US3759621A

US3759621A - Window cleaning apparatus

Info

Publication number: US3759621A
Application number: US00187369A
Authority: US
Inventors: Carlo J De
Original assignee: CARLO J DE
Current assignee: CARLO J DE
Priority date: 1971-10-07
Filing date: 1971-10-07
Publication date: 1973-09-18
Anticipated expiration: 1990-09-18

Abstract

Apparatus for simultaneously cleaning both sides of a window comprises master and slave magnetic cleaning units. When the cleaning units are aligned on opposite sides of the window, magnetic attraction causes the slave cleaning unit to press against and to slide on the outside of the window in response to corresponding movement of the master cleaning unit on the inside of the window. Self-contained cleaning fluid reservoirs in alternative embodiments of the cleaning units controllably dispense cleaning fluid onto the window surfaces in response either to gravity forces, controlled by orientation of the master unit with respect to the vertical, or to magnetic biasing means actuated by a control means on the master unit.

Description

limited States Patent [1 1 De Carlo WINDOW CLEANING APPARATUS [76] Inventor: Joseph P. De Carlo, 200 Harding CL,

Centerport, NY. 11721 22 Filed: Oct. 7, 1971 [21] Appl. No.: 187,369

Related US. Application Data [63} Continuation-impart of Ser. No. 21,050, March 19,

1970, abandoned.

[52] 11.8. C1, 401/10, l5/250.03, 15/220 A see) [ Sept. 18, 1973 Primary Examiner-Robert W. Michell Assistant Examiner-J. H. Wolff Att0rneyJames 1-1. Callahan et a1.

[57] ABSTRACT Apparatus for simultaneously cleaning both sides of a window comprises master and slave magnetic cleaning units. When the cleaning units are aligned on opposite sides of the window, magnetic attraction causes the slave cleaning unit to press against and to slide on the outside of the window in response to corresponding movement of the master cleaning unit on the inside of the window. Self-contained cleaning fluid reservoirs in alternative embodiments of the cleaning units controllably dispense cleaning fluid onto the window surfaces in response either to gravity forces, controlled by orientation of the master unit with respect to the vertical, or to magnetic biasing means actuated by a control means on the master unit.

4 Claims, 8 Drawing Figures PAIENTEDSEPI 8'975 sum. 1 or 5 wwwm W w @l WT mpW w @TW PAIEmEnsEH 3.759.621

SHEEI 2 OF 3 INVENTOR M05591 10. .06 @QQo WINDOW CLEANING APPARATUS I This is a continuation-in-part of copending application Ser. No. 21,050, filed Mar. 19, 1970, now aban- 'doned.

BACKGROUND OF THE INVENTION This invention relates to window cleaning apparatus and more specifically to devices for simultaneously cleaning both sides of a window by an operator on one side of the window.

Prior devices in this field have generally required special guide bars permanently mounted on the window frame. To avoid the expense of permanentinstallations, completely portable apparatus has been developed. An example is the window washing device disclosed in U.S. Pat. No. 3,120,672, issued to P. F. DiCostanzo on Feb. I l, 1963. In the DiCostanzo device, an elongated tank unit carrying a supply of cleaning fluid is designed to travel on rollers along the top of the window frame. Extensible flexible tubes extend from the tank unit on either side of the window and have permanent magnets covered by absorbent cleaning padsattached to their lower ends. Attraction between the magnets causes the pad on the outside of the window to press against the glass and to follow the interior pad as it is moved across the inner surface of the winow. Squeezing a rubber air bulb connected to the top of the tank forces fluid out of the tank, down the tubes, and out small. holes adjacent the cleaning pads onto the window surfaces. 7

Although the DiCostanzo device provides an effective means for simultaneously washing both sides of'a window, it is limited to windows which can be opened to provide space to mount the tank unit on top of the frame. Thus, it cannot be used, for example,'on doublehung windows having a fixed upper sash or on the fixed center pane of so-called picture windows. Further- SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved portable apparatus for simultaneously cleaning both sides of a window of any type by a person standing on the inside.

It is a further object of the invention to provide window cleaning apparatus of the type described with which the cleaning step can be performed with the window shut.

A still further object of the invention is to provide improved apparatus for simultaneously cleaning both sides of a window of the type using magnetic attraction to hold separate cleaning units in aligned relation on opposite sides of a window, in which the cleaning units have absorbent pads which are easily replaceable to permit rapid change-over from washing to drying steps.

It is a still further object of the present invention to provide window cleaning units of thetype described and having self-contained cleaning fluid reservoirs from which cleaning fluid can be selectively and controllably dispensed upon both sides of the window.

It is a still further object of the present invention to accomplish all of the foregoing purposes with apparatus of extremely simple design and having a minimum number of parts for utmost economy of both initial cost and operation.

The foregoing and other objects are achieved in the present invention by an apparatus comprising a pair of cleaning units. Each cleaning unit includes a lightweight housing having a substantially flat face with means for detachably mounting an absorbent cleaning pad thereon. Magnetic holding means are mounted in the housing of each unit, the magnetic holding means of each unit being positioned relative to the magnetic holding means of the other unit so as to hold the units in place by magnetic attraction after they have been placed in opposed alignment on each side of a window.

For convenience, one of the cleaning units is designated the master unit and is intended to be placed in contact with the more accessible surface of the window, usually the inside. The other cleaning unit is designated the slave unit and is placed opposite the master unit on the more remote surface of the window, usually the outside. The flux density produced by the magnetic holding means of both units is chosen to provide sufficient attractive force between the units to cause the slave unit on the outside of the window to follow the master unit as the operator moves it across the inside of the window.

Furthermore, the magnetic flux is distributed in at least two laterally spaced-apart zones, the separation of the zones permitting themagnetic attraction forces between units 'to develop moment forces on the slave'unit to cause it to remain in alignment when the master unit is given rotary as well as translatory motion.

Preferably, each cleaning unit includes a reservoir for cleaning fluid within the housing and a conduit leading from the reservoir to adjacent the window surface for dispensing the cleaning fluid upon the window as needed during the cleaning operation.

An important feature of the present invention is that simultaneous dispensing of cleaning fluid from the reservoirs of both units is controllable by the operator. In one preferred embodiment cleaning fluid can be dispensed in response to gravity forces through a conduit leading from a reservoir within the housing of each unit to a location adjacent one edge of the unit. When additional fluid is needed, the operator merely rotates the master unit against the window until the dispensing location is below the reservoir. Because the magnetic flux between the units is distributed in at least two spacedapart zones, the slave unit will rotate in alignment with the master unit so that cleaning fluid will be dispensed therefrom simultaneously with that from the master unit. 1

In other preferred embodiments, cleaning fluid can be dispensed in response to magnetic biasing means actuated by a control means on the master unit. Magnetic forces produced by the magnetic biasing means may be used either directly to operate pumping means for forcing cleaning fluid from the reservoir through the dispensing conduit or indirectly to control potential energy storage means which, in turn, produce the pumping pressure for forcing cleaning fluid through the dispensing conduit.

The pumping means in those embodiments where the magnetic biasing means acts directly to force cleaning fluid through the dispensing conduit may comprise collapsible containers for expelling fluid in response to collapsing force exerted by the biasing means when actuated by a manually operated control means on the master unit. The collapsible containers are preferably of the type having flexible walls, such as rubber or plastic bags. Alternatively, they may have one or more movable walls, such as a cylinder containing a movable piston.

Regardless of the Structure of the collapsible containers, the magnetic force between biasing magnets in the master and slave cleaning units provides the direct pumping force for dispensing cleaning fluid. The dispensing pressure available varies directly with the field strength, and consequent attractive force, of the biasing magnets.

On the other hand, when the magnetic biasing means are used indirectly to control potential energy storage means, the pumping pressure is determined by the stored potential energy. The function of the potential energy storage means is to amplify the effectiveness of the magnetic biasing means in dispensing the cleaning fluid, so that comparatively small and light-weight magnets can control relatively large pumping pressures.

The potential energy storage means preferably includes a container of pressurized propellant of the type used for so-called aerosol dispensers. The propellant, when released by a valve operated by movement of the magnetic biasing means, provides the pressure for dispensing the cleaning fluid.

The potential energy storage means alternatively may comprise, for example, electrical energy storage means, such as a battery, for powering an electrically driven pump for dispensing cleaning fluid in response to a switch operated by movement of the magnetic biasing means.

In one preferred embodiment the cleaning fluid reservoir of each unit itself comprises a collapsible container. A movable compression member is disposed in the housing on the opposite side of the container from the face of the housing. Biasing magnets are mounted on the compression members of each unit, the magnet in the slave unit being fixed and the magnet in the master unit being movable by manually operable control means to selectively produce either attraction or repulsion magnetic forces between the master and slave biasing magnets when their respective units are in alignment on opposite sides of a window.

In using this embodiment, the operator places the master and slave cleaning units in aligned relation on opposite sides of the window to be cleaned with their absorbent pads contacting the window surfaces. As the operator moves the master unit over the inside of the window, the slave unit will follow in substantial alignment as a result of the attraction between the respective magnetic holding means in each unit.

When additional cleaning fluid is needed the operator merely adjusts the control means on the master unit to position the biasing magnet in the master unit in proper relation to the biasing magnet in the slave unit to produce attraction force between them. This attraction force biases the compression members against the respective collapsible containers, thus forcing cleaning fluid out through the dispensing conduits of each unit. When sufficient cleaning fluid has been dispensed, the operator readjusts the control means to position the biasing magn et of the master unit in repulsion relation with the biasing magnet of the slave unit.

DESCRIPTION OF THE DRAWINGS Other objects, aspects and features of the present invention will be apparent from the following description and drawings, in which:

FIG. 1 is a perspective view of a preferred embodiment of the invention employing direct-acting magnetic biasing means for dispensing cleaning fluid as it may be used to simultaneously clean both sides of a window;

FIG. 2 is a cutaway perspective view along lines 2--2 of FIG. 1, showing the details of the fluid dispensing mechanism;

FIG. 3 is a cross-section along lines 33 of FIG. 1, showing the position and operative relation of the magnetic holding means and fluid dispensing means;

FIG. 4 is a cross-section along lines 44 of FIG. 3;

FIG. 5 is a cross-section similar to FIG. 4 of an alternate embodiment of the invention;

FIG. 6 is a cross-section similar to FIG. 3 of another modified embodiment of the invention;

FIG. 7 is a cutaway perspective view of another embodiment of the invention employing potential energy storage means controlled by magnetic biasing means for dispensing cleaning fluid; and

FIG. 8 is a cross-section along lines 88 of FIG. 7, showing the operating linkage between the magnetic biasing means and the pOtential energy storage means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, and particularly to FIGS. 1-4, the present invention comprises a master cleaning unit 11 and a slave cleaning unit 12. In FIG. 1, the units are shown in operative relation on opposite sides of windowpane 14. The two units are identical except for handles and a cleaning fluid dispensing control means included in the master unit-only. Identical elements in each unit are designated by the same reference number. For convenience, only the construction of master cleaning unit 11 will be described, it being understood that slave cleaning unit 12 is identical except where noted.

Master unit 11 includes an elongated hollew housing 15, which differs from housing 16 of slave unit 12 only in the provision for mounting

handles

17 and 18 and control means 20, elements which are not part of slave unit 12. Housing 15 may be made of any suitable lightweight nonmagnetic material, such as molded plastic or aluminFm.

Housing 15 has a substantially flat face 22 with inwardly sloping flanges 24 extending along each longitudinal edge to form a shallow slot for slideably receiving and holding an absorbent cleaning pad 26.

Inside housing 15, collapsible container 28 serves As a reservoir for cleaning fluid 30. Dispensing conduit 32 leads from collapsible container 28 outside housing 15 to a dispensing location adjacent absorbent cleaning pad 26.

At either end of housing 15, magnetic holding means such as

magnets

34 and 36 are mounted with their pole faces adjacent housing face 22. Holding

magnets

38 and 40 are similarly mounted in housing 16 of slave unit 12 but, as shown in FIG. 4 with their pole faces arranged oppositely to the pole faces of holding

magnets

34 and 36. That is, the North seeking pole of holding magnet 38, for example, will be opposite the South 34, 38 and 36, 40 will exert attractive forces upon each other when cleaning

units

11 and 12 are aligned with each other on opposite sides of window-pane 14.

As shown particularly in FIG. 2, a fluid dispensing means includes a control means 20, found onlyon master cleaning unit 11, such as, for example, a dialing means such as disc 41 rotatably mounted on the back 42 of housing by means of bushing 44. One end of square shaft 46 is slideably keyed into square hole 48 passing along the axis of bushing 44 and partially into disc 41. The other end of square shaft 46 is fixed to a magnetic biasing means, such as magnet 50, which in turn contacts movable compression member 52.

Slave cleaning unit 12 also has a magnetic biasing means such as magnet 54 identical to magnet 52 of master cleaning unit 1 1 except that it is rigidly attached to an identical compression member 52. As shown more clearly in FIG. 3, compression member 52 is free to slide toward and away from housing face 22 on

support rods

54 and 56.

Referring again to FIG. 2, biasing magnet 50 is affixed onto square shaft 46 with an orientation such that when arrow 58 on disc 41 is aligned with dispensing conduit 32, the pole faces of magnet 50 are positioned to produce maximum magnetic attraction with the pole faces of bias magnet 54 in slave unit 1-2 when the two units are in alignment on opposite sides of window 14. That is, the North seeking pole of magnet 50-will'be positioned opposite the South seeking pole of magnet 54, and vice versa.

Since both biasing

magnets

50 and 54 are free to move with compression members 52 in a direction per-v pendicular to housing faces 22, the magnetic attraction between them will urge compression members 52 of both master and slave cleaning units toward their respective housing faces 22 and forcecleaning fluid 30 out of both collapsible containers 28 through dispensing conduits 32 for application simultaneously to both sides of windowpane 14. When sufficient cleaning fluid has been dispensed, the operator can rotate disc 41 a half turn so that alike poles of

magnets

50 and 54 are then in opposed relatiOn, thereby producing repulsion magnetic forces between them and releasing the biasing pressure on the collapsible containers 28.

In the magnetically actuated fluid dispensing means described above the magnetic force between the master and slave biasing magnets provides the direct pumping force for dispensing the cleaning fluid from the reservoirs. In the specific embodiment of FIGS. 1-4, this magnetic force acts on the entire fluid in each reservoir through the pressure of compression members 52 against collapsible containers 28. The pressure transmitted to the fluid is equal to the attractive force between the magnets divided by the effective piston area ervoir. This allows the effective piston area of the compression members to be quite small and, correspondingly, the biasing magnets to be relatively small and lightweight while still providing the desired fluid dispensing pressure. For example, the collapsible container can be sized to hold one squirt of cleaning fluid and be connected through a check-valved conduit to the rest of the reservoir so as to be replenished from the main portion of the reservoir after each actuation of the control means.

Furthermore, although the collapsible containers 28 shown in the embodiment of FIGS. 14 are in the form of flexible-walled bags, the invention is not limited to such construction. The collapsible containers, for example, can be in the form of a cylinder and piston combination. The piston is connected by a piston rod to the biasing magnet and serves as a compression member to pump fluid contained in the cylinder through the dispensing conduit in response to actuation of the control means. The cylinder can be replenished through conduits leading to the main reservoir, and various valve arrangements well known in the art can be used to provide single-acting or double-acting pump operation, as desired.

Referring next to FIG. 5, a modification of the preferred embodiment is shown which makes use of gravitational rather than magnetic force to dispense cleaning fluid from a self-contained reservoir. In this embodiment, the master and slave cleaning units are identical in all respects, with the possible exception of optional handles (not shown) for use with the master unit. All elements of these units are the same as corresponding elements in the previously described embodiment except for the.,dispensing mechanism, so only that feature will be described.

Within elongated hollow housings 60 of both master and slave cleaning units, cleaning fluid reservoirs 62 are filled through any suitable means, such as a stoppered hole (not shown), with cleaning fluid 30. For use, the master and slave units are placed, as described above, in aligned relation on opposite sides of windowpane 14.

In normal use, the units will be aligned with dispensing conduits 32 positioned above the housings 60. When additional cleaning fluid is needed, the operator merely rotates the master unit a half turn on the inside of the window to place dispensing conduit 32 below housing 60, as shown in FIG. 5. Because of the attraction between the magnetic holding means Of the two units, as previously described, the slave cleaning unit will remain in substantial alignment with the master unit so that its dispensing conduit will also be rotated to a position below its housing 60. When sufficient cleaning fluid 30 has flowed down and out of dispensing conduits 32 under the influence of gravity, the operator will rotate the master unit another half turn to again position both dispensing conduits 32 above housing 60.

In this connection it should'be noted that by using a pair of holding magnets, one in each end of the housing, spaced-apart magnetic flux zones are created which permit substantial moment force to be developed tending to rotate the slave unit when the master unit is rotated. It will be apparent that the same effect could be obtained by using a single magnet in each unit having its poles spaced a substantial distance apart, such as a long bar magnet.

Referring to FIG. 6, another modification of the preferred embodiment is shown having no self-contained reservoir. This embodiment is intended for use with smaller windows which can be cleaned by saturating the absorbent cleaning pads once in a bucket of cleaning fluid; positioning the cleaning units in aligned relation on opposite sides of the window; wiping the master unit over the inside surface, with the slave unit simultaneously washing the outside surface; removing the units from the window; replacing the wet pads with clean dry pads; and then drying the washed window by the same process.

The embodiment of FIG. 6, by not including a selfcontained reservoir of cleaning fluid, can use smaller magnets than the previously described embodiments, but in other details it is similar to those embodiments and will not be separately described here. It should be noted, however, that the embodiment of FIG. 6 includes the feature of the spaced-apart magnetic holding means of the invention; so that the slave unit will easily follow any rotary motion of the master such as nor mally occurs in the sweeping motions used in washing windows. Thus, the holding magnets of the two units will not rotate out of alignment during the cleaning operation with resultant loss of holding force exerted o the slave unit.

In the alternate preferred embodiment shown in FIGS. 7 and 8, the fluid dispensing means includes potential energy storage means, such as identical containers 70 for pressurized propellant fluid, in both the master unit 72 and slave unit 74 for providing the pumping pressure to dispense cleaning fluid from identical reservoirs 76. Magnetic biasing means, such as

magnets

78 and 80 in the master and slave units, respectively, are connected through identical levers 82 to control the operation of conventional aerosol valves 84 mounted on containers 70. When opened, valves 84 permit pressurized propellant to flow from containers 70 through conduits 86 to reservoirs 76, thereby displacing cleaning fluid from the reservoirs through dip tubes 88 and out dispensing conduits 32.

To actuate the fluid dispensing means of this embodiment, the operator presses a control means, such as push button 90 extending through the top of master unit 72 and conneted to biasing magnet 78. Downward motion of biasing magnet 78 pivots lever 82 about pin 92 to open valve 84 in the master unit. At the same time, the magnetic attraction forces between biasing magnet 78 in the master unit and biasing magnet 80 in the slave unit cause magnet 80 to move downward and actuate Valve 84 in the slave unit in a similar manner. All other features necessary for operation of the embodiment of FIGS. 7 and 8, such as magnetic holding means (not shown) mounted in end compartments 94 and 96 are similar to those in previously described embodiments and will not be described here.

Although the preferred embodiments have been illustrated and described above, it will be understood that the invention is not so limited. For example, electromagnets can be used in place ofthe permanent magnets shown, and the shape of the housing and positioning of the magnets can be varied without departing from the principles of the invention.

Furthermore, different linkage arrangements are possible for using movement of the magnetic biasing means to control, either directly or through the amplifying power of potential energy storage means, the selective dispensing of cleaning fluid from the reservoirs of both master and slave unit. For example, the fluid dispensing means of master and slave units can be made to operate independently by disconnecting the magnetic biasing means of the master unit from the fluid dispensing linkage of that unit and having separate control means for each.

Similarly, in those embodiments in which the magnetic biasing means operate pumping means directly to dispense cleaning fluid, the pumping means of the master unit can be independent of operation of the magnetic means and be manually operable as, for example, by a push button acting against a collapsible container. The magnetic biasing means of both units would then be used only to operate the pumping means of the slave unit.

In addition, the invention covers the placement of fluid dispensing means in the slave unit only, if desired for reasons of economy, since the window surface cleaned by the master unit is accessible for application of cleaning fluid from a separate container or applicator.

What is claimed is:

1. Apparatus for simultaneously cleaning both interior and exterior surfaces of a window comprising a master cleaning unit and a slave cleaning unit, each unit including:

a' housing having a substantially flat face;

an absorbent cleaning pad removably mounted on said housing face;

magnetic holding means mounted in said housing, the

magnetic holding means of the master cleaning unit being disposed relative to the magnetic holding means of the slave cleaning unit so as to produce sufficient attractive force between said magnetic holding means when said master and slave cleaning units are aligned on opposite sides of the window with said absorbent cleaning pads contacting the window surfaces to cause said slave unit to move in alignment with the master unit when the master cleaning unit is moved over one surface of the window;

fluid dispensing means in at least said slave unit comprising a collapsible container for cleaning fluid contained within said housing and a conduit leading from said container to the exterior of said housing adjacent the cleaning pad for applying the cleaning fluid to the surface of the window; said master cleaning unit further comprises a master magnetic biasing means mounted within the housing for providing a selectively variable magnetic flux field and manually-operated control means connected to the master magnetic biasing means for selectively varying the magnetic flux field induced by the master magnetic biasing means; and

said slave cleaning unit further comprises a slave magnetic biasing means disposed within the housing for movement toward or away from said collapsible container of said slave unit in response to the selectively variable magnetic flux field induced by the master magnetic biasing means when the master and slave cleaning units are aligned on opposite sides of the window with said absorbent pads contacting the window surfaces, whereby the cleaning fluid in the collapsible container of the slave unit is selectively forced through the conduit for application to the surface of the window in response to operation of the master unit control means.

2. The apparatus of claim 1 wherein said manually-operated control means selectively reverses the magnetic flux field induced by the master magnetic biasing means, and

said slave magnetic biasing means comprises a magnet disposed on the opposite side of the collapsible container from the slave unit housing face for movement selectively toward and away from said face in response to the selectively reversible magnetic flux field of the master magnetic biasing means.

3. The apparatus of claim 2 wherein said master and slave units both include said fluid dispensing means and said master magnetic biasing means comprises a magnet slideably mounted within the master unit housing on the opposite side of the collapsible container from the housing face for movement toward and away from said face.

4. The apparatus of claim 3 wherein the manually operable control means of the master cleaning unit comprises:

dialing means rotatably mounted on the back of the housing for rotation one-half turn about an axis substantially perpendicular to the face of the housing and means for mounting the master biasing magnet, said mounting means being slideably keyed to said dialing means for rotation by said dialing means and translation toward and away from said dialing means along said axis of rotation, whereby the biasing magnet of said master cleaning unit is selectively rotatable one-half turn in response to rotation of said dialing means to reverse the position of the poles of the master biasing magnet.

Claims

1. Apparatus for simultaneously cleaning both interior and exterior surfaces of a window comprising a master cleaning unit and a slave cleaning unit, each unit including: a housing having a substantially flat face; an absorbent cleaning pad removably mounted on said housing face; magnetic holding means mounted in said housing, the magnetic holding means of the master cleaning unit being disposed relative to the magnetic holding means of the slave cleaning unit so as to produce sufficient attractive force between said magnetic holding means when said master and slave cleaning units are aligned on opposite sides of the window with said absorbent cleaning pads contacting the window surfaces to cause said slave unit to move in alignment with the master unit when the master cleaning unit is moved over one surface of the window; fluid dispensing means in at least said slave unit comprising a collapsible container for cleaning fluid contained within said housing and a conduit leading from said container to the exterior of said housing adjacent the cleaning pad for applying the cleaning fluid to the surface of the window; said master cleaning unit further comprises a master magnetic biasing means mounted within the housing for providing a selectively variable magnetic flux field and manually-operated control means connected to the master magnetic biasing means for selectively varying the magnetic flux field induced by the master magnetic biasing means; and said slave cleaning unit further comprises a slave magnetic biasing means disposed within the housing for movement toward or away from said collapsible container of said slave unit in response to the selectively variable magnetic flux field induced by the master magnetic biasing means when the master and slave cleaning units are aligned on opposite sides of the window with said absorbent pads contacting the window surfaces, whereby the cleaning fluid in the collapsible container Of the slave unit is selectively forced through the conduit for application to the surface of the window in response to operation of the master unit control means.

2. The apparatus of claim 1 wherein said manually-operated control means selectively reverses the magnetic flux field induced by the master magnetic biasing means, and said slave magnetic biasing means comprises a magnet disposed on the opposite side of the collapsible container from the slave unit housing face for movement selectively toward and away from said face in response to the selectively reversible magnetic flux field of the master magnetic biasing means.

4. The apparatus of claim 3 wherein the manually operable control means of the master cleaning unit comprises: dialing means rotatably mounted on the back of the housing for rotation one-half turn about an axis substantially perpendicular to the face of the housing and means for mounting the master biasing magnet, said mounting means being slideably keyed to said dialing means for rotation by said dialing means and translation toward and away from said dialing means along said axis of rotation, whereby the biasing magnet of said master cleaning unit is selectively rotatable one-half turn in response to rotation of said dialing means to reverse the position of the poles of the master biasing magnet.