ARRANGEMENT FOR SYMBOLS BACKGfiOUND OF THE INVENTION The present invention is applicable whenever it is desired to select symbols or characters as desired and display them for viewing to either individual observers or large masses of persons at a predetermined distance. Thus, the present invention is applicable whenever the symbols or characters are to be changeable and installable in machines and equipment indicating measurements or parameters as a function of time. In-
- cluded among these, are applications for traffic signals, bulletins or announcements, and paging systems.
When considering such optical display of the preceding species, there are two separate classes of construction. In one class the displayed symbols will emit light by operating in conjunction with separate illuminating sources and thus distinguish the symbol from the background. A typical arrangement may be one in which the characters or symbols are displayed in white on a dark background and thus provide the necessary contrast by which they are distinguished from their surroundings. In the other class of display devices, no light is emitted from the display. This last species has the advantage over the first species whereby the symbols may be seen equally well in bright sunlight as in relative darkness, as they are independent of the light intensity of their surroundings, In all display arrangements of the second class, the changing of the symbols or characters is accomplished through mechanical motion of the supporting members upon which the display symbols or parts thereof are mounted. The mechanical motions are generated and controlled preferably through electrical, electromagnetic or electrical motor means. In such systems are included those associated with rotatable counter drums, linearly moving tapes, and arrangement of boards hinged at one end in the form of a book and turned in sequence from one end to another. These systems, however, have the disadvantage that in order to display a desired symbol, an entire sequence of symbols will first pass through the viewing area before the desired symbol appears. As a result, the changing of the symbols may take a relatively long period of time. In other systems with movable supporting members for the symbols to be displayed, the displayed symbols may be comprised of a number of different elements mounted on relatively small supporting members. These miniature supporting members may then be moved rapidly to and from, and the symbol is displayed by the combination of various characters and symbol elements through a coding arrangement. in this manner, a substantially rapid display arrangement may be realized.
The present invention relates, in particular, to an arrangement in which a symbol may be displayed as desired through the use of a grid system in which movable symbol supporting members are situated within the area ordered by the lines and columns of the grid. The symbol supporting members are moved through magnetic means into one or the other of its two positions, and are viewed with the aid of a separate illuminating source.
In an arrangement known in the art, the symbol supporting members are in the form of circular discs having a black and a white side. The discs are rotatable through an angle of I80", whereby they can display either the black or the white side. By combining the black and white sides of the discs, numerous different symbols may be displayed. These symbols may be displayed in white on a dark background or in black on a white background.
In the arrangement known in the art, the discs contain a permanent bar magnet inclined to the axis about which the discs are rotatable. Each of the disc is a magnetic actuating device comprised of two pole pieces separated from each other by the diameter of the disc and a ferromagnetic rod located behind the disc and having a coil wound about it. The magnetic actuating devices are designed basically to generate a magnetic field in the plane of the disc and transverse to the movable axis of the disc. The arrangement is such that the bar magnet built into the disc, tends to align itself with respect to the magnetic field similar to that of a compass needle. When the magnetic field is applied to the actuating device in a predetermined direction, the viewing surface or display area is of one type. The opposite viewing surface becomes displayed, on the other hand, when the magnetic field is reversed in polarity. The ferromagnetic parts of the actuating device can be made of soft iron which may be magnetized in one or the opposite polarity depending upon the direction of the current through the coil. in this manner, the reversal of the disc can be accomplished simply by reversing the current through the coil. The permanent bar magnet within the disc tends to maintain the disc in the new position to which it was moved, when the current in the coil is turned off, through means of the magnetic ef fect between the poles of the bar magnet and the ferromagnetic material of the coil. Such a system, however, has so many difficulties and disadavantages that it is not practically. One basic disadvantage is that the bar magnets within the discs are not actuated with sufficient force to be precisely retained in a well-defined position. Thus, the discs may readily oscillate about the desired position when relatively small disturbing forces are applied. When the magnetic flux is reversed the discs are unfavorably balanced so that the flipping of the disc takes place only after the elapse of a relatively long period of time. When the disc has been flipped to the new position, on the other hand, it is not retained in position, but instead, it oscillates slowly and inaccurately about the new position. Furthermore, the magnetic lines of force are subject to stray fields to the extent that neighbor discs are also influenced to a considerable extent. Since it is desirable to locate the discs very close to each other within the grid display area, for purposes of providing a high resolution display, the interacting effects of the magnetic devices is especially undesirable. Aside from this, it is also essential that the magnetic actuating devices or the discs have coil windings of large ampere turns with their accompanying electrical losses. These disadvantages also cannot be avoided through the use of magnetic material with relatively high residual magnetism, such as permanent magnet material, in the actuating devices. When using such magnetic material, it is possible to achieve magnetization effects in the actuating devices with relatively low currents in the coils, and the new positions on the discs are maintained with greater accuracy when the current is turned off between the time that the symbols are changed. However, all other disadvantages still remain.
The present invention, on the other hand, achieves the desired object without any of the aforementioned disadvantages. In accordance with the present invention, permanent magnets are mounted in at least one plane transverse to the axis about which the symbol supporting member rotates. The permanent magnets operate in conjunction with at least one well-defined airgap of relatively large area within the normal magnetic path through the soft iron magnetic actuating device. The permanent magnets are preferably constructed in the form of discs. The arrangement is such that when the magnetic lines of flux are in one direction, the permanent magnet is drawn into or attracted into the airgap. When, on the other hand, the magnetic flux is in the opposite direction, the permanent magnet is repelled from the airgap.
In such a system, the stray magnetic field may be maintained very small without difficulty. Furthermore, the magnetizing current may be maintained substantially small so that with relatively small current pulses the supporting members of the display symbols may be moved to the opposite position from which it is in a relative rapid manner and with considerable precision. This applies particularly to the construction in which the two end positions are well defined through limit stops. The limit stops are located so that when the display mounting member is moved to one position, the permanent magnet disc is not entirely within the airgap. At the same time, when the display mounting member is moved to the opposite position, the permanent magnetic disc is not entirely outside of the airgap. ln this manner, the end positions of the display mounting member are well defined. The display supporting members are. thereby maintained in the end positions with a predetermined amount of force and are brought to their opposite positions rapidly without oscillating effects when the magnetic field through the airgap is reversed.
In the construction of an advantageously designed display, two" viewing planes which are in optical contrast are situated parallel to the axis of rotation and are mounted at an angle of 60 with respect to each other. A plate connecting the two viewing planes is transverse or perpendicular to the axis of rotation and has mounted on it two permanent magnet discs separated from each other by an angle of I. The permanent magnetic discs are of opposite polarity. The angle of 120 is formed by radial lines drawn from the axes of the permanent magnetic discs .to the axis of rotation.
In a preferred design, the soft iron core of each actuating system is in the form of a symmetrical double ring with two airgaps. The soft iron ring is U-shaped and in the form of a yoke. A rod situated parallel to the poles of the yoke and secured to the connecting member or bar of the legs of the yoke, carries an end or crossmember at its free or unsupported end. Between the end surfaces of the crossmember and the inner surfaces of the yoke poles, remains a disc-shaped air space. The airgap is such that a uniform magnetic field may be established through it and depending upon the direction of the field, will attract or repel one of the two permanent magnetic discs.
The rod between the U-shaped legs of the yoke may, for example, be within a space in which it is movable as a result of the action of the permanent magnet in one or the other direction. Through a rotation of 120, the magnetic flux within both airgaps becomes reversed and thereby transfers the symbol supporting members from one position into the other posi tion. It is usually provided that one section of the soft iron ring or path, preferably the rod, has at least one coil wound about it. This arrangement prevails for every actuating device for each movable symbol supporting member or flap. By applying a DC current to the coil, a magnetic field is established through the airgap which forces the symbol supporting member or flap in one or the other direction. The direct current is not required to prevail except for a relatively short period of time. Once the symbol supporting member has achieved or arrived at its new position, the permanent magnetic discs will act to maintain the supporting member or flap in the position.
The selection of predetermined symbols for purposes of display can beconsiderably simplified, when the rod of every actuating device is provided with a plurality of excitation coils. In such an arrangement, the coils are connected so that each one is associated with a particular supporting member in the grid field or grid of the display area. All of the coils associated with a particular combination of symbols or characters to be displayed at any one time, are connected electrically in series. When a momentary DC current is applied to the series circuit, all of the symbols will become actuated and the predetermined or desired combination of characters or symbols are displayed.
SUMMARY OF THE INVENTION A display arrangement for displaying any desired symbol in an arbitrary manner and in any desired sequence. The symbol is generated by means of a grid system in which the individual areas ordered by lines and columns are either one or another visually contrasting surfaces. These visually contrasting surfaces are mounted upon a supporting member which is rotatably movable and which will position one or the other of the visually contrasting surfaces upon being actuated by a magnetic actuating system. The magnetic actuating means includes a yoke-shaped ferromagnetic core and a coil which when energized by an electrical current will produce a magnetic field directed along the ferromagnetic core and through at least one airgap. The movable symbol support has mounted on it at least'two permanent magnetic discs which are movable into and out of the airgap with the rotation of the support. Depending upon the direction of the current through the coil, the magnetic field through the airgap will attract one of the permanent magnetic discs and repel the other. In this manner, the supporting member may be rotated into one or the other position and thus display one or the other visually contrasting display surfaces.
The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiment when read in connection with the accompanying drawing,
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view of the display, in accordance with the present arrangement, and shows the principle of the grid pattern through which any desired symbol may be formed and displayed,
FIG. 2 is a cross-sectional view taken along line llll in FIG. 1 and shows the principle ofthe structural configuration of the present invention with the omission of some detail for purposes of clarifying the illustration;
FIG. 3 is a cross-sectional view taken along line Ill-III in FIGS. I and 2 and shows a magnified portion, in accordance with the present invention;
FIGS. 4 and 5 are cross-sectional views showing the construction of the movable supporting member which supports the symbols to be displayed;
FIG. 6 is an electrical schematic diagram which shows the interconnection of a control circuit including electromagnetic coils shown in FIGS. 2 and 3 whereby a predetermined symbol is displayed; and
FIG. 7 is a cross-sectional view taken along line VII-VII in FIG. 6 and shows the relative mountings and interconnections of the electromagnetic coil windings.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, two U-shaped yokes 1 made of ferromagnetic material of relatively high permeability are adjacently located. Rods 2 made of similar ferromagnetic material or soft iron, are secured to the crossmember of the yokes l. The arrangement is such that the rods 2 project from their mounted location in between and parallel to the pole pieces of the yokes 1. At the free end or unsupported end, each rod carries a crossmember 3 also made of ferromagnetic material. The ends of these crossmembers 3 face the interior surfaces of the pole pieces, these being the lower surface of the upper pole and the upper surface of the lower pole piece. An air space 4 prevails between the ends of the crossmembers and the interior surfaces of the pole pieces. Furthermore, the magnetic path is closed through the ferromagnetic members I, 2 and 3. Between the horizontal front ends of the yoke pole pieces 1, a snap member in the form of a picture-carrying part is inserted. These snap members are movable about a vertical axis 9. The snap members are designed to serve as front shutters of the carrying yoke l which shows one of two rectangular shaped viewing surfaces 5 and 6. These viewing surfaces provide optical contrast effects when illuminated. By means of an electromagnet and a controlling signal applied to it, the snap members may be moved to another position in which they serve as front shutters of the carrying yoke and show the other rectangular shaped viewing surface. The configuration is such that all rectangular shaped viewing surfaces of the snap members which are shown, lie in the common front plane corresponding to FIG. 1. In accordance with the selected distribution of the viewing surfaces of both types, a different number of symbols may be displayed to an observer in accordance with the grid design of FIG. 1. Thus, the number 3 is being displayed in the configuration of FIG. I.
As illustrated by the drawing, the snap member or flap isconstructed of two plates 5 and 6 oriented at an angle of 60 with respect to each other. These plates from the viewing surface of the flap'or snap member. Associated with these viewing surfaces 5 and 6 are also top and bottom plates 7. The parts 5 and 6 comprising the viewing surfaces, and the plates 7 which when combined with the viewing surfaces, form the flap or snap member, are not made of ferromagnetic material. Instead, these parts are preferably made of synthetic pressed or wrought material. Each one of the flaps includes two flat permanent magnetic discs 85' and 86 at the upper and lower plates 7. These magnetic discs are of opposite polarity as shown in FIGS. 3 and 5. The flaps 5-6-7 are arranged between the unsupported ends of the carrying yoke 1 so that they are rotatable about the vertical axis 9. As may be seen from FIG. 2, the flap is rotatable from one position in which one of the viewing surface, as, for example, surface 5 is exposed, to another position in which the other surface 6 is exposed for viewing purposes. In order to provide rotatable bearings suitable for this type of construction, the free pole ends of the yokes are equipped with bearing projections or gimbels 90 at the inner surfaces of the yoke, as shown in FIG. 3. The end plates 7 of the flap have corresponding indentations cooperating operatively with these projections 90. The permanent magnetic discs 85 and 86 attached to the flap are arranged so that upon rotation of the flap comprised of elements 5,6 and 7, they will be either repelled or attracted by the magnetized crossmember 3, depending on the direction of the magnetic flux and the polarity of the discs 85 and 86.
Assume that the core rod 2 of a yoke I is magnetized so that it has a magnetic south pole S at its base where it is secured to the yoke, and that a north pole N is induced in the crossmember 3. .In accordance with FIG. 3, the permanent magnetic discs $5 are then drawn or attracted to the crossmember 3 and into the airgap 4. In this manner, the flap displays the viewing surface 5 as shown in FIG. 4. When on the other hand, the core rod 2 is magnetized in an opposite manner so that a south pole S is induced in the crossmember 3 and a north pole N, prevails at the base or supported end of the rod, the permanent magnetic discs 85 are forced out of the airgap 4 and the permanent magnetic discs 86 are drawn into or attracted into the airgap 4. Under these conditions, the flap is rotated to the position in which it displaced the viewing surface 6. Through proper selection of the materials used for the yoke l, the core rod 2, and the crossmember 3, as well as selective specification of the permanent magnets and air space 4, it is possible to maintain securely the flap in a desired position through a relatively short magnetizing current pulse. The direction of the current pulse or its polarity is dependent upon the polarity arrangement of the permanent discs and the viewing surface desired. Thus, a relatively small amount of magnetizing current is required to maintain the flap in position until it is to be conveyed to another position by a magnetizing pulse of opposite polarity. In order not to contribute any disturbing or undesirable forces, the permanent magnetic discs are made of material having preferably a relative permeability of u=l similar to that of air.
In what follows, the process is described for displaying numerous different symbols and characters in the grid pattern of FIG. I. It is also shown that every displayed symbol can be changed within an extremely short period of time of the order of 0.1 sec. maximum. Thus, within this time interval, any symbol may be changed to any other desired symbol.
In accordance with FIGS. 2 and 3, disc-shaped coils are mounted on the core rods 2. Thus, the drawing shows that each one of the 5X7=35 core rods 2 are provided with coil carrying plates in order to show one of the numerous symbols'possible. In practice, it is possible to mount 40 to 50 of the coils on each core rod 2 when the dimensions of the yoke I are approximately mm. l5 mm.X45 mm. All coils of the same order in all magnetic positioning arrangements for all flaps in the grid pattern of FIG. 1, are combined on one plate or folio 10. Such a flat coil carrying plate 10 the configuration shown in FIGS. 6 and 7. All coils are the same in construction with the exception of the winding. Coils which are wound in the direction for displaying the viewing surface 5 on the flap are designated with the reference numeral 15. The coils which are wound in the opposite direction for the purpose of displaying the viewing surface 6 of the flap 5-6-7 are designated with reference numeral 16. All coils I5 and 116 have, in accordance with FIG. 7, a flat coil from the front side and an additional coil on the backside of the corresponding plate 10. Furthermore, all 35 coils 15 to 17 ofa plate 10 are connected in series through interconnecting lines 11. Therefore, when the pushbutton T is depressed momentarily, a current pulse of positive polarity flows through all coils of the plate 10 to ground. Power is supplied by a direct current supply A providing two opposite polarities. Depending upon the winding direction of the coil, each coil has induced a magnetic north pole directed toward or from the cross link of the U-shaped yoke. The crossmember 3 of the core rod 2 has therefore a magnetic north pole induced in it. The coils l5 and I6 are distributed in the plate 10, in accordance with FIG. 6, so that the distribution of viewing surfaces 5 and 6 is realized in a desired manner, as shown in the grid pattern of FIG. 1.
In a preferred embodiment; the viewing surfaces 5 of the flap are made of a deep black color. The viewing surfaces 6 on the other hand are made of a bright white. In this manner, the numeral 3 on a coil carrying plate 10 of FIG. 6, appears on a black background. When the switch U in FIG. 6 is moved to the position opposite to that shown in. the drawing, this numeral 3 is displayed in black on a white: background when the pushbutton 3 is depressed. This is because the coil 16 then position the viewing surfaces 5 and the coils 15 position the viewing surfaces 6, since the magnetic fields are reversed.
In this manner, it is possible, without difficulty, to provide every desired symbol for displaying purposes by distributing correspondingly the coils l5 and 16 with the proper winding direction on the coil-carrying plate 10. All coil-carrying plates being provided can be selectively switched in as desired through actuation of an associated switch or pushbutton T. As a result, the desired symbol will appear in the grid display area at all times without delay.
It is possible to provide 35 similar disc coils or flat coils transversed to their interconnecting lines. The coils are then irregularly connected within a particular plane of the grid distribution in one or the other direction and then fixed. In this manner, no soldered connections are required between individual coils of a plate 10.
The selection of the symbols to be displayed in a changing manner can be accomplished through electrical type or printing machines or through punched cards of tape.
The construction and retention of the flap containing the viewing surfaces 5 and 6 should be such. that no other forces or moments act on the flap other than the turning moment about the axis 9. This is the turning moment generated through the magnetic flux in the airgaps 4. In order to inhibit the possibility that the attractive forces increase the frictional effect in the bearings of the flap 57, these forces are preferably sustained by additional structural parts. With regard to this aim, it is also desirable to make the permanent magnetic discs 85 and 86 out of material having a relative permeability of one, as already mentioned.
It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of electromagnetic display devices differing from the types described above.
While the invention has been illustrated and described as embodied in electromagnetic display devices, it is not inart, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.
What is claimed as new and desired to be protected by Letters Patent is set forth in the appended We claim:
1. An arrangement for the selective display of symbols comprising, in combination, a plurality of symbol portion carrier means each with two optically contrasting display plates, each of said plates being movable into the plane of a screened grid having lines and columns, each of said plates being visible on said screen grid when moved into said plane, said plates being mounted in said symbol portion carrier means at an angle of 60" relative to each other, the axis of intersection of said plates being the vertex of said angle and being parallel to an axis of rotation about which said plates are movable, said plates being movable about said axis of rotation from moving said plates into and out of said plane; wall members perpendicular to said axis of rotation and secured to said plates, said wall members being perpendicular to said plates; U-shaped yoke members of ferromagnetic material in each of said symbol portion carrier means and behind said plane, said yoke members being plates perpendicular to said axis of rotation and extending over said wall members; bearing means on said yoke members for rotatably holding said wall members so that said wall members with said plates secured thereto are rotatable about said axis of rotation; a coil carrying core secured at one end to the transverse portion of said U-shaped yoke member and arranged parallel between said plates of said yoke member, said core being of ferromagnetic material; a crossmember of ferromagnetic material secured to the other end of said coil carrying core, said crossmember being parallel to said axis of rotation, the axis of said crossmember being spaced by a predetermined amount from said axis of rotation, the end surfaces of said crossmember being parallel to said yoke plates and being spaced from said yoke plates through a predetermined airgap, said wall members extending into said airgap; a pair of permanent magnetic discs in each wall member and of opposite polarity, said permanent magnetic discs being spaced by said predetermined amount from said axis of rotation, the axes of said permanent magnetic disc subtending an angle of 120 from said axis of rotation, one of said permanent mag:
netic discs being within said airgap when one of said plates is moved within said plane and the other one of said permanent magnetic discs being within said airgap when the other plate is moved within said screened grid plane; coil means on each of said core means for inducing when energized a magnetic field through said core means; and control circuit means connected to said core means for selectively inducing a magnetic field through said core means so that one of said permanent magnetic discs is selectively moved into said airgap, the selection of said permanent magnetic discs depending upon the polarity of said induced magnetic field through said core means, said control circuit means moving selectively said permanent magnetic discs into said airgap through a current pulse applied to said coil means, the permanent magnetic disc moved into said airgap through said current pulse remaining within said airgap until a current pulse of opposite polarity is applied to said coil means by said control circuit means.
2. The arrangement as defined in claim 1 including limit stop means on each symbol portion carrier means for limiting the motion of said permanent magnetic discs when moved into and out of said airgap.
3. The arrangement as defined in claim 1 including a plurality of electromagnetic coils on each of said cores, said coils of one core being interconnected with coils of other cores so that when applying a current pulse from said control circuit means a predetermined symbol is displayed on said plane.
4. A display arrangement for displaying symbols arbitrarily as desired comprising, in combination, rotatably movable supporting members for individual matrix elements of a symbol and having two visually contrasting display surfaces; means for mounting said supporting members in a screened grid pattern, each of said supporting members being rotatably movable about an axis between two predetermined positions, each position being associated with one of said display surfaces so that one of said surfaces is displayed in one of said positions of said supporting member and the other surface is displayed in the other one of said positions; at least two permanent magnetic elements of opposite polarity and mounted on each of said movable symbol supporting members, said magnetic elements being spaced from each other and being eccentrically mounted with respect to said axis; magnetic field generating means for generating a magnetic field linked through at least one predetermined airgap to said permanent magnetic elements; ferromagnetic means magnetized by said field and directing said magnetic field through said airgap, so that one of said permanent magnetic elements is attracted to said ferromagnetic means and the other one of said permanent magnetic elements is repelled from said ferromagnetic meansyand selection means for selecting the polarity of said magnetic field of each supporting member to select the magnetic element attracted to said ferromagnetic means whereby a predetermined symbol is displayed on said grid pattern through selective positioning of said supporting members in said predetermined two positions.
5. The display arrangement for displaying symbols arbitrarily as defined in claim 4, wherein said magnetic field generating means includes at least one coil carrying plate, and at least one electromagnetic coil mounted on said coil carrying plate for determining the direction of said magnetic field through said airgap when energized by a DC current.
6. The display arrangement for displaying symbols arbitrarily as defined in claim 4, wherein said ferromagnetic means in cludes a U-shaped yoke having two parallel legs connected by a connecting base member; rod means secured to said connecting base member and extending between said legs of said yoke; and a crossmembersecured to the other end of said rod means and spaced by an airgap from each external end surface of said crossmember and the inner surface of the leg of said yoke facing said end surface of said crossmember, said magnetic field being directed uniformly through said airgap in one of two possible directions and selectively attracting one of said permanent magnetic elements into said airgap while repelling the other one ofsaid permanent magnetic elements.
7. The display arrangement for displaying symbols arbitrarily as defined in claim '6, including a plurality of coil carrying plates each provided with a plurality of apertures, a plurality of excitation coils mounted on said plates and each surrounding one of said apertures, and said rod means extending through and interconnecting in series associated ones of said -coi|s on the respective plates, said excitation coils being operative for displaying a predetermined symbol when energized through a DC-current.
8. The display arrangement for displaying symbols arbitrarily as defined in claim 4, including switching means connected to said coils for controlling the polarity state of said excitation coils by transmitting DC current thereto.
9. The display arrangement for displaying symbols arbitrarily as defined in claim 4, including mounting members for mounting said visually contrasting display surfaces parallel to said axis about which said movable supporting member rotates, said display surfaces being mounted at an angle of 60 with respect to each other.
10. The display arrangement for displaying symbols arbitrarily as defined in claim 9, including connecting plate means connecting said mounting members of said visually contrasting display surfaces and being transverse to said axis about which said movable symbol supporting members rotate, said permanent magnetic elements being magnetic discs mounted on said mounting members and spaced from each other, the axes of said discs being spaced from said axis by a predetermined amount so that radial lines between said axes of said discs and said axis of rotation subtend a central angle of with vertex at said axis of rotation.
10 bitrarily as defined in claim ll, including additional limiting means for limiting the movement of said symbol supporting member when the other one of said permanent magnetic elements is repelled from said airgap.