JPH0736252A - Photoconductor charging device - Google Patents

Abstract

(57) [Abstract] [Purpose] A charging device capable of ensuring stable charging ability without deteriorating the charging agent even during long-term charging in particle charging [Configuration] The photosensitive member 1 is sandwiched on the downstream side of the photosensitive member charging area. Magnetic field generating means 2A, 5A (5A ') made of a magnetic body or a magnet body are arranged substantially opposite to each other on the back side and the front side, and a vertical magnetic field 4A is formed in the photoconductor normal direction, while the photoconductor charging area is upstream. On the side, the circulation means 2 for the magnetic particle group 4
B, 11, and 9 are arranged so that the magnetic particle group 4 located in the charging area can be circulated between the circulation means 2B, 11, and 9 and the vertical magnetic field 4A formation position.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging device in an electrophotographic apparatus for charging a belt-shaped or drum-shaped photosensitive member by particle charging.

[0002]

2. Description of the Related Art Conventionally, process means for exposing, developing, transferring, cleaning (removing residual toner), discharging, and charging are arranged on the outer peripheral surface of a photosensitive drum, and an image is formed by a predetermined electrophotographic process. Image forming apparatuses based on the so-called Carlson process are well known.

In recent years, a light-transmitting conductive layer and a photoconductor layer are laminated on a cylindrical light-transmitting support to form a photosensitive drum, and an optical output corresponding to image information is provided in the drum. At the same time as or immediately after the exposure means (for example, an LED head) to be generated is inserted, and the light output of the exposure means is exposed through the focusing lens on the photosensitive drum charged on the photosensitive drum using a predetermined charging means. Image formation in which the latent image is formed into a toner image (developed) through a developing sleeve facing the photosensitive drum, and then the toner image can be transferred onto a recording sheet through a transfer roller or other transfer means. Apparatuses (Japanese Patent Laid-Open No. 58-153957, etc.) are also known.

The charging means used in this type of apparatus is generally a corotron system in which a high voltage is applied to a thin tungsten wire to carry out corona discharge, or a conductive roller is charged with a voltage of several hundreds of volts to contact and charge the photosensitive drum. In addition, there is one in which a voltage is applied to a conductive brush so that the brush is charged while being in contact with the photosensitive drum. However, the corotron method uses a high voltage, and in terms of safety such as generating ozone,
There are many environmental problems. Further, since the charging roller is in line contact with the photosensitive drum, charging is unstable. Further, in the brush charging method, since the drum and the brush contact each other to perform charging, the charging deterioration of the brush is likely to occur.

In order to eliminate such a drawback, as shown in FIG. 5, a non-magnetic sleeve 103 having a photosensitive drum 101 and a magnet assembly 102 inserted therein is used, and a charging bias 108 is applied to the sleeve 103. , The sleeve 10
3, a magnetic particle group 104 is attached to the magnetic particle group 3, the brush-shaped magnetic brush is rubbed against the photosensitive drum 101, and a charging bias 108 is applied to the magnetic particle group 104 via the sleeve 103 to perform charging. Is proposed.
(JP-A-59-133569, JP-A-63-18726
7 and others)

In the charging method, in order to uniformly charge the photosensitive drum 101 with the conductive fine particles, it is necessary to make the contact area and the contact density between the magnetic particle group 104 and the photosensitive drum 101 in the charging region sufficient conditions. But,
The contact area of the magnetic brush is the photosensitive drum 1 and the magnet assembly 10
2 is determined by the outer diameter of the non-magnetic sleeve 103 in which the photosensitive drum 101 and the sleeve 1 are inserted.
The smaller the size of 03, the narrower the contact nip, and the faster the rotation speed of the photosensitive drum 101, the more likely the contact nip becomes unstable. Therefore, in order to ensure the stability of the contact nip, the gap tolerance between the photosensitive drum 101 and the non-magnetic sleeve 103 is set to be strict. However, with this configuration, the number of assembling steps and the like increase, which leads to an increase in cost. In addition, the magnetic force and the magnetic flux density fluctuate due to an assembly error such as eccentricity between the fixed magnet assembly 102 and the non-magnetic sleeve 103 and expansion / contraction due to heat and other environmental factors, so that stable charging cannot be performed.

Further, in the above-mentioned prior art, since the magnetic pole S of the magnet assembly 102 is fixed in the charging area, the magnetic particles in the charging area are structured to continue charging without moving. The long-term charging operation causes a problem that the magnetic particles are deteriorated and the charging ability is lowered.

In order to prevent such a drawback, there is a method of rotating the magnet assembly. However, in this method, the structure of the drive system is complicated and the magnetic pole moves at a high speed, so that eddy current is generated. However, problems such as scattering of magnetic particles occur.
Further, in the above-mentioned prior art, when a magnetic ear is formed by the fixed magnet assembly 102, the magnetic field is attenuated in proportion to the square of the distance from the fixed magnet assembly 102, and therefore the position on the surface of the photosensitive drum 101 is reduced. The magnetic coercive force of the magnetic particles is weakest. On the other hand, the magnetic particle groups 104 are charged by being rubbed with each other or with the photosensitive drum 101 by being rubbed in the charging region. Therefore, when the photosensitive drum 101 is rotated in this state, a centrifugal force acts on the surface of the drum 101, and the magnetic particles 104 electrostatically attached to the photosensitive drum 101 are retained by the fixed magnet assembly 102. A force acts against the (magnetic field) in the direction away from the magnetic brush, that is, the charged area, and the particles 104a attached to the photosensitive drum 101 adversely affect the exposure and development in the next step. For this reason, in the above-mentioned conventional technique, a blade is arranged on the downstream side of the charging region in the moving direction of the photoconductor to collect the magnetic particles. A mechanism for collecting the magnetic particles attached to the blade is required, which complicates the configuration.

In view of the above-mentioned drawbacks of the prior art, it is an object of the present invention to provide a charging device which can maintain a stable charging ability without being affected by the fluctuation of the charging gap and the assembly error. Another object of the present invention is to provide a charging device which can effectively prevent leakage of magnetic particles to the outside of the charging region with a simple structure. Still another object of the present invention is to provide a charging device capable of ensuring stable charging ability without deterioration of the charging agent even after long-term charging.

[0010]

According to the present invention, a magnetic field generating means 2 for magnetically carrying a magnetic particle group 4 as shown in FIG.
A first feature is that A, 5A (5A ′), that is, a magnet body or a magnetic body is arranged not only on the back side of the non-magnetic sleeve 3 but also on the back side of the photoconductor 1. In this case, the surface magnetic force of the sleeve 3 at the position where the vertical magnetic field is formed is preferably larger than the surface magnetic force of the photosensitive member. As a result, the magnetic poles for magnetically retaining the magnetic particles are not arranged on only one side of the charging gap but on both sides thereof, so that the magnetizing force (ΔH / Δ) between the charging gaps is increased.
t) can be set substantially evenly, and as a result, even if there is some assembly error in the charging gap or a large gap tolerance is taken into consideration, the eccentricity of the photosensitive drum 1 or the non-magnetic sleeve 3 is taken into consideration. Stable magnetic particle group 4
The magnetic holding power of can be maintained.

Further, the rough setting of the assembly tolerance, the amount of eccentricity, etc. leads to a reduction in the assembly man-hour and the manufacturing cost. In this case, the magnetic field generating means 2A, 5A (5A ′) may be magnet bodies of opposite polarities or a combination of magnet bodies and magnetic bodies. Further, it is generally preferable that the magnetic particle group 4 is composed of conductive magnetic particles or a mixed particle group of conductive particles and magnetic particles, and the particle group 4 is charged through the sleeve 3 or the blade. The bias 8 can be applied.

The second characteristic is that the magnetic field generating means 2A, 5A.
(5A ') are arranged so as to face each other to form a vertical magnetic field in the normal direction of the photoconductor, and the non-magnetic sleeve 3 is rotatable in the opposite direction to the moving direction of the photoconductor, that is, in the against direction. It is in. That is, by arranging the vertical magnetic field 4A in the normal direction of the photoconductor 1 on the downstream side of the charging region, a so-called vertical magnetic barrier can be formed on the downstream side of the charging region, so that the magnetic force electrostatically attached to the photoconductor 1 is formed. It is possible to prevent the particles from leaking out of the charged area, and as a result, the blade used in the above-mentioned prior art and the mechanism for collecting the magnetic particles attached to the blade are not required, and the structure is simplified. Further, according to the present invention, by rotating the non-magnetic sleeve 3 in the against direction with respect to the photoconductor, the magnetic particle group retained on the vertical magnetic field 4A can be circulated to the upstream side of the charging area.

In this case, the circulation can be performed more smoothly by constructing the sleeve surface magnetic force at the position where the vertical magnetic field 4A is formed to be larger than the photosensitive member surface magnetic force. As a result, the magnetic particles are not deteriorated and the charging ability is not deteriorated even after long-term charging. And preferably, by providing means 2B, 11 and 9 for circulating or stirring the magnetic particle group 4 on the upstream side in the moving direction of the photoconductor from the position where the vertical magnetic field 4A is formed, the magnetic particles magnetically sealed by the vertical magnetic field 4A. The group 4 can circulate or stir more smoothly in the charging area. The circulation means 2B, 11 and 9 form a non-magnetic field on the non-magnetic sleeve 3 on the upstream side of the charging area by, for example, a repulsive magnetic field provided in a fixed magnet assembly in the non-magnetic sleeve 3. The magnetic releasing means 2A, 2B formed of a band may be configured to release the magnetic restraint of the magnetic particle group 4 carried on the non-magnetic sleeve 3 and drop it on the photoreceptor side. A mechanical peeling means 9 such as a film or a blade that mechanically peels the magnetic particle group 4 supported on the upstream side of the charging area may be used.

In order to smoothly carry out the above-mentioned operation, for example, on the downstream side of the charging area of the photoreceptor, magnetic field generating means 2A composed of a magnetic body or a magnet body on the back side and the front side of the photoreceptor 1 with the photoreceptor 1 sandwiched therebetween, 5A (5A ′) are arranged substantially opposite to each other to form a vertical magnetic field 4A in the normal direction of the photoconductor, while on the upstream side of the photoconductor charging area, circulation means 2B, 11 for the magnetic particle group 4 are provided.
9 is arranged and the magnetic particles 4 located in the charging area can be circulated between the circulating means 2B, 11 and 9 and the position where the vertical magnetic field 4A is formed, so that the non-magnetic sleeve 3 is not necessarily used. It is possible.

[0015]

Embodiments of the present invention will now be illustratively described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative positions and the like of the components described in this embodiment are not intended to limit the scope of the present invention thereto, but are merely examples, unless otherwise specified. Not too much.

The structure of the charging device according to the first embodiment of the present invention will be described with reference to the drawings. As described above, the charging device 20 is arranged so that the photoconductor drum 1 rotating to the right in the figure has a charging gap (0.5 mm) with respect to the photoconductor drum 1.
A non-magnetic sleeve 3 is rotatably provided in the rotation direction and the opposite direction (left direction in the drawing), and a fixed magnet body 2A fixedly arranged on the downstream side of the charging area on the back side of the non-magnetic sleeve 3; A repulsive magnet body 2B having the same polarity as that of the fixed magnet body 2A is arranged on the upstream side of the charging area of the fixed magnet body 2A, in other words, on the downstream side in the sleeve rotation direction. Incidentally, 8
Is a bias power source for applying a developing bias to the conductive magnetic particle group 4 via a conductive blade or a non-magnetic sleeve 3 not shown.

A group of conductive magnetic particles 4 is interposed on the charged area. The magnetic particles are not particularly limited as long as they are conductive, but they are composed of conductive magnetic particles such as ferrite, iron powder, magnetite or the like coated on the surface of a magnetic core with a conductive resin, or a mixture of conductive particles and magnetic particles. It may be composed of the particle group 4. For example, a magnetic particle base material having an average particle size of about 30 μm and a conductive particle material having an average particle size of about 15 μm may be mixed in an appropriate ratio. In this embodiment, the average particle size is 20 to 35 μm and the resistivity is 10 ^3.
Ferrite core particles having a density of about 10 ⁶ Ω · cm are used, and magnetic properties thereof are set to 60 to 70 emu / g (1 k Oe).

On the other hand, on the rear surface side of the photosensitive drum 1, the first magnet body 5A and the first magnet body 5A are arranged so as to be substantially opposed to the fixed magnet body 2A located on the downstream side of the charging area. The second magnet body 5B is disposed adjacent to the upstream side of the charging area, the first magnet body 5A is set to the S pole having a polarity opposite to that of the fixed magnet body 2A, and the second magnet body 5B is set to the first magnet body 5B. The magnet body 5A of No. 1 is set to the N pole having the opposite polarity, and both magnet bodies 5A, 5B
A horizontal magnetic field is formed on the photosensitive drum 1 in between. That is, specifically, the first magnet body 5A and the fixed magnet body 2 which are opposed to each other are arranged on the downstream side of the charging area in the rotation direction of the photosensitive drum 1, and the vertical direction formed between the two magnet bodies 2A and 5A. The magnetic particles 4A are magnetically held by the magnetic field 4A, and
The magnet body 5B is arranged adjacent to the first magnet body 5A on the upstream side of the charging area, and the magnetic particles are generated by the horizontal magnetic field mainly formed between the first magnet body 5A and the second magnet body 5B. The group 4B is brought into close contact with the photosensitive drum 1.

The repulsive magnet body 2B set to the N pole having the same polarity as the fixed magnet body 2A is laid out so that the non-magnetic field due to the repulsive magnetic field formed between the two is located on the upstream side of the charging area. The magnetic particle group 4C carried on the non-magnetic sleeve 3 and conveyed to the upstream side of the charging area from the position of the vertical magnetic field 4A is magnetically released and dropped to the photosensitive drum 1 side. The magnetic force of each of the magnet bodies is, for example, about 1200 to 1400 Gauss at the end face of the N pole facing the photosensitive drum 1 in the fixed magnet body 2A,
Further, when the first magnet body 5A is set to, for example, 800 gauss on the end face of the S pole on the side of the photosensitive drum 1, the magnet body 5A
A magnetic force of about 300 to 400 gauss can be obtained on the surface of the photosensitive drum 1 on A, and 800 to 10 on the sleeve 3.
A magnetic force of 00 Gauss was obtained.

By setting the magnetic force of the second magnet body 5B to be substantially equal to that of the first magnet body 5A, a magnetic force of about 300 to 400 gauss can be obtained on the surface of the photosensitive drum 1, and the repulsive magnet body can be obtained. The magnetic force of 2B is applied to the sleeve 3 immediately above it.
It is set so that a magnetic force of 800 to 1000 gauss is obtained above. That is, the magnetic force on the surface of the photoconductor of the first and second magnet bodies 5A and 5B should be as large as possible in order to achieve uniform charging and improve the adhesion, but the nonmagnetic sleeve from the side of the photoconductor drum 1 on the vertical magnetic field 4A. In order to facilitate the movement of the particles to the 3 side, it is necessary to make the magnetic force on the sleeve 3 side stronger than on the photosensitive drum 1 side.

Therefore, the limit value is the position directly above each magnet body on the surface of the photosensitive drum 1, the first magnet body 5A and the second magnet body 5B are each 1000 gauss or less, the fixed magnet body 2A and the repulsive magnet body. 2B is preferably set to 1200 gauss or less. According to this embodiment, the fine particle group in which the surface of the photosensitive drum 1 is smoothly charged while the magnetic particle group 4B is brought into close contact with the horizontal magnetic field on the photosensitive drum 1 has a vertical magnetic field according to the rotation of the photosensitive drum 1. 4A, where the vertical magnetic field 4A magnetically seals the fine particle group 4A to prevent leakage to the outside of the charging area and, due to the magnetic force difference, from the photosensitive drum 1 side to the sleeve 3 side. The magnetic particle group 4A is adsorbed, and is moved to the upstream side of the charging area while being held by the sleeve 3 by the against rotation of the non-magnetic sleeve 3. Here, when the magnetic particle group 4C reaches the non-magnetic band due to the repulsive magnetic field with the repulsive magnet body 2B,
The magnetic particle group 4C carried by the non-magnetic sleeve 3 is magnetically released, drops to the photosensitive drum 1 side, and the above-mentioned operations are repeated.

Therefore, the magnetic particles 4 on the charged area are always circulated even if they are in close contact with each other due to the horizontal magnetic field, and there is no room for deterioration of the charged particles even after long-term use. In addition, the above-mentioned embodiment is our LED printer (trade name: ECOSYS)
When 20,000 sheets of paper were incorporated in the above-mentioned printer and the deterioration state of the charging potential of the photosensitive drum 1 was examined, a charging potential which was completely the same as that at the initial printing was obtained.

FIG. 2 shows a modification of the above embodiment, in which ferrite or other magnetic material 5A 'is used instead of the first magnet. Even with such a configuration, the closed circuit composed of the vertical magnetic field 4A between the ferrite 5A 'and the fixed magnet body 2A can further strengthen the magnetic force on the sleeve 3 side from the photosensitive drum 1 side, and further, the photosensitive drum 1 A closed circuit composed of a horizontal magnetic field is formed between the ferrite 5A 'and the second magnet body 5B which are adjacent to each other on the back side.

FIG. 3 shows another modification of the above-described embodiment, which is constructed without using the repulsive magnet body 2B and using ferrite or other magnetic body 5A 'in place of the first magnet body, and charging A particle peeling means 9 such as a blade or a film is arranged in the vicinity of the starting point of the area, and is supported by the sleeve 3 to support the vertical magnetic field 4
A group of magnetic particles 4C that has been conveyed to the upstream side of the charging area from A
Is dropped to the photosensitive drum 1 side.

FIG. 4 shows a printer incorporating a charging device according to the second embodiment of the present invention which does not use the non-magnetic sleeve 3, and its construction will be described. In FIG. 1A, a charging device 20, which is the gist of the present invention, an exposure unit 40 including an LED unit, a developing unit 50 including a developing sleeve, and the like, and a transfer roller 6 around the photosensitive drum 1 along the rotation direction.
0 and a cleaning means 30, and the stirring means 1
After uniformly charging the surface of the photosensitive drum 1 with the conductive magnetic particle group 4 to which the charging bias is applied via 1, the latent image is formed by the exposing means 40 and the toner image developed by the developing unit 50 is formed. Recording paper 70 through transfer roller 60
Side, and the unfixed toner image is fixed by a fixing roller (not shown). On the other hand, the residual toner that could not be transferred to the transfer roller 60 is removed by the cleaning blade 31, and the above operation is repeated. It is configured.
The charging device 20 and the cleaning means are integrally incorporated in a fan-shaped frame body 21, and a substantially central portion of the frame body 21 is partitioned by a partition wall 21a to partition the photosensitive drum 1 on the upstream side in the rotation direction. The cleaning blade 30 is attached to the partition wall 21a in the space to form the cleaning means 30.

On the other hand, as shown in FIG. 1B, the charging device 20 is rod-shaped so as to extend in the normal direction from the most downstream end of the frame 21 in the rotation direction of the photosensitive drum 1 toward the photosensitive drum 1 side. The fixed magnet body 2A is provided and the fixed magnet body 2A is provided.
And a first magnet body 5A, and a second magnet body 5B adjacent to the first magnet body 5A on the upstream side of the charging area.
Are arranged in the photosensitive drum 1, respectively. A stirring member 11 is disposed in the storage space above the photosensitive drum 1 between the first magnet body 5A and the second magnet body 5B so as to be rotatable in the arrow direction. In this embodiment, the stirring member 1
1 is connected to a developing bias power source 80, and a developing bias can be applied to the magnetic particle group 4 via the stirring member 11.

The conductive magnetic particles have an average particle size of 35.
The maximum magnetization in a magnetic field of 5 kOe (Oersted) is 55 to 80 emu / g, and the maximum magnetization in a magnetic field of 1 kOe is 45 to 60 emu using ferrite core particles having a resistivity of 5 × 10 ² Ω · cm. / G. The magnetic force of each of the magnet bodies is approximately the same as in the above-described embodiment, in the fixed magnet body 2A2, for example, about 1000 to 1200 gauss at the end face of the N pole facing the photosensitive drum 1, and the first magnet. When the body 5A is set to, for example, 800 gauss on the end surface of the S pole on the photoconductor drum 1 side, a magnetic force of about 300 to 400 gauss can be obtained on the surface of the photoconductor drum 1 on the magnet body 5A. By setting the magnetic force of the magnet body 5B to be substantially equal to that of the first magnet body 5A, a magnetic force of about 300 to 400 gauss can be obtained on the surface of the photosensitive drum 1 in the charging area. The stirring member 11 is made of a non-magnetic material, and it is preferable that the stirring member 11 has a small diameter and is rotated in the direction of the arrow in order to facilitate the circulation of the magnetic particle group 4 and the stirring in the horizontal magnetic field region.

According to this embodiment, the stirring member 11 stirs the magnetic particle group 4B which is in close contact with the horizontal magnetic field on the photosensitive drum 1 while magnetically generating the magnetic field 4A on the downstream side. The magnetic particle groups 4A in the sealed charging area are gently migrated in the direction of the alternate long and short dash line to allow circulation and replacement in the charging area. In this case, the stirring member 11 may be rotated at all times during the charging operation,
Further, it may be appropriately performed at the time of non-printing.

[0029]

[Effect] As described above, according to the present invention, without being affected by the fluctuation of the charging gap and the assembly error,
A uniform magnetic field can always be obtained, which can prevent the magnetic particles from adhering and leaking to the photosensitive drum side with stable charging ability, and the magnetic particles can be smoothly circulated and stirred within the charging area for a long time. Therefore, deterioration of the charging agent can be prevented.
It has various remarkable effects.

[Brief description of drawings]

FIG. 1 is an overall schematic view (A) of a charging device according to a first embodiment of the present invention and an enlarged view (B) of its main part.

FIG. 2 is an enlarged view of a main part showing a modified example of FIG.

FIG. 3 is an enlarged view of an essential part showing a modified example of FIG.

FIG. 4 is an overall schematic view (A) showing an arrangement of a printer incorporating a charging device according to a second embodiment of the present invention and an enlarged view (B) of a main part thereof.

FIG. 5 is an overall schematic view of a charging device according to a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2A, 5A (5A ') Vertical magnetic field forming means 3 Non-magnetic sleeve 4 Magnetic particle group 80 Charging bias power supply 2B, 11, 9 Circulation or stirring means of magnetic particle group

Front Page Continuation (72) Inventor Ryushi Imo 2-14-9 Tamagawadai, Setagaya-ku, Tokyo Kyocera Corporation Tokyo Yoga Works (72) Inventor Yasuo Nishiguchi 2-14-9 Tamagawadai, Setagaya-ku, Tokyo No. Kyocera Corporation Tokyo Yoga Works

Claims (6)

[Claims] 1. A charging device in which magnetic particles are carried between a non-magnetic sleeve facing a photoconductor and the photoconductor can be charged through the particles, and a photoconductor located downstream of a charging region of the photoconductor. Magnetic field generating means composed of a magnetic body or a magnet body are arranged substantially opposite to each other on the back side of the body and the back side of the non-magnetic sleeve to form a vertical magnetic field in the direction normal to the photoreceptor, and the non-magnetic sleeve is attached to the photoreceptor. Charging device for photoconductor characterized by being configured to be able to rotate against the moving direction 2. A means for circulating or stirring magnetic particle groups upstream of the vertical magnetic field forming position in the moving direction of the photosensitive member, wherein magnetic particle groups magnetically sealed by the vertical magnetic field circulate or stir in a charging region. Claim 1 constituted as possible
Charging device for photoreceptor 3. A charging device for a photosensitive member according to claim 2, wherein the circulating means is a magnetic opening means formed on a non-magnetic sleeve on the upstream side of the charging area. 4. The charging device according to claim 2, wherein the circulation means is a peeling means for mechanically peeling the magnetic particle group carried on the non-magnetic sleeve on the upstream side of the charging area. 5. The charging device according to claim 1, wherein the magnetic force density of the magnet body is set so that the magnetic force on the surface of the sleeve on the vertical magnetic field is larger than the magnetic force on the surface of the photoconductor. 6. A charging device configured to charge a photoconductor through a group of magnetic particles that rub against the surface of the photoconductor. The photoconductor charging area has a downstream side, and a back side and a front side thereof sandwiching the photoconductor. Magnetic field generating means composed of a magnetic body or a magnet body are arranged substantially opposite to each other to form a vertical magnetic field in the normal direction of the photoconductor, and on the other hand, circulation means for the magnetic particle group is arranged on the upstream side of the photoconductor charging area, A charging device for a photoconductor, characterized in that a group of magnetic particles located in a charging region can be circulated between a circulation means and a position for forming a vertical magnetic field.