US3625358A

US3625358A - Random filing system

Info

Publication number: US3625358A
Application number: US851673A
Authority: US
Inventors: Keith D Duperron
Original assignee: KEITH D DUPERRON
Current assignee: KEITH D DUPERRON
Priority date: 1969-08-20
Filing date: 1969-08-20
Publication date: 1971-12-07
Anticipated expiration: 1988-12-07

Abstract

Apparatus for selecting one or all similar cards from a plurality of randomly distributed and different cards, the cards having magnetizable tags along an edge, in which there is a plurality of magnets which can be selectively and independently energized and deenergized. There are means for moving the cards into contact with the magnets so that ones with tags corresponding to the energized magnets are held by those magnets and those which do not have corresponding tags can be moved from the magnets. In this way selected cards, regardless of their positions in a stack of cards can be separated from nonselected cards.

Description

ilnited States Patent [56] References Cited UNITED STATES PATENTS 2,795,226 6/1957 Bralove 209/! l 1.8 3,524,545 8/1970 Doundoulakis 209/805 Primary Examiner-Richard A. Schacher Attorney-Fetherstonhaugh and Co.

ABSTRACT: Apparatus for selecting one or all similar cards from a plurality of randomly distributed and different cards. the cards having magnetizable tags along an edge, in which there is a plurality of magnets which can be selectively and independently energized and deenergized. There are means for moving the cards into contact with the magnets so that ones with tags corresponding to the energized magnets are held by those magnets and those which do not have corresponding tags can be moved from the magnets. in this way selected cards, regardless of their positions in a stack of cards can be separated from nonselected cards.

PATENIEB DEC 7 I97! 1625358 sum 1 OF 4 l I m l N VI-NTOR KEITH D DUPERRON woman PATENTED DEC 7 I97! SHEET 3 [1F 4 mvcm'on KEITH D. DUPERRON ATTORNIYQ RANDOM FILING SYSTEM BACKGROUND OF THE INVENTION This invention is concerned with a random filing system, i.e. with an apparatus for selecting one or all similar cards, from a plurality of randomly distributed and different cards. It is also concerned with cards for use in conjunction with the apparatus.

The invention has particular, but not necessarily exclusive application to apparatus for selecting file cards from randomly distributed banks of file cards. However it is to be appreciated that the information contained on the cards is secondary to the function of the apparatus.

The advantages of a filing system in which the order in which the cards are located is not important are apparent and there have been attempts to produce such systems but the results of those attempts have not been entirely satisfactory.

Some devices have cards one edge of which is notched or cammed and cooperates with a plurality of bars or pins which are selectively movable into the notches of the cards to prevent displacement of all cards not selected and to allow the displacement of the selected card(s) by such means as a magnet, the magnet drawing the selected card(s) from the bank of remaining cards. Examples of such devices are illustrated in Smitzers U.S. Pat. No. 3,273,566 and Townsleys U.S. Pat. No. 3,273,565 both of which were issued on Sept. 20, 1966 to the Bell & Howell Company ofChicago.

These devices have the disadvantage common with many of the prior art patents, of a great number of moving parts and are therefore costly and difficult to maintain.

BRIEF SUMMARY OF THE INVENTION According to this invention it is proposed that the cards be arranged in packs in the apparatus and should have magnetizeable projections along an edge, the numbers and positions of the projections distinguishing the cards from one another. The apparatus includes a plurality of magnets extend ing transversely over the edges of the cards and operable selectively to hold all the cards except those having projections corresponding to the particular selection magnets made and means for supporting the pack and movable relatively to the magnets to carry with it a card or cards not held by the magnets to a dispensing position. It will be understood that preferably each card in the pack is different from all the others but clearly this may not always be possible. If for example the cards are identified by the name of a person to whom the information on the card relates then with common names such as Brown and, where, to select a card magnets corresponding to the letters of the alphabet are used, then there may be several cards selected but clearly the system does not lose its value for it is very much simpler manually to select the particular card needed from those selected than it is to select the card from all of those in the bank. Further, it is to be appreciated that the numbers of selections which can be made is related to the number of magnets available for the selection and with magnets of the kind hereinafter described, the number of magnets can be made large so that this will become even less of a problem if indeed it is a problem at all.

In one embodiment the cards would be supported on a platform which is operable to move the bank of cards into a position against the magnets and then to return the selected card(s), i.e. the cards not held by the magnet to a position away from the magnet at which they can be pushed mechanically from the bank or merely extracted from that position.

It is preferred that the magnets be electromagnets and in such a case they would be located in a fixed position and would simply be magnetized according to the operation of electrical switches. However, it is possible and is within the ambit of this invention to have permanent magnets which are selectively movable into operative and inoperative positions i.e. into positions in which they would or would not hold particular cards in the pack of cards. Another possibility would be to combine fixed permanent magnets with coils so that the coils would vary the field of the magnets according to the direction and magnitude of current through the coils.

Also according to this invention there is provided a card for use in conjunction with the apparatus and which is of laminated construction and has secured between the lamina tions and along one edge, a strip of magnetizeable material which is thin so that it can be easily cut to remove certain sections of the strip thus to provide magnetizeable projections at appropriate intervals along that edge. Preferably the strip is transversely slit from that edge most remote from the remainder of the card so that longitudinal cuts can separate the material between slits from the remainder of the strip.

From another aspect there is provided a bank of cards each card in the bank having a plurality of magnetizeable projections along one edge and each card being distinguished from the remainder in the bank by the numbers and/or positions of the projections on that card.

BRIEF DESCRIPTION OF THE DRAWINGS A simple embodiment of the invention is illustrated schematically in the accompanying drawings in which:

FIG. 1 is a plan view of the apparatus of this invention FIG. 2 is a side view, partly in section, of the apparatus in FIG. 1

FIG. 3 is a section on the line 3-3 of FIG. 2

FIGS. 4, 5, 6, 7 and 8 are details ofthe apparatus in FIGS. I, 2 and 3,

FIGS. 9 and 10 are front and edge views respectively of the cards for use in conjunction with the apparatus of FIGS. 1 to FIG. 11 is a simplified circuit diagram of for activating the magnets, and

FIG. 12 is a diagram illustrative of the operation of the apparatus.

DESCRIPTION OF AN EMBODIMENT The apparatus in FIGS. I to 3 comprises essentially a bank of cards indicated generally at 10 and a coder mechanism indicated at 11, the apparatus for selecting the cards being operable, as is the coder mechanism, by a keyboard indicated generally at 12.

The bank of cards 13 is supported upon a platform 14 which is mounted for vertical sliding movement on guides 15 of which there are two to each side of the platform.

The cards are supported in their vertical movement by end plates 16.

The vertical movement of the platform 14 is effected by a means of a bellcrank lever 17 of which one arm 18 is operated by a crank element 19, which is described in more detail hereinafter, to raise the platform and is returned to its lower position by a spring 20. The other arm 21 of the bellcrank lever carries a horizontal lug 22 to which the guide 15 at the right-hand end of FIG. 2 is secured. Thus as the crank 19 moves the bellcrank lever anticlockwise the guide 15 and with it the platform will be raised.

A link 23 is secured between the arm 18 of bellcrank lever 17 and an arm 24 of a bellcrank lever 25. The bellcrank lever 25 is pivoted as at 26 to a frame member 27 and has an arm 28 which is connected, in similar fashion to the connection between arm 21 and guide 15, to the guide 15 at the left-hand side of FIG. 2 by a horizontal lug 29.

From the outer end of the arm 24 of bellcrank lever 25 a link 30 extends upwardly to a lever 31 pivoted as at 32 to a frame element 33.

Movable with the lever 31 is a plate 34 which is mounted on the frame unit 32 as is lever 31 and cooperates with the upper forward edge of the bank of cards to push the bank of cards rearwardly against an abutment 35 to ensure that the cards are properly located relatively to the magnets.

The upward movement of the platform is to bring the upper edges of the cards into contact with a plurality of magnets which extend tranversely of the top edges of the cards and the magnets are then activated to maintain all but the selected card or cards in contact with the magnets and as the platform moves down those cards which are selected and which are not held by the magnets move with the platform and it is from this lowered position that they are pushed forwardly i.e. to the left in FIG. 2 by a bar member 40 which is carried by a pair of levers 42 pivoted as at 43 to a frame member 33. The bar member is moved forwardly by a crank element 45 which is described in more detail hereinafter and is returned to the position shown in FIG. 2 by the return string 46.

As the crank element 45 moves the bar 40 forwardly this sweeps under the lower edges of the cards supported by the magnets and above the platform when the platform is in the lower position so that any cards supported by the platfonn or put another way, not held by the magnets, will be pushed forwardly from the pack of cards. From this position the card can be extracted by hand.

To drive the crank elements 19 and 45 there is provided a motor 50 the drive shaft of which is indicated at 51 and carries a pinion 52 which drives gear 53 which is journaled on a bearing post 54.

The gear 53 carries, close to its periphery. a pin 56 which cooperates with a stirrup-shaped section 57 of the crank element 45, see FIG. 4.

Projecting rearwardly from the stirrup-shaped section 57 are a pair of parallel sections 58 which turn upwardly as at 59 and then forwardly again as at 60 to their interconnection with the lower ends of the levers 42. Between the lower ends of the levers 42 and the open ends of the section 60 of the crank member 45 is the pusher bar 40.

The stirrup-shaped member comprises two part circular lengths the most widely spaced parts of which are connected by a diagonal section so that he post 56 cooperates with the diagonal so that as the post moves forwardly of the position shown in FIG. 2 so the stirrup is pushed forwardly to cause the pusher bar 40 to sweep across the platform. As the post reaches its most forwardly position the crank element 45 is returned under the action of return spring 46 to be maintained in contact with the stirrup.

The crank element 19 comprises a pair of sleeves 70 which are mounted for telescopic movement over a pair of guide posts 71 projecting forwardly from the rear wall of the casing. lnterconnecting the rearward ends of telescopic sleeves 70 is a bar 72 which coacts with a post 73 which projects downwardly from close to the periphery of the gear 53 so that in the movement of that post rearwardly from the position shown in F IG. 2 it draws the sleeves 70 over the guide posts 71 and thus raises the platform Contact between the bar 72 and the post is maintained by the action ofsprings 20.

Extending transversely and supported by

frame members

80 and 81 are a plurality of magnets x,y, and a, b, and c.

In this particular arrangement the magnets a, b, and c are arranged normally to be energized and magnets x and y are arranged normally to be deenergized. The magnets cooperate with metallic tags which project upwardly from the upper edges of the cards.

With the particular arrangement illustrated a selection may be made from seven cards and for the purposes of describing the invention it is convenient to refer to those cards as being card a, b, c, ab, ac, be, and abc.

In FIG. 12 which is a simple diagram of the arrangement it will be seen that card a has metallic projections which correspond to magnets x, a, and y; card b has projections which correspond to magnets x, and y and b and card c has projec- I tions which correspond to magnets x, y and c.

Card ab has projections which correspond to magnets y and a and b; card ac has projections corresponding to magnets y, a, and c; card be has projections corresponding to magnets y, b, and c; and card abc has projections corresponding to magnets a, b, and

Thus if it is desired to select card a then the magnet a is deenergized and as the magnets x and y are normally deenergized then no magnet will support the card a but magnets will support all of the other cards.

If it is desired to select card ac then it is required that magnets y and b be energized and deenergized respectively so that all cards except card ac would be held against the magnets as the platform lifts the cards into contact with the magnets.

To select card abc it is necessary to energize magnets x and y and deenergize magnets a, b, and c so that for example card a is held by magnets x and y as are cards b and c and cards ab, ac, and bc are held by magnet y. Thus card abc is the only card free to fall from the magnets with the downward movement of the platform.

A simple electrical circuit for effecting the control of the magnets is illustrated in FIG. 11. In that figure there is a power source for the magnets a, b, c, x, and y. There is a separate power source 90A for relays 91 and 92 which control the operation of magnets x and y. While the power sources are indicated as DC sources in this particular embodiment and they are illustrated as separate sources for the solenoids and for the magnets it will be appreciated that for an operational model it is desirable to use a main supply power source rather than to rely on batteries.

There are pushbuttons a, b, and c which correspond to each of the magnets a, b, and c. Button 0 controls switches 93 and 94, button b, switches 95 and 96, button c, switches 97 and 98.

The switches 94, 96 and 98 are each arranged in the respective supply circuits to magnets a, b and c and are so arranged that those circuits are normally closed, maintaining the magnets a, b and c energized and those circuits are only broken upon actuation of the buttons a, b and c.

It will be appreciated that to conserve power there will be an on-off switch such as is indicated at 100.

Connected across the terminals of

switches

93, 95 and 97 respectively are resistors 10], 102 and 103 and the arrangement is such that when any one of the resistors is shorted from the circuit including the three resistors and the

solenoids

91A and 92A insufficient current flows through that circuit to energize the solenoids and cause the relays to operate. The resistors are so selected that when any two buttons are operated and the corresponding resistors shorted from the circuit including the

solenoids

91A and 92A then sufficient current flows in that circuit to energize the solenoid 92A and cause relay contact 928 to be thrown thus to energize the electromagnet x. When the three buttons are pressed sufi'rcient current flows through that circuit to energize both relays 91 and 92 and thus to energize both electromagnets x and y.

Thus in selecting either of cards a, b and c neither of the electromagnets x and y is energized, in selecting any of cards ab, ac and be the electromagnet x is energized by throwing the switches 92B of relay 92 and in selecting card abc both the relays 91 and 92 are energized accordingly to energize the electromagnets x and y.

The power source 90 is also the supply to the electric motor 50 and that motor is energized by operation of a pushbutton 110.

The arrangement of the keyboard is to be seen in FIG. 1.

It will be appreciated that using conventional electromagnets the selecting device will have considerable size for the magnets must extend over a considerable number of cards, many more than the seven illustrated in this exemplary embodiment, and to effect selection there must be many magnets, say, in a working embodiment, about 30. Thus the magnets must be long and thin and must not develop stray fields which might interfere with adjacent ones of the magnetizeable projections on the cards. Such a magnet has been developed and is illustrated in FIGS. 7 and 8 and comprises a comblike structure having a spine 121 and a plurality of spaced teeth 122. Windings are formed around the teeth of the comb and the comb is then located in a U-shaped channel 123 with the tips of the teeth of the comb secured in spaced opening along the length of the base of the channel. This construction has been found to be one which is remarkably effective for this particular application.

In F108. 9 and 10 a preferred card construction is illustrated and the card is laminated as can be seen in FIG. 10 and secured between the laminations at the upper edge of the card is a metallic strip 125 which is slit as at 126 at regular intervals along the length of the strip so that by making cuts at right angles to the slits and extending between slits the metal of the strip can be removed to leave projecting magnetizeable tabs in appropriate positions.

lt is convenient to effect the removal of the magnetizeable tags in the same machine as the selection of the cards is made and to this end there is provided a coder section 11 into which a card is located to have appropriate ones of the projections removed.

The coder comprises an opening 130 into which a card to be coded is inserted and the end of that opening is defined by an abutment wall 131 against which the magnetizeable strip on the edge of the card is pushed to ensure proper location. A plurality of knives 132, there being as many knives as there are magnets, is located to snip off the protruding part of a section of the metallic strip on the edge a card between the slits 126 of the card.

Thecoder mechanism comprises a pushrod 140 which carries on its rear end i.e. the right-hand end as viewed in FIG. 2 a cam-following roller 14! which cooperates with a cam 142 carried on the shaft of the gear 53. The pushrod 140 is mounted for reciprocating movement in a bearing block 145 and terminates at a plane face 146. In the most retracted position of the pushrod, as seen in FIG. 2, the plane end 146 of the pushrod is spaced from a corresponding end face 147 of a continuation of that pushrod which carries a wedge-shaped bladeoperating member 148.

During selection operations of the apparatus the pushrod 1140 will simply reciprocate within the bearing block 145 and will cause no movement of the blade-operating member 148. When it is required that a coding operation be performed a switch is thrown to energize relays 149 which cause a connector member 150 to be moved into the space between the

faces

146 and 147 so that the reciprocation of the pushrod 140 upon rotation of the gear 53 causes a corresponding movement of the wedge-shaped blade-operating member 148. The blade-operating member has an inclined face 151 which cooperates with an appropriately guided beam 152 which, in the blade-operating members forward horizontal movement is caused to move vertically downwards.

The beam 152 has a lower plane face 153 for cooperation with actuating elements 155 which comprise a raised operating segment 156 and a cutaway portion indicated generally at 157. The actuating member 155 is connected as by links 158 to the pushbuttons of the keyboard so that as a key is depressed so the actuating member is shifted between an operative position in which the upper surface of the operating segment 156 and the lower surface 153 of the element 152 are in an abutting relationship or an inoperative position in which the beam 152 is free to move into the cutaway section 157. It will be appreciated that in the operating position of the actuating member 155, the downward movement of beam 152 will cause the blade associated with the particular actuating member to be moved downwardly in a cutting operation but that in the inoperative position the blade will remain stationary during the downward movement of the beam 152.

To return the blade to its raised position the blade has a rearwardly extending abutment piece 160 between which and between the base ofa blind bore 161 in the member 131 there acts a spring 162.

Also to return the blade-operating member 148 and the pushrod 140 there is a return spring 1641 Clearly the return spring 164 could be replaced by a return cam which can conveniently be embodied in the cam for effecting forward movement ofpushrod 140.

To ensure that the card is properly pressed up against the abutment face 131 there is provided a simple linkage between the beam 152 and a pivoted foot 170 so that as the beam moves downwardly so the foot moves rearwardly pressing with it the card located in the slot 131.

It is to be appreciated that the apparatus illustrated in the accompanying drawings is strictly an apparatus for illustrating the principle of the invention and is not a production model. However a production model would simply be an enlargement of the embodiment illustrated in the drawings or a modification of that embodiment in the drawings. For example the circuitry of a production model could be printed or solid state circuits.

One modification which is clear is the replacement of the electromagnets by permanent magnets which are movable into operative and inoperative positions rather than with electromagnets, rendering them energized and deenergized.

Another modification could be to combine permanent magnets with electric coils as for example with the magnets described with reference to FIGS. 7 and 8 the combs or the channel 123 can be formed of a permanently magnetizable material and the coils used simply to modify the field produced by those magnets to cause them to hold or to release a cord.

Clearly many of the operations illustrated as being effective by linkages and levers could be effected by simple electromechanical operations such as for example those of solenoids. This is particularly the case in the operation of the actuator elements 156 of the coder mechanism.

It will be appreciated that the compartments for the cards could be duplicated so that several banks of cards could be controlled from a single keyboard or alternatively there could be several keyboards controlling remote banks of cards.

The advantages of the first arrangement are those of economy whereas with the second arrangement many people could have access to the system from remote positions and coupled with an appropriate system such as for example, a closed circuit television system could draw information from the filing system without moving from their remote locations.

I claim:

1. For use in selecting cards from a plurality of randomly distributed cards which have magnetizeable portions at fixed positions along an edge and which are distinguished from each other by the numbers and locations of the portions, apparatus comprising a compartment for the cards, a plurality of magnets extending across the compartment and, when the cards are located in the compartment, across said edge of each card, said magnets being selectively operable to attract and not to attract the magnetizeable portions on the cards, a support for said cards, said support and said magnets being relatively movable between positions in which said edge of the cards contact the magnets and in which said edge is spaced from the magnets, thus by making that movement to separate selected and nonselected cards.

2. Apparatus as claimed in claim 1 in which said magnets are permanent magnets and are selectively movable relatively to said support.

3. Apparatus as claimed in claim 1 in which said magnets are electromagnets and selectively energized and deenergized.

4. Apparatus as claimed in claim 3 in which said support comprises a platform forming the base of the compartment and the magnets extend across the top of thecompartment. said platform being movable towards and away from said magnets.

5. Apparatus as claimed in claim 4 in which said nonselected and selected cards are held and not held by the magnets respectively and said selected cards are movable with the platform away from said magnets.

6. Apparatus as claimed in claim 5 in which means is provided for displacing said selected cards from said lowered position of said platform.

'7. Apparatus as claimed in claim 6 in which said means comprises a pusher element movable between the lower edges of the nonselected cards and the platform.

8. For use in the apparatus as claimed in claim 3, an electromagnet comprising a comblike core having a spine and a plurality of teeth projecting from that spine and spaced apart along said spine and electrical windings on each of said teeth.

9. For use in the apparatus as claimed in claim 3 an electromagnet comprising a comblike core having a spine and a plurality of teeth projecting from that spine and spaced apart along said spine and electrical windings on each of said teeth, wherein said teeth are enclosed within a U-shaped channel member of magnetizeable material.

10. Apparatus as claimed in claim 1 in which the magnets comprise fixed permanent magnets having electric coils associated therewith said coils being selectively actuated to modify the field of the permanent magnets to attract or not to attract the magnetizable portions on the cards.

11. Apparatus as claimed in claim 1 including means for forming said magnetizeable portions along said edges of said cards.

12. Apparatus as claimed in claim I including means for forming magnetizeable portions along an edge of a card having a magnetizeable strip along that edge which comprises means for locating a card in a fixed position relative to a plurality of independently and selectively operable knives so that said strip is below said knives, and means operating said knives selectively to remove sections of said strip and leave magnetizeable portions between said removed sections.

13. For use in conjunction with the apparatus as claimed in claim 1, a card of laminated material and a metallic strip set in one edge of said card between said laminations.

14. Apparatus as claimed in claim 13 in which said metallic strip is slit from its edge spaced from the card transversely of the strip towards said cards so that portions between said slits can be removed by a longitudinal cut.

15. For use in apparatus as claimed in claim 1, a set of cards comprising magnetizeable portions along an edge of each card, each card in the set being distinguished from the remainder by the numbers and locations of said magnetizeable portions along said edge.

16. For use in selecting cards from a plurality of randomly distributed cards having magnetizeable portions fixed along an edge thereof and which are distinguished from each other by the numbers and locations of those portions. apparatus comprising a compartment for said cards, a plurality of electromagnets extending across the compartment and across said edge of each card when said cards are located in said compartment. said magnets being selectively energized and deenergized to attract and not to attract the magnetizeable portions of the cards, and means for moving said cards into contact and out of contact with said magnets thus to separate cards having portions corresponding to the selected energized and deenergized magnets from cards not so corresponding.

17. Apparatus as claimed in claim 16 including means for forming said magnetizeable portions along said edges of said cards.

18. Apparatus as claimed in claim 16 including means for forming magnetizeable portions along an edge of a card having a magnetizeable strip along that edge which comprises means for locating a card in a fixed position relative to a plurality of independently and selectively operable knives so that said strip is below said knives, and means operating said knives selectively to remove sections of said strip and leave magnetizeable portions between said removed sections.

19. For use in conjunction with the apparatus as claimed in claim 16, a card of laminated material and a metallic strip set in one edge of said card between said laminations.

20. Apparatus as claimed in claim 19, in which said metallic strip is slit from its edge spaced from the card transversely of the strip towards said cards so that portions between said slits can be removed by a longitudinal cut.

21. For use in apparatus as claimed in claim 16, a set of cards comprising magnetizeable portions along an edge of each card, each card in the set being distinguished from the remainder by the numbers and locations of said magnetizeable portions along said edge.

22. For use in the apparatus as claimed in claim 16, and electromagnet comprising a comblike core having a spine and a plurality of teeth projecting from that spine and spaced apart alon said spine and electrical windings on each ofsaid teeth.

2 For use in the apparatus as claimed in claim 16, an electromagnet comprising a comblike core having a spine and a plurality of teeth projecting from that spine and spaced apart along said spine and electrical windings on each of said teeth. wherein said teeth are enclosed within a U-shaped channel member of magnetizeable material.

i i k k

Claims

4. Apparatus as claimed in claim 3 in which said support comprises a platform forming the base of the compartment and the magnets extend across the top of the compartment, said platform being movable towards and away from said magnets.

7. Apparatus as claimed in claim 6 in which said means comprises a pusher element movable between the lower edges of the nonselected cards and the platform.

12. Apparatus as claimed in claim 1 including means for forming magnetizeable portions along an edge of a card having a magnetizeable strip along that edge which comprises means for locating a card in a fixed position relative to a plurality of independently and selectively operable knives so that said strip is below said knives, and means operating said knives selectively to remove sections of said strip and leave magnetizeable portions between said removed sections.

16. For use in selecting cards from a plurality of randomly distributed cards having magnetizeable portions fixed along an edge thereof and which are distinguished from each other by the numbers and locations of those portions, apparatus comprising a compartment for said cards, a plurality of electromagnets extending across the compartment and across said edge of each card when said cards are located in saiD compartment, said magnets being selectively energized and deenergized to attract and not to attract the magnetizeable portions of the cards, and means for moving said cards into contact and out of contact with said magnets thus to separate cards having portions corresponding to the selected energized and deenergized magnets from cards not so corresponding.

22. For use in the apparatus as claimed in claim 16, an electromagnet comprising a comblike core having a spine and a plurality of teeth projecting from that spine and spaced apart along said spine and electrical windings on each of said teeth.

23. For use in the apparatus as claimed in claim 16, an electromagnet comprising a comblike core having a spine and a plurality of teeth projecting from that spine and spaced apart along said spine and electrical windings on each of said teeth, wherein said teeth are enclosed within a U-shaped channel member of magnetizeable material.