US3834350A

US3834350A - Electrographic apparatus for magnetic printing

Info

Publication number: US3834350A
Application number: US00084947A
Authority: US
Inventors: R Hodges
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1967-11-23
Filing date: 1970-10-28
Publication date: 1974-09-10
Anticipated expiration: 1991-09-10

Abstract

Electrographic apparatus wherein a rotatable surface is interposed between the recording surface of the apparatus and means is provided for applying printing powder to the rotatable surface, the rotatable surface providing the means to develop a latent image on the recording surface. The thickness of the layer of printing powder is regulated by a plurality of magnetic drums spaced apart from one another by an amount which decreases near to the recording surface.

Description

United States Patent Field of Search 346/74 MP Hodges a Sept. 10, 1974 ELECTROGRAPHIC APPARATUS FOR [56] References Cited MAGNETIC PRINTING UNITED STATES PATENTS [75] Inventor: Robert James Hodges, Cheshunt, 3,250,636 5/1966 Wilferth 346/74 MP England 3,370,546 2/1968 Muller 346/74 ES 3,419,884 12/1968 Betts et a1. 346/74 ES Asslgneer Standard Telephones and Cables 3,437,074 4/1969 l-lagopian et a1 346/74 MP Limited, London, England [22] Filed. Oct 28 1970 Primary Examiner-Bernard Konick Assistant Examiner-Jay P. Lucas [211 App]. No.2 84,947 Attorney, Agent, or FirmA. Donald Stolzy Related US. Application Data [63] Continuation of Ser. No. 770,691, Oct. 25, 1968, [57] ABSTRACT abandoned Electrographic apparatus wherein a rotatable surface is interposed between the recording surface of the ap- 30 Foreign Application priority Data paratus and means is provided for applying printing Nov 23 1967 Great Britain 53328/67 powder to the rotatable surface, the rotatable surface 8 1968 Great Britain 415/68 providing the means to develop a latent image on the recording surface. The thickness of the layer of print- [52] Us Cl 118/637 346/74 MP ing powder is regulated by a plurality of magnetic [51] d 15/12 Bosh 3/02 drums spaced apart from one another by an amount [58] which decreases near to the recording surface.

1 Claim, 8 Drawing Figures PATENIEB SEP 1 0 I974 SHEET 1 Bf 4 Inventor ROBERT J. #0055 A Home y PATENIEBSEPIOW 3334.350

SHEET 2 OF 4 I v J5 I I Inventor ROBERT J. #00565 www- A Home y PAIENTEDSEP10I974 3.884.350

SHEU SN 4 Inventor ROBERT J. #00 55 @Mw-l A Home y PAIENIED EPI 3.884.350

sass: u or 4 Inventor ROBERT J. #00 55 Aflorne y ELECTROGRAPHIC APPARATUS FOR MAGNETIC PRINTING This application is a continuation of a copending parent application, now abandoned, Ser. No. 770,691, filed Oct. 25, .1968, and of the-same title.

The invention relates to electrographic printing and- /or display apparatus.

Electrographic'printing and/or display apparatus is defined as apparatus of the kind wherein the recording surface of either a print drum or band is selectively magnetised or electrostatically charged to form a pattern or latent image thereon representative of information contained in a signal applied to the apparatus, and wherein the print drum .or band is passed through or relative to a powder applicator containing a powder that is attracted to the electromagnetically .or electrostatically formed latent image to develop same and to form a powder image which may be viewed at a display position and/or transferred to a permanent record by printing means which form part of the apparatus.

In order to achieve a good print out or display, it is necessary to carefullypowder the latent image so as to achieve a clean background to the powdered image and to provide a powder image of even powder density. Fluid powder beds have been used to apply the powder directly to the latent image but difficulties with regard to irregular powder built up on the latent image have been experienced which are in the main due to a jet ting effect occurring in the powder contained in the fluid powder beds.

The present invention provides an electrographic printing and/or display apparatus of the kind as hereinbefore defined including a permanently magnetised or electrostatically charged rotatable surface interposed between'the recording surface of said apparatus and means for applying printing powder to said rotatable surface, said rotatable surface providing the means to develop a latentimage or images on said recording surface.

The foregoing and other features according to the invention will be better understood from the following description with reference to the drawings accompanying the Specifications,- in which:

' FIG. 1 diagrammatically illustrates a perspective view in part section of part of an electrographic printing apparatus and a transfer powdering device.

FIG. 2 diagrammatically illustrates a sectioned front view of part of an electrographic printing apparatus and another transfer powdering arrangement.

FIG. 3 diagrammatically illustrates a perspective view in part section of part of an electrographic display apparatus and the transfer'powdering device shown in the drawing according to FIG. .1,

FIG. 4 diagrammatically illustrates a sectioned front view of an electrographic printing and/or display apparatus and another transfer powdering device, a

FIG. 5 diagrammatically illustrates a sectioned front view of part of either the electrographic printing apparatus shown in thedrawing according to FIG. 1 or2 or the electrographic display apparatus shown in the drawing according to FIG. 3 and an impeller powdering device,

FIG. 6 diagrammatically illustrates a cross-sectioned end view of part of an impeller powdering device.

FIG. 7 diagrammatically illustrates a sectioned front view of part of an electrographic printing and/or dis- 2 play apparatus and another impeller powdering device, and

FIG. 8 diagrammatically illustrates a plan view of an electrographic display apparatus which utilizes a transfer and/or impeller powdering device.

Referring to FIG. 1, part of an electrographic printing apparatus and a transfer powdering device are diagrammatically illustrated therein in a part sectioned perspective view. The transfer powdering device basically consists of a band or roller 1 interposed between a powder reservoir 3 and the recording surface 4 of the print drum 5 which forms part of the electrographic apparatus and which is rotated in the direction of arrow D by a drive shaft 8 attached thereto by means not shownin FIG. 1.

It will be assumed by way of example that the recording surface of the print drum 5 is selectively magnetized to form the pattern thereon therefore the outer surface 2- of the band or roller 1 will need to be permanently magnetised in a manner such as to collect thereon an evenly distributed layer of magnetic printing powder 6 contained in the reservoir 3 when it is rotated therein in the direction of arrow E by a drive shaft 7 which is attached to the band or roller 1 by means not shown in FIG. 1.

The permanently magnetised transfer surface 2 is separated from the recording surface 4 by an amount such that consistent powder transfer to the latent magnetic image which is formed on the recording surface 4 of the print drum 5 by means of the magnetic recording head unit 9, is obtained, i.e., the recording surface 4 of the print drum 5 is arranged to just contact the outer surface of the evenly distributed layer of printing powder 6 on the surface 2 of the band or roller 1 and such that the magnetised surface 2 does not have a demagnetising effect on the latent image. The strength of the magnetic field of the band or roller 1 therefore determines the separation distance between the transfer surface 2 thereof and therecording surface 4 since the stronger the I field of the transfer surface 2 then the greater the build up of the printing powder 6 and therefore the gap between the transfer surface 2 and the recording surface 4 will need to be larger in order to ensure the recording surface 4 just contacts the. outer surface of the layer of printing powder 6. Therefore, the stronger magnetic field will have the least demagnetisa- 'tion effect on the latent magnetic image.

Normally, the surface 2 of the roller 1 and the surface 4 of the print drum 5 would have the same linear velocity but if any irregularities exist at the outer surface of printing powder 6 then it may be necessary for the surface 2 to have a higher linear velocity than the surface 4.

When the latent magnetic image is powdered with the printing powder 6, the print drum surface 4 comes in contact with a strip of paper 10 which is moving in the direction of the arrow A at the same linear velocity as the surface 4 of the print drum 5. The paper strip 10 is dispensed from a roller of paper 13 wound on a shaft 14 which is roatated in the direction of the arrow B. A pressure roller 11 which is rotated in the direction of the' arrow C by a driving shaft 12 attached thereto by means not shown in FIG. 1, presses the paper strip 10 against the drum surface 4, and the powdered image is transferred under pressure from the drum surface 4 to the surface of the paper strip 10. It is the usual practice in such processes to include a therthe surface of the paper strip 10.

mal fixing agent, for example, resin or wax, in the powder formulation so that the powdered image may be fixed by the application of heat subsequent to the formation of the powdered image, therefore the paper strip after passing between the drum 5 and the roller 11 is passed through heating means (not shown in FIG. 1) wherein the powdered image is thermally bonded to The thickness of the layer of printing powder on the permanently magnetised transfer surface 2 of the band orroller 1 may be more accurately controlled in order toeffect better control of the transfer-of the printing powder6 by utilizing a plurality of bands or rollers 1 interposed between the recording surface 4 of the print drum 5 and the powder reservoir 3 which may be arranged for example as shown in the drawing according to FIG. 2.

Referring to FIG. 2 part of an electrographic printing apparatus and another transfer powdering arangement are diagrammatically illustrated therein in a sectioned front view. The transfer powdering arrangement of this apparatus utilizes three transfer bands or rollers, i.e., the

rollers

1, 1A, and 1B interposed between the recording surface 4 and the powder reservoir 3, the gap between the surface 2 of the roller 1A and the surface 20f the roller 13 being less than the gap between the storage is required due to leakage problems associated with the magnetic images.

' 3 FIG. 3 diagrammatically illustrates a perspective view in part section of part of an electrographic display apparatus and the transfer powdering device outlined in preceding paragraphs-with reference to the drawing according to FIG. 1. It will of course be appreciated that the transfer powdering device may also in this application be modified and adapted as outlined in preceding paragraphs. The operation of this apparatus in basically the same as the apparatus outlined with reference to P16. 1 except the print out facility is eliminated and the latent magnetic image which is formed on'the surface of a band 15 by means of a magnetic recording head unit 19 and rendered visible by powdering with the band or roller 1 in association with the powder reservoir 3 in a manner as previously described, is presented in visible form to an observer by arranging for that part of the surface 20 of the band 15 which ap- 20 pears between the transfer powdering device and an surface 2 of the roller 1A and the surface 20f the roller l but the same as the gap between the surface 2 of the roller 18 and the surface 4 of the print drum 5. The strength of the magnetic field at the surface 2 of the roller 1A is arranged to be greater than the magnetic field at the surface 2 of the roller 1B but less than the magnetic field at the surface 2 of the roller 1 therefore the thickness of thelayer of printing powder 6 on the surface 2 of theroller 1B i.e., the roller nearest to the recording surface 4 will be accurately controlled, the printing powder 6 being evenly distributed by the succession'of rollers since each subsequent roller in the chain of rollers between the powder reservoir 3 and the recording surface 4 reduces the thickness of the layer of printing powder on the previous roller by a fixed amount and tends to even out-any irregularities at the outer surface of the powder layers.

Alternatively, the thickness of the layer of printing powder 6 on the permanently magnetized transfer surface 2 of the roller 1 may be more accurately controlled by utilizing stripper blades placed at the appropriate distance from the transfer roller 1 in order to carefully skim off any excess powder thereby obtaining the required thickness of the printing powder 6. With this arrangement any surface irregularities tend to be eliminated and the printing powder 6 is therefore tended to be more evenly distributed on thesurface 2 of the roller 1. It must however be noted that the amount of printing powder skimmed off must be carefully controlled in order to ensure that the permanently magnetized surface 2 of the roller 1 does not affect the latent magnetic image on the recording surface 4 of the print drum 5.

erase unit 21 to be made viewable. The band 15 which is guided by

rollers

16 and 18 and driven by means of the roller 16 which is rotated in the direction of the arrow J by a drive shaft 17 attached thereto by means not shown in the drawing may be similar to that of magnetic tapes used in conventional recording apparatus, for example a supporting band of an insulating and non-magnetic material such as a polymer film, coated on the surface 20 with a thin film of magnetic material such as iron oxide or a nickel cobalt alloy. 1

The colour of the printing powder is preferably dark to provide the highest possible contrast with the surface 20; it is preferred to provide the band with a white, matt silver or reflecting coating which need only be a few microns thick, in order to increase the contrast and hence the legibility of the displayed image or images. The surface 20 of the band 15 may be illuminated when in the display position by means of a light source. It willbe seen from the drawing that the magnetic recording head unit 19 operates on the opposite surface of the In certain instances, for example when lengthy storage of the magnetic image on the recording surface 4 is required, it may prove necessary to control the relative strengths of the magnetic fields of the transfer surface 2 and the magnetic images on the recording sur-v face 4,'since the demagnetization effect of the transfer surface 2 will become very important when lengthy band 15 from the surfaceto which the powder is applied. This arrangement is preferred in order to reduce the possibility of abrasion of the contrasting surface by the recording head, but since the magnetic coating on the film must then be on the side remote from the magnetic recording head the polymer film itself must be no more than about 0.001 inches thick. In order to achieve a more durable band 15, the supporting band of insulating and non-magnetic material may be replaced by a thin non-magnetic metal supporting band for example of beryllium copper.

\ It should be noted that the electrographic printing apparatus'and display apparatus shown in the drawings according to FIGS. 1 -to 3 may be combined as a single unit to provide both the display facilities and a permanent copy of the display either continuously or at will.

The transfer powdering devices outlined in preceding paragraphs are primarily for use with horizontal recording surfaces, when it is required to powder a vertical recording surface utilizing this principle then the transfer powdering device may take the form shown in the drawing according to FIG. 4 which diagrammaticaly. illustrates a sectioned front view of an electrographic printing and/or display apparatus and a transfer powdering device. This transfer powdering device basically consists of a bevelled disc 22 i.e., frustoconical in form interposed between the powder reservoir 3 and the vertical recording surface 25 of the machines print drum or band 24. j

The bevelled surface 26 of the disc 22 is permanently magnetised in a manner such as to collect thereon an evenly distributed layer of the magnetic printing powder 6 contained in the reservoir 3 when it is rotated therein in the direction of arrow K by a drive shaft 23 which is attached to the disc 1 by means not shown in FIG. 4. All operating considerations etcetera which apply to the transfer powdering devices outlined in preceding paragraphs with reference to FIGS. 1 and 3 also apply to the transfer powdering device of FIG. 4.

The powder formulation which is utilized in the apparatus outlined in preceding paragraphs includes a magnetic powder such as magnesium-nickel ferrite or nickel-zinc ferrite which have a dielectric property that may be utilized in the magnetic powder transfer process. For example, the rotatable surface of either the band or roller 1 (FIG. 1), the

rollers

1, 1A and 1B (FIG. 2), theband (FIG. 3) or the bevelled disc 22 (FIG. 4) may be arranged such that -it can be charged electrostatically thereby causing the magnetic powder to be attracted thereto due to its dielectric properties to form an evenly distributedlayer of powder thereon. When the apparatus is to be operated in a printing mode the magnetic powder, as previously stated, is coated with a thermal fixing agent, for example, resin or wax so that the powdered image may be fixed? by the application of heat subsequent to the formation of the powdered image therefore the dielectric properties of the fixing agent assists in the attraction of the magnetic powder particles to the electrostatically charged rotatable surface.

With this arrangement the problems outlined in preceding paragraphs which arise due to the demagnetising effects on the latent magnetic image or images by a permanently magnetised rotatable surface no longer exist and the displacement of the rotatable surface relative to the recording surface is no longer critical.

When the latent image or images on the recording surface of the print drum or band are formed electrostatically, the recording surface will be of a material such as a dielectric material and the powder formula tion will include .a dielectric powder or ferro electric powder, for example barium titanate to form the image or pattern. Therefore with this arrangement of the apparatus the magnetic or dielectric properties of the powder may be utilized to effect the powder transfer i.e., the rotatable surface may be either permanently magnetised or electrostatically charged.

The fluid powder bed or the fluid powder bed and the transfer powdering device may be replaced by an impeller powdering device such as the ones illustrated in the drawings according to FIGS. 5 and 7.

Referring to FIG. 5 part of either the electrographic printing apparatus shown in the drawing according to FIG. 1 or 2 or the electrographic display apparatus shown in the drawing according to FIG. 3 and an impeller powdering device are diagrammaticallyillustrated therein in a sectioned front view. The impeller powdering device which is suitable for use with horizontal recording surfaces basically consists of a rotating impeller 27 having its axis parallel to the surface 28 which may be the recording surface of a print drum or the permanently magnetised or electrostatically charged transfer surface of a transfer powdering device, which p is rotated in the direction of the arrow G by a drive shaft 29 attached thereto by means not shown in FIG. 5.

The impeller 27 which is partially submerged in the printing powder 6 contained within a container 30, is of a length at least equal to the length of the surface 28 and is provided by an appropriate length of rod with a very coarse gear tooth from 31 cut along its length. Whilst the impeller shown in the drawing according to FIG. 5 is provided with a gear tooth form 31, other forms would work equally well, for example the rod may be formed along its length in a manner as shown in the drawing according to FIG. 6 which illustrates a cross-sectional end view of the impeller. Referring to FIG. 6, the tooth form which is cut along the length of the rod is, as can be seen from the drawing, of a special form in order to facilitate the collection of the printing powder 6 when the impeller is rotated therein in the direction of the arrow G. Each of the teeth 32 are shaped so as to provide recesses 33 which are of a form such that they are capable of collecting a larger quantity of the printing powder 6 than would be the case with the coarse gear tooth form 31 shown in the drawing according to FIG. 5.

The teeth formed on the impeller 27 act as paddles in that as the impeller 27 is rotated theycollect the printing powder and throw it vertically upwards towards the surface 28 thereby forming along the length of the impeller 27 an arcuate sheet of powder.

The surface 28 is placed as shown in the drawing at apoint at the top of the arc where the powder and surface are moving in the same direction and relatively at zero velocity with respect to each other therefore the impact force of the powder to the surface 28 is a minimum.

Printing powder is attracted to either the latent image or the permanently magnetised or electrostatically charged surface depending whether the surface 28 is considered to be the recording surface of a print drum or the permanently magnetised or electrostatically charged surface of a transfer powdering device to form as the case may be either a visible image or an evenly distributed layer of the printing powder.

In order to produce an arcuate sheet of printing powder of constant cross-section along the whole length of the surface 28 a deflector plate 34 is provided which is situated under the leading edge of the impeller along its whole length and which is secured to the container 30 by means not shown in the drawing and supported therein at one end by means of a support member 35. The other end of the deflector plate 34 is provided with a number of adjustable members 36 which are spaced at regular intervals along the whole length of the deflector plate 34 and which are supported by a member 37 which is attached to the container 30 by means not shown in the drawing. Each of the adjustable members 36 basically consist of a female threaded bush 38 which is secured to the deflector plate 34 and an adjusting screw 39 which screws into the bush 38, the headless end of the screw 39 being supported by the member 37.

Adjustment of the screws 39 cause the gap between the deflector plate 34 and the leading edge of the impeller 27 to be varied thus by selective adjustment of the screws 39 the height of the arcuate sheet of printing powder may be adjusted at any point along the whole length of the surface 28 thereby ensuring that the printing powder contacts all parts of the surface 28 situated directly above the top of or the highest point of the arcuate sheet of printing powder.

Variation of the speed of the impeller 27 allows adjustment of the mean height of the arcuate sheet of printing powder.

Referring to FIG. 7 part of an electrographic printing and/or display apparatus and another impeller powdering device are diagrammatically illustrated therein in a sectioned front view. This impeller powdering device which is suitable for use with vertical recording surfaces basically consists of a shaft 40 for example 0.25 inches diameter with a spiral flute 41 (approximately t.p.i.) which is rotated by means not shown in the drawing in the direction of the arrow H within a close fitting sleeve 42 having a narrow slit 43 for example 0.032 inch wide in the wall thereof which is parallel to the axis of the shaft 40. The sleeve 42 is mounted vertically within a printing powder reservoir 44, for example any one of the fluid powder beds outlined in our copending British Patent Application No. 56619/66 and secured thereto by means of a mounting bracket 46.

The skirted end 45 of the sleeve 42 is suspended in the printing powder 6 and the slit 43 is situated adjacent to a band or roller 47 which may be the printing drum of an electrographic printing and/or display machine or the band or roller which forms part of a transfer powdering device.

The shaft 40 is rotated at a speed of the order of 1,500 rpm. and the direction of the shaft rotation is such that the printing powder is drawn up the tube 42 by means of the spiral flute 41 and due to centrifugal action a controlled amount of printing powder 49 is ejected from the slit 43 onto the surface 48 of the band or roller 47. The skirting at the end 45 of the tube 42 ensures that a constant amount of printing powder is drawn up by the action of the rotating shaft 40.

Whilst the arrangement of FIG. 7 utilises a fluid powder bed, a powder bed having deflectors i.e., as shown in the drawing according to FIG. 5 may be utilised providing a powder feed mechanism is connected thereto to ensure that an adequate powder level is maintained which is sufficient to ensure efficient operation i.e. the skirted end 45 of the sleeve 42 is always covered with the printing powder 6. 1

It should be noted that a horizontal impeller having an action similar to the one described in preceding parwidth being 0.1 inches, a deflector plate would be used to deflect the powder onto the recording surface and the impeller speed and deflector system would need to be arranged such that the mean height of the sheet of printing powder thrown upwards by the action'of the impeller is sufficient to powder the requisite parts of the recording surface.

FIG. 8 diagrammatically illustrates aplan view of an electrographic display apparatus which utilises a transfer and/or impeller powdering device. The display appara'tus shown in FIG. 8 includes a band 50 which is guided in a closed loopv by a series of rollers 51, a guide roller 52 and a drive roller 53 for example a sprocket roller, the sprockets of which engage with holes provided at the outer edges of the band 50. A motor 54 drives the sprocket roller 53 and thereby the tape loop,

either continuously or under the control of input signals.

The correct tension of the band 50 is regulated by means of a roller 56 which is rotatable mounted at one end of a pivot arm 57, the other end of this arm is pivoted at the position 58. The pivot arm 57 is attached to a rigid support block 60 via a variable spring tension unit 59 by means of which the band tension is regulated.

A recording head is placed adjacent to the band 50 so that when energised by suitable electrical signals the head forms a pattern in the band as the latter moves in the-direction of the arrows X. This pattern corresponds to the visible image to be presented.

The band then passes through a powdering device 61 which may be provided by any one of the transfer or impeller powdering device outlined in preceding paragraphs which aresuitable for powdering a vertical recording surface, for example, the transfer powdering device shown in the drawing according to FIG. 4. The powdering device 61 applies to the surface of the band 50 a finely divided powder whose colour provides adequate contrast with the background of the band. The

- powder adheres to the band 50 in a pattern corresponding to the pattern formed by the recording head 55, and renders this pattern visible and suitable for direct viewlng.

The band 50 then passes to a display position where it can be viewed in the direction of the arrow Y through a window 63 in the casing 64 of the apparatus.

The tape loop 50-completes its circuit by passing through a cleansing stage 62 where the printing powder is removed. It then returns to the recording head 55; depending on the mode of operation of the latter, a preliminary erase process, indicated in the drawing according to FIG. 8-by an erase head 65, may or may not be required.

The apparatus therefore provides a means by which information received over an electrical input line can be rendered visible. The term information is here used in its widest sense and the apparatus may be used equally to display alphabetical or numerical matter, pictorial matter such as graphs, diagrams or maps, or facsimile. In most cases code conversion will be required in the driving circuits of the recording head 55 in order to create the pattern corresponding, for example, to a numerical input signal; on the other hand with certain forms of head and with an input derived from the line by line scanning of an original the input signals may be applied directly to the elements of the recording head 55.

If a permanent record of the displayed information is required either continuously or at will, then the apparatus shown in the drawing according to FIG. 8 may be modified to inlcude this facility by adding the printing arrangement outlined in preceding paragraphs with reference to the drawing according to FIG. 1. The printing arrangement would need to be interposed between the viewing stage and'the cleansing stage.

It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitawherein latent magnetic charge patterns created on said recording surface are developed into visible images, a magnetic powder developing arrangement including a permanently magnetized rotating surface, positioned proximate to the recording surface and in operative contact with a source of magnetic developing powder particles, for evenly transferring from said source the magnetic particles to said latent magnetic image, developing same substantially independent of interference to the magnetic image by the magnetism of said rotating surface; said recording and'rotatable surfaces being separated one from the other by an amount which is dependent on the relative strengths of the magnetic field associated with said latent magnetic image and said permanently magnetized rotatable surface; means for regulating the thickness of the layer of faces are to said recording surface. l

Claims

1. In a printing and/or display apparatus, the combination comprising: a body having a recording surface wherein latent magnetic charge patterns created on said recording surface are developed into visible images, a magnetic powder developing arrangement including a permanently magnetized rotating surface, positioned proximate to the recording surface and in operative contact with a source of magnetic developing powder particles, for evenly transferring from said source the magnetic particles to said latent magnetic image, developing same substantially independent of interference to the magnetic image by the magnetism of said rotating surface; said recording and rotatable surfaces being separated one from the other by an amount which is dependent on the relative strengths of the magnetic field associated with said latent magnetic image and said permanently magnetized rotatable surface; means for regulating the thickness of the layer of magnetic powder applied to the recording surface, said regulating means including a plurality of drums which are interposed between the recording surface of said apparatus and said source of magnetic powder, wherein the surfaces of said plurality of drums are permanently magnetized, the adjacent rotatable surfaces of said drums being spaced apart from one another by an amount which decreases the nearer said adjacent surfaces are to said recording surface.