US 3707134 A
Apparatus for monitoring and controlling the ratio of toner to carrier particles of a dry developer mix includes an inductive sensing coil positioned in contact with the developer mix. The voltage output of circuitry connected electrically to the sensing coil changes in response to a change in the inductive reactance of the sensing device due to changes in the ratio of toner to iron particles in the mix. A toner dispenser replenishes toner material to the mix in response to a predetermined change in the voltage output of the circuitry.
Description (OCR text may contain errors)
[451 Dec. 26, 1972 United States Patent Gawron [541 AUTOMATIC TONER CONCENTRATE OTHER PUBLICATIONS DETECTOR AND CONTROL DEVICE Detecting Apparatus for Suspension Particles by  Inventor: Toa Tokushu Denki Co., Ltd. Japan Div. 280.
Stanley A. Gawron, Arlington Heights, Ill.
 Assignee: Addressograph-Multigraph Primary Examiner-Mervin Stein poration, Mount Prospect, Ill. Assistant Examiner-Leo Millstein Aug 21 1970 Attorney-Sol L. Goldstein  Filed:
21 App1.No.: 65,902
cuitry connected electrically to the sensing coil changes in response to a change in the inductive reactance of the sensing device due to changes in the 9305 006 7 l 001 l 7 1 14 7012. 7 1,24 l7 .9... WM ,4 12 m 13" 1 H w s, m W6C mm m "1 n "12 n ""2 H mm m mh "c m "m m mS L h 0 d s mm U IF l 1] 2 00 5 55 I References Cited UNITED STATES PATENTS ratio of toner to iron particles in the mix. A toner dispenser replenishes toner material to the mix in 11 7 response to a predetermined change in the voltage .......324/71 output of the circuitry. ..............113/7 Oberg et 18/7 3,527,651 9/1970 Shelffo et a1. 2,671,200
9 Claims, 2 Drawing Figures 3/1954 Lederer........... 3,475,316 10/1969 DeVittorio 3,418,163 12/1968 PAIENTED um 26 m2 SHEET 1 BF 2 m VENTOR can/E ufawlon PATENTEDHEBZB 191 3.707; 134
0 Bmm/ wm AUTOMATIC TONER CONCENTRATE DETECTOR AND CONTROL DEVICE BACKGROUND OF THE INVENTION larly to apparatus of the above-described type which monitors an electrical property of the developer mix to maintain the ratio of toner-to-carrier particles substantially constant.
Various types of apparatus are known for monitoring and controlling the ratio of toner-to-carrier particles in dry developer mix used in electrostatic copiers. Some of the types of apparatus monitor the developer mix for changes in resistance as toner is depleted in the development of copies, while others use optical sensing devices to measure light reflectance of toner deposited on a probe to determine and control the ratio of tonerto-carrier therein. Still others monitor capacitive changes in a capacitor device placed into the developer mix.
While the above-described types of apparatus function satisfactorily for the most part to maintain the toner-to-carrier ratio in a dry developer mix substantially constant, there are drawbacks which detract from the desirability of using the apparatus. For instance, the various apparatus described tend to be complex and require frequent inspection so as to be maintained in working order. Some of the heretofore known types of monitoring apparatus function best in developer mixes wherein the ratio of toner-to-carrier particles is in a restricted range.
SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide new and improved apparatus for monitoring and controlling the ratio of toner-to-carrier particles in a dry developer mix used for developing copies in an electrostatic copying machine.
It is another object of the present invention to provide apparatus of the last-described type which is simple in construction, efficient in operation and can be used satisfactorily in dry developer mixes of various ratios of toner-to-carrier.
It is still another object of the present invention to provide an improved method of developing electrostatic images by monitoring and controlling the ratio of toner-to-carrier particles.
Briefly, a preferred embodiment of the apparatus for monitoring and controlling the ratio of toner-to-carrier particles of a dry electrostatic developer mix comprises an inductive sensing coil having an iron core. The coil is placed in the surroundings of the developer apparatus of an electrostatic copying machine so as to be in contacting relation with the dry developer mixture containing toner and magnetizable carrier particles. The inductive reactance of the coil is a function of the amount of magnetizable particles per toner particles in the mix. Thus, as the toner is depleted, the inductance of the coil changes. The frequency of an oscillator circuit connected to the coil changes as the inductance of the coil is varied. The change in frequency produces a corresponding output of additional circuitry which in turn operates a toner dispenser unit, causing toner to be added to the mix to restore the toner-to-carrier ratio to a predetermined level.
DESCRIPTION OF THE DRAWINGS A better understanding of the present invention and its organization and construction may be had by referring to the description below in conjunction with the accompanying drawings, wherein:
FIG. 1 is a schematic diagram of a preferred embodiment of the circuitry used in the monitoring and controlling apparatus of the invention; and
FIG. 2 is a side sectional view of a magnetic brush' type developer arrangement employing the inductive monitoring and controlling apparatus.
DETAILED DESCRIPTION Referring now to the drawings in greater detail, FIG. 1 thereof illustrates a preferred embodiment of the apparatus according to the invention for monitoring and controlling the ratio of toner-to-carrier particles of a dry electrostatic developer mixture including a schematic representation of the control circuitry 10 and inductive sensing means 12 employed therein.
The control circuit 10 comprises several stages connected in tandem and shown enclosed in dotted lines. A first Variable Frequency Oscillator stage (VFO) 14 is connected to an Amplifier and Discriminator stage 16, which in turn is connected to a Buffer stage 18. Outputs 20, 22 of the last-mentioned Buffer stage 18 are connected to interconnected switching networks 24, 26, respectively, which in turn operate a toner replenisher drive 28 and an out-of-toner indicator arrangement 30, respectively. A voltage regulating circuit portion 32 connected to the Buffer stage 18 serves to stabilize the control circuit and compensates'for voltage variations in the input voltage.
A more detailed description of the control circuit 10 including its various stages will now be presented.
Power to the control circuit is provided at input electrode 34. The positive potential side of a 26 volt DC power supply (now shown) is connected thereat to power the circuit 10. A ground connection is made at ground point 36.
The VFO stage 14 consists essentially of a standard Collpitts Oscillator circuit including a transistor 38 connected in an emitter follower configuration. Input voltage to the transistor 38 is provided through resistors 41, 43 connected to the base electrode 45 of transistor 38. The collector electrode 40 of the transistor is connected through a tank circuit including a tuneable inductor 42 and capacitors 44, 46. The capacitors are connected via lead 48 to one side of the inductive sensing means 12 and the tuneable inductor 42 is connected via lead 50 to the other side of the sensing means 12. The last-mentioned sensing means comprises an inductive sensing coil 13 preferably having an iron core. A preferred embodiment of the sensing coil will be discussed when referring to FIG. 2 wherein such a coil is illustrated.
The primary frequency determining portion of the VFO stage 14 is the tank circuit arrangement including capacitors 44, 46 and tuneable inductor 42. In practice the inductor 42 can be adjusted to permit a matching of the oscillator circuit 14 to the sensor coil 13 which thereafter is placed into the environment of a I The base electrode 64 of the amplifier configuration is connected through a first resistor 68 to the positive potential lead 70 and through a second resistor 72 to ground lead 74; and at-its emitter 76 through a parallel connected-resistor '78 and capacitor'80 toground lead 74. Connected'at the collector electrode of transistor 60 is atun'ed network 84,.comprising a capacitor 86 and coil 88, the latter of which serves asthe primary winding of a transformer 90. The network in practice is tuned to a normaloperating frequency of about KB.
' The primary winding 88 is loosely coupled inductively' through a pair of secondary'windings 92, 94 of the Discriminator portion of stage 16. Capacitors'96, 98, connected in parallel configuration with respective secondary windings comprise tank circuits 100,102, tuned to frequencies of approximately 16 KB and 14 KH' respectively.
' The diode and capacitor combinations 104, 106, and 108, 110, connected to the respective tank circuits '100, 102, rectify and filter the outputs, respectively,
taken across respective capacitors 96, 98. Resistors 105, 107 are connected in parallel with tank circuits 100, 102, respectively.
' In a normal situation, prior to placing the sensing coil 13 in the toner-carrier mixture, the outputs across the respective capacitors 106, 110, are equal but opposite in polarity and cancel out. Consequently, the output of the Discriminator circuitry is zer Upon placi ng' the coil 13 into the environment of the toner-carrier mixture, the frequency of the oscillator circuit decreases due to the magnetizable character of the carrier particles in the mix. As the frequency decreases, the coupling of the primary winding 88to the secondary windings 92, 94, causes a greater coupling with the 14 KB secondary tank circuit since the oscillator frequency drops from its original 15 KB setting, and a looser coupling with the other secondary winding tank circuit. Since the positive charge on the capacitor 106 is greater than the opposing negative charge on capacitor 1 10, the output of the Discriminator circuitry is positive and of a magnitude that depends upon the relationship of the frequency of the VFO stage to the 14 KB frequencyof the tank circuit 100.
A thermistor device 112 is connected in parallel across capacitor 110 to provide stability to the circuit regardless of temperature changes. The thermistor compensates for temperature variations essentially between 40 to l60 F. h
Connected to the output leads 114, 116 of the Amplifier Discriminator stage 16 is a Field Effect Transistor 118. A Zener diode 120 connected in series with a resistor 122, is connected across the output of the Amplifier Discriminator stageat leads 114, 116, to protect the Field Effect Transistor by limiting the voltage supplied to the latter.
A resistor-capacitor combination 124, 126, connected to the gate electrode 128 of the Field Effect Transistor 118serves as a Time Delay Network 130, to delay'the voltage output impressed on the transistor 1 18,thereby to prevent the, latter transistor frombeing subjected to instantaneous changes in the voltage caused by frequency changes detected by coil 13. In this manner, a relatively smooth change in voltage is impressed on the transistor 1 18. v Y
The Field Effect Transistor 118, or Darlington transistor as it is sometimes called, is connected in an emitter follower configuration with' the*dra'in electrode 132 connected to the positive-DC voltage lead and the sourcef'electrode connected througha pair of series connected resistors 136, 138, acrosswhich outputs to the Switching Networks 24, 26, to be described, are taken at leads 20, 22, respectively. I v
The Field Effect Transistor v118 acting as an emitter follower avoids [further amplification of the output from the Amplifier r Discriminator stage 16 and converts thehigh output impedance into a low output impedance, high power signal which can be used to operate the Switching Networks 24,26.
As mentioned heretofore, Voltage Regulator circuit 32 stabilizes any voltage variations occurring in the DC voltage source. The Voltage Regulator '32 includes resistor connected to the positive potential electrode 30 34 and in series with a parallel connected Zener diode 142 and capacitor 144. The regulator circuitry maintains the voltage'at approximately 22 volts DC.
Referring nowto SwitchingNetwork 24, there is included therein a conventional solid state switching arrangement comprising first and second transistors 146, 148. The voltage from output 20 is impressed'at the base electrode of transistor 146. A resistor 152 is interposed between the base electrode and output 20 of the Buffer stage 18 of the control circuit. Base electrode 150 is also connected -to' a capacitor 157, in turn connected to ground at 36. The emitter 147 of transistor 146 is connected via Zener diode 154- to ground 36.'The collector 155 is connected to a pair of current'limiting resistors 156, 158, and to the base of transistor 148. The Zener diode 154 establishes the operating voltage for the transistor 146. The voltage at the base of transistor 146 must exceed the Zener break down voltage of diode 154 plus the voltage drop across the emitter 147 before transistor 146 will conduct. In practice the break down voltage of diode 154 is approximately 9.1 volts and the voltage drop across the emitter 147 is approximately 0.5 volts.
The emitter of transistor 148 is connected to a positive potential lead 165, connected itself to the positive electrode 34 of the DC power supply (now shown). The collector 164 of transistor 148 is connected to an electric clutch 166 of toner replenisher drive 28 which drives an auger (FIG. 2) in a toner dispenser reservoir (FIG. 2) for the purpose of dispensing toner to the developer unit containing developer mix. A resistor 169 is connected between the base 150 of transistor 146 and the collector electrode 164 of transistor 148. A Zener diode 168 is connectedin parallel relation with the clutch 166 to prevent damage to transistor 148 upon the cessation of operation of clutch 166 after sufficient toner is replenished in the developer mix.
The voltage output taken across resistor 138 at output 22 is impressed on Switching Network 26 at 22. Switching Network 26 also includes a pair of transistors 170, 172. The emitter 174 of transistor 170 is connected via lead 175 to emitter 147 of transistor 146 of Network 24 and to diode 154, and the base electrode 176 of transistor is connected through a capacitor 178 to lead 180 connected to the opposite side of diode 154. The collector 182 of transistor 170 is connected through a pair of current limiting, series connected resistors 184, 186, to the positive potential lead 165.
Base electrode 188 of transistor 172 is connected between resistors 186, 184. The emitter electrode 190 of the last-mentioned transistor is connected to the positive potential lead 165, and the collector 192 thereof is connected through a resistor 194 of a pair of resistors 194, 196, to which base 176 of transistor 170 is also connected. The resistor 196 is in turn connected to the output 22 of Buffer stage 18.
A toner-out indicator lamp 198 is connected via lead 200 to the collector 192 of transistor 172, and a fault signal producing circuit arrangement 30, comprising series resistors 202, 204, is connected to collector 192 for producing a signal over lead 206 to interrupt the operation of the copying machine in which. the apparatus is used.
A resistor 208 connected between the switching networks 24, 26, in parallel relation thereto at leads 165, 175, establishes a common reference voltage for both Networks. Zener diode 154 is used for both networks, and transistor 170 will also not operate until the voltage at input 22 at the base 176 of transistor 170 exceeds the breakdown voltage of the Zener diode 154 plus the voltage drop across emitter 174. This will be explained in greater detail when the operation of the apparatus, including control circuit is described.
Turning now to FIG. 2 of the drawings, there is illustrated therein a sectional view of a developer unit 210 of the type shown in copending patent application, Ser. No. 9,316, filed Feb. 6, 1970, used in a high speed electrostatic copier.
The developer unit 210 comprises a hollow cylinder or drum 212 rotatably mounted on a fixed shaft 214, and suitably supported on the shaft by bearing members 216.
Affixed to the shaft 214 is a magnetic flux generating assembly 218 including an array of permanent magnets, such as 220, disposed within the cylinder 212 for magnetically arranging about the outer surface of the cylinder, dry developer mix comprised of toner and magnetizable iron carrier particles into a magnetic brush configuration 222 for developing copy material transported adjacent thereto.
The hollow cylinder 212 is mounted in an enclosure 224 in which there are also included two pairs of feed rollers, only one pair 226 of which is shown, for transporting copy material along a path indicated by arrow 228, past the cylinder 212.
Developer mix including a predetermined ratio of toner and iron carrier particles is initially deposited in a trough (not shown) below the cylinder 212 for use in developing copy material fed along path 228. As the copy material is developed, toner particles are depleted from the mix, changing the ratio of toner and iron therein, and toner must be added thereto to maintain the ratio substantially constant. To provide additional toner to the developer mix, there is incorporated in the developer unit a hopper 230, filled with toner. A member 232 is provided in the hopper to thoroughly mix the toner, preventing caking, and an auger 234 mounted for rotation beneath the churning member 232, is provided to dispense additional toner from hopper 230 when it is indicated that the ratio of the mix has changed a predetermined amount. For a more complete understanding of the developer unit shown in FIG. 2, the reader's attention is directed to the abovementioned copending patent application.
Included in the developer unit adjacent the magnetic brush cylinder 212, and downstream of the developing area 228, is a preferred embodiment of the sensing coil 13 described. This particular embodiment of the sensing coil according to the invention includes a central, cylindrically shaped, elongated iron core 236. At the end 237 of the sensing coil 13 nearest the rotating cylinder 212 there is provided a winding 238. A plastic inner housing 240 surrounds the winding and iron core. Permanent magnets 244 are positioned at the opposite end 242 of the coil, surrounding the iron core 236. A dielectric spacer 246 is interposed between the permanent magnets 244 and the coil housing 240. Additional insulative plastic spacers 248, 250, surround the coil and iron core at end 237, and a plastic outer housing 252 surrounds the magnets 244. The insulative plastic serves to shield the sensing coil 13 from outside stray fields. A thin Mylar layer 254 is interposed between sensing coil 13 and the brush of the developer unit to avoid a build up of toner adjacent the coil.
In use, the magnets 244 attract developer mix remaining on the cylinder surface after passing the developing area toward the coil end 237 so that a monitoring of the mix can be carried out. The magnets, while being of sufficient Gauss strength to attract developer mix, are not sufficiently strong to attract developer mix to the coil end 237 If the latter occurs, a false indication of the ratio of toner to carrier particles of the mixture is provided.
An iron core coil arrangement is preferred for use as a sensing means since the iron core provides a greater sensitivity to change in the inductance of the coil due to the outside influence of the mix. An air core coil does not provide as great a sensitivity for efficient appraisal of the iron-to-toner ratio in a dry developer mix.
While the sensing coil 13 is shown located at a preferred position in the developer unit 210, downstream of the development zone, the coil may be located in other areas and provide equally good results. In developer units other than the magnetic brush type shown in FIG. 2, such as for example cascade developers, the sensing coil is normally placed in a location wherein it will be contacted by the developer mixture used in the developer unit.
For purposes of affording a more complete understanding of the invention, it is advantageous now to provide a functional description of the mode in which the component parts thus far described cooperate.
As described heretofore, upon placing the sensing means 12 (coil 13) in the vicinity of the dry mixture comprising toner and iron particles in a predetermined ratio, a positive output voltage is produced at 20 and 22 of Buffer Stage 18. The output voltages, however, are
normally less than required to cause the conduction of transistors 146, 170.
As copy material passes through the developer unit for development, toner is applied thereto and removed from the mixture, changing the ratio of toner-to-carrier particles of the latter.
As the ratio of toner-to-iron particles changes, leaving a greater amount of iron in the mix per toner particle, the inductance of coil 13 changes to in turn vary the frequency of the Variable Frequency Oscillator Stage 14. The frequency change is amplified and produces a decreased frequency in the primary winding tank circuit 84. The inductive coupling of the secondary winding is increased and hence greater voltage outputs at 20, 22 are realized. The output voltage at 20, however, is approximately 2 volts greater than that at 22.
. When the voltage at point 20, which is fed into the base of transistor 146 of Switching Network 24, reaches a predetermined value, i.e. the Zener breakdown voltage of diode 154 (9.1 volts) plus the voltage drop of approximately 0.5 volts across the emitter 147 of the transistor, transistor 146 is rendered conductive or is turned on. The conduction of transistor 146 causes transistor 148 to conduct also. The last-mentioned transistor in turn operates electric clutch 166.
In the case of the developer unit illustrated in FIG. 2, when the clutch 166 is operated, auger 234 is rotated to dispense toner from hopper 230 into the vicinity of the rotating cylinder 212, thereby to reinstate the predetermined ratio of toner-to-iron particles of the mix.
Upon replenishing sufficient toner material to the mix, the inductance of the sensing coil 13 changes to a value, whereat the output voltage at point 20 falls below the 9.6 volts required to operate transistor 146, and the operation of clutch 166 is discontinued.
In the event the supply of toner in the replenishing hopper 230 is depleted so that no toner is added to the mix, the clutch 166 continues to turn the auger 234. Within a short time, however, the inductance of the sensing coil 13 has changed to such a degree that the voltage taken at point 22, which as mentioned heretofore, is approximately 2 volts less than that taken at point 20, reaches the predetermined voltage value, i.e. 9.6 volts, and transistor 170 of Switching Network 26 conducts, causing transistor 172 to conduct also.
The operation of transistor 172 lights lamp 198, indicating to the operator of the copying machine in which the developer unit is housed, that the toner replenisher hopper is depleted of toner. In addition, a signal at lead 206 is provided which may be used to interrupt the operation of the copying machine itself until toner is replenished in the hopper.
Upon replenishment of toner in hopper 230, Switching Network 26 ceases to operate and lamp 198 is turned off. The copying machine likewise is restarted. Switching Network 24, however, remains operable until sufficient toner is replenished in the developer unit. When the predetermined ratio of tonerto-iron particles is reinstated, the last-mentioned Switching Network 24 also is rendered inoperative until a further change in toner-to-iron particle concentration is detected.
The inductive toner sensing device according to the invention can be used with developer mixtures of widely varying toner-to-carrier particle ratios with little or no adjustment of the circuit. If necessary, inductor 42 can be adjusted easily for greater sensitivity.
The use of the toner-to-carrier ratio monitoring and controlling apparatus of the invention insures that co pies made in an electrostatic copier in which the apparatus is used, will be of uniform density and contrast and of high quality. The apparatus of the invention is especially useful in new high speed electrostatic copying machines in which toner is depleted rapidly and in great quantities. The apparatus is designed to provide fast replenishment of toner to maintain easily the ratio of toner-to-carrier particles in the mix substantially constant regardless of the speed in which toner is removed from the mix.
While a particular embodiment of the invention has been shown and described, it should be understood that the invention is not limited thereto since many modifications may be made. It is therefore contemplated to cover by the present application any and all such modifications as fall within the true spirit and scope of the appended claims.
What is claimed is:
1. In an electrostatic copying machine in which a recording member is imparted with an electrostatic charge and is developed by contact with a charge attractable toner powder included in a mixture of magnetically attractive carrier particles and toner powder combined in a predetermined mixture ratio of particles to powder and including a developer assembly having a quantity of said mixture for applying the latter to said image which is developed by the selective adherence of the powder, the improvement comprising:
sensing means in said developer assembly includin inductive reactance generating means and magnetic means for attracting said mixture to said sensing means,
said generating means having a predetermined inductive reactance when the ratio of toner powder to carrier particles of said mixture is at said predetermined ratio, and
control circuit means connected to said sensing means for monitoring changes in the inductive reactance of said generating means as said toner powder is depleted from said mixture and, in response thereto, for indicating a change in the toner-to-carrier ratio of said mixture.
2. A copying machine as claimed in claim 1 further including a toner powder dispensing assembly responsive to said control circuit means for dispensing toner powder to said developer assembly in response to changes in the inductive reactance of said generating means to maintain the ratio of carrier particles and toner powder at said predetermined level to provide optimum print density.
3. In an electrostatic copying machine as recited in claim 2, the improvement further comprising means responsive to said control circuit means for indicating the depletion of toner powder from said dispensing assembly.
4. In an electrostatic copying machine as recited in claim 2, the improvement further comprising means for enabling the interruption of the operation of said copying machine in response to the depletion of toner powder from said dispensing assembly.
5. A copying machine as claimed in claim 2 wherein said control circuit means includes an oscillator circuit having a predetermined frequency when said toner powder and carrier particles ratio is at said predetermined ratio and in response to a change in the inductive reactance of said generating means, changing said frequency, the output of said circuit controlling said toner powder dispensing assembly so that upon a predetermined change in frequency, said dispenser is actuated to deliver toner powder to said developer means. I
6. A copying machine as claimed in claim 1 wherein said generating means comprises an electrical coil located in the vicinity of said developer assembly in contacting relation with at least a portion of said developer mixture.
7. In an electrostatic copying machine in which a recording member is imparted with an electrostatic charge and is developed by contact with a charge attractable toner powder included in a mixture of magnetically attractive carrier particles and toner powder combined in a predetermined mixture ratio of particles to powder and including a developer assembly having a quantity of said mixture for applying the latter to said image which is developed by the selective adherence of the powder, the improvement comprising:
inductive sensing means in said developer assembly having a predetermined inductive reactance when the ratio of toner powder to carrier particles of said mixture is at said predetermined ratio, and control circuit means connected to said inductive sensing means for monitoring changes in the inductive reactance of said sensing means as said toner powder is depleted from said mixture and in response thereto, for indicating a change in the toner-to-carrier ratio of said mixture, said sensing means comprising a coil wrapped about a first end of an iron core and a magnet positioned adjacent the opposite end thereof, said magnet being of a predetermined Gauss strength to attract developer mix toward said coil wrapped end without retaining said mixture thereat permanently.
8. In an electrostatic copying machine which includes developer means for applying toner from a dry developer mixture including toner and magnetizable carrier particles in a predetennined ratio to copy material having a charged image thereon,
means for monitoring said developer mixture thereby to maintain the ratio of toner-to-carrier particles of said mixture substantially at said predetermined ratio, said monitoring means comprising inductive sensing means located in the vicinity of said developer means in contact with at least a portion of said developer mixture, said sensing means having a predetermined inductive reactance when the quantity of toner and carrier particles are at said predetermined ratio, the inductive reactance of said sensing means changing in accordance with the change in ratio of toner-to-carrier particles of said mix, thereby to indicate a need for replenshing toner particles to said mixture.
9. In a copying machine of the type in which a recording member is imparted with an electrostatic charge and is developed by applying a mixture of carrier and toner materials combined in a predetermined mixture ratio of carrier material to toner material and including a magnetic brush assembly for forming bristles of the mixture and moving the bristles into engagement with the member to develop the electrostatic image, the improvement comprising:
inductive sensing means mounted in engagement with a portion of the bristles of said magnetic brush and having a predetermined electrical inductance, and toner dispensing means connected and controlled by said inductive sensing means for selectively adding toner material to said developer mixture in dependence on the change in inductance of said sensing means due to the depletion of toner in said bristles.