referring to fig1 there is shown an electrophotographic copying machine of a wet type including a photosensitive member formed of a photoconductive material such as selenium . the photosensitive member 1 is initially charged by a corona discharger 2 of a known construction and is then exposed to a light image of a document to be copied by an optical system 3 to form an electrostatic latent image on the peripheral surface of the photosensitive member 1 . the latent image formed is converted into a toner image in a wet developing unit 4 , which includes a developing electrode 5 in the form of a conductive member . the developing electrode also serves as a tray for carrying a quantity of the developing solution . an excess amount of developer is removed by a squeeze roller . subsequently , the toner image is transferred onto a record paper by means of a transfer corona discharger 6 . after the transfer , the photosensitive member 1 , which may contain a slight amount of remaining developer , is cleaned by a cleaning roller and a cleaning blade in preparation for the next charging step . in the example shown , the developing electrode 5 of the developing unit 4 is of a forced bias type and is supplied with a constant potential of , say + 100 volts , which is of a magnitude equal to and a polarity the same as the potential to which the photosensitive member 1 is charged . specifically , one end of the electrode 5 is connected to an output terminal e of a power supply circuit 10 which also includes a supply terminal c . if the power supply circuit 10 is absent , the terminal e would be directly connected with the terminal c which would be connected to a conductive member 20 located inside the corona discharger 2 . in this way , corona ions or charged particles generated within the corona discharger 2 can be conducted from the surface of the conductive member 20 to the developing electrode 5 , thus supplying a potential of from 100 volts to several kilovolts to the developing electrode . the power supply circuit as shown in fig1 comprises a load resistor 11 and a constant voltage element , for example , a varistor or a zener diode 12 , having a threshold value of 100 volts , connected in series with the terminal c and ground . the output terminal e is connected to the junction between the resistor and the zener diode , and in this manner , a given bias potential of about + 100 volts can be maintained at the output terminal e during the discharge operation of the corona discharger 2 . the load resistor 11 may be omitted where a discharge resistance is high . referring to fig3 and 5 , the conductive member 20 comprises a metal film such as , for example , a foil layer of copper or aluminium or an insulating material formed on the surface thereof with a film of conductive paint , such as that containing silver and carbon powder . the conductive member is threadably secured to an inner wall surface of a shield casing 7 of the corona discharger 2 with an insulating plate 22 , such as can be provided by an acryl sheet or mylar film , interposed between the conductive member and the casing 7 so as to electrically isolate the conductive member from the casing 7 . a connection piece 21 which is electrically insulated by an insulating element 23 engages the central portion of the conductive member 20 , and forms the terminal for connection with the power supply circuit 10 . the size of the conductive member 20 and its position on the casing 7 are not critical . depending on conditions to be described later , it may be located on the rear surface of the shield casing 7 ( see fig1 ) or may be located on the lateral side thereof ( see fig2 ). since the magnitude of the current generated is generally proportional to the length of the conductive member 20 , its length is determined in accordance with a desired value of the current . by experiments , it was found that by using a conductive member which measures 12 mm × 70 mm , a d . c . current of 15 μa was derived when a corona discharger having a discharge voltage of + 6 . 3 kv was employed . in this example , the spacing between the conductive member and the electrode wire was 13 mm , the width of the opening of the casing was 20 mm , and the distance between the electrode wire and the photosensitive member was 9 mm . what should be considered when determining the mounting location of the conductive member 20 is its influence upon the uniformity of the primary charging by the corona discharger . for this reason , the location cannot be universally stated , but varies with the particular prameters of the corona discharger . it is necessary to test the charging characteristic of the corona discharger when the final location is to be determined . it is to be understood that the configuration of the conductive member 20 is not critical , and may comprise a metal plate , for example . the discharger which is used as a power source therefor may be any discharger other than that used for charging the photosensitive member . while an application to the developing unit of constant bias potential type has been described in the above embodiment , the described power supply may be utilized in a number of biasing techniques . by way of example , fig6 shows an application of the present invention to a developing unit of an auto bias type in which the surface potential of the photosensitive member 1 is detected and a corresponding bias voltage is applied to the developing electrode 5 . an operational amplifier 13 of a conventional design is connected between the power supply circuit 10 and a developing electrode 8 , and has its input terminal connected with the output terminal e of the power supply circuit . the amplifier includes a terminal b which is connected with the electrode 8 , and an input terminal d which is connected with a detection electrode 9 which is insulatingly formed on one end of the electrode 8 for detecting the background potential of the image , thereby applying a corresponding bias potential to the electrode 8 in order to assure a developed image of high quality . a more reliable biasing effect can be achieved by replacing the amplifier 13 by another operational amplifier 14 shown in fig7 . operational amplifier 14 includes circuitry for compensating for any variation in the corona discharge and a corresponding variation in the electromotive force applied to the conductive member 20 so as to maintain the current from the bias supply at a constant value . several embodiments of the present invention will be described below as applied to an electrophotographic copying machine of a wet type which is provided with a discharger for charge elimination in addition to the primary corona discharger . referring to fig8 a photosensitive member 31 is provided on its periphery with a layer of photoconductive material , which is selenic in the present example , and is mounted on a shaft 32 for rotation with a uniform speed in the direction indicated by the arrow . a primary corona discharger 33 serves to uniformly charge the surface of the photosensitive member 31 to a polarity which depends on the layer of photoconductive material thereon , which polarity is positive in the present example . after having been charged , the photosensitive member 31 is exposed to an image of a light original through an exposure system 4 , thereby forming an electrostatic latent image corresponding to the light image on the photosensitive member . the latent image formed is converted into a visual image by applying a toner thereto which is contained in a developing solution 36 and which , in the present example , is charged to the negative polarity . the developing solution 36 is contained in a reservoir 37 and is pumped up to the upper portion of a developing electrode 38 disposed close to and in conformity with the periphery of the photosensitive member 31 . after contact with the latent image , the flooding developing solution 36 is collected in the reservoir 37 , thus circulating through the developing unit 35 . a squeeze roller 39 is shown for removing any excess amount of developer off the peripheral surface of the photosensitive member 31 . a transfer paper 41 , which is supplied through a guide assembly 40 , is brought into superimposed relationship with a surface area of the photosensitive member carrying the visualized toner image , and a transfer corona discharger 42 applies , to the rear surface of the transfer paper 41 , a charge of the same polarity as , but of a greater magnitude than , tht applied to the photosensitive member 31 . in this way the toner image is transferred onto the transfer paper 41 . the transfer paper 41 having the toner image transferred thereto is discharged by a feed roller 43 . any toner which may remain on the peripheral surface of the photosensitive member 31 is cleaned by a cleaning roller 44 and a cleaning blade 45 . after completion of one copying cycle , there remains a slight amount of potential on the surface of the photosensitive member 31 , which is eliminated in preparation for the next uniform charging by a discharger 46 which , in the present example , is an a . c . corona discharger . each of the a . c . corona discharger 46 and the primary corona discharger 33 comprises an electrode plate 47 , 48 insulated from a shield casing 49 common to both corona dischargers . these electrode plates can be formed of a metal film such as a foil of copper , aluminium or the like , or an insulating material on the surface of which a film or conductive paint such as that containing silver or carbon powder is formed . both of these electrode plates 47 , 48 are electrically connected with the developing electrode , and are connected with the ground through a pair of back - to - back zener diodes 55 and 56 . the a . c . corona discharger 46 includes a wire electrode 50 connected to an a . c . source 51 , and the primary corona discharger 33 includes a wire electrode 52 connected with a d . c . source 54 through a timing relay 53 . in a conventional electrophotographic copying machine , the primary corona discharger is turned off as the projection of a light image of an original onto the present surface of the photosensitive member is terminated . however , in accordance with the invention , the timing relay 53 is adjusted so that the primary corona discharger 33 is turned off only after the latent image formed on the surface of the photosensitive member 31 has passed by the developing electrode 38 . generally , a selenic photosensitive member which is provided with an electrostatic latent image thereon by projection of a light image of an original having an increased non - image area will have a potential on the order of + 50 to + 70 volts in the non - image area , so that the potential of the developing electrode must be on the order of + 80 to + 100 volts in order to avoid the background smearing of the copy as a result of deposition of negatively charged toner to the non - image area . in accordance with the embodiment of the present invention as shown in fig8 the provision of the electrode plates 47 and 48 inside the a . c . corona discharger 46 and the primary corona discharger 33 , respectively , so that they supply an individual discharge current to feed the zener diode 55 permits a constant voltage of v 1 =+ 100 volts to be applied to the developing electrode 38 as a bias potential , thus preventing the background smearing . in the embodiment of the present invention shown in fig8 the potential of the developing electrode is constrained to an average d . c . potential of v 1 =+ 100 volts by superimposition of an a . c . component of nearly 60 % from the a . c . corona discharge on the d . c . component from the primary corona discharge , but it is found that little difference is noted in the optical density or image quality as compared with the application of a pure d . c . voltage of + 100 volts . since the primary corona discharge is timed to be turned off after the latent image has passed by the developing electrode , i . e ., subsequent to the developing of the latent image , the developing electrode 38 is supplied only with the a . c . component by the a . c . corona discharge after the developing of the latent image takes place . in particular , when a selenic photosensitive member is used which is charged to the positive polarity , the a . c . corona discharge which is used for the elimination of charge has a negative component primarily , so that the developing electrode 38 will be supplied with a negative a . c . component , which is supplied by the zener diode 56 , producing a constant negative potential of v 2 . thus , the developing electrode 38 which is supplied with a positive potential to attract a negatively charged toner during the developing of the latent image is supplied with a negative potential during the time after the developing of the latent image has taken place , thus releasing the toner attracted to the developing electrode and preventing toner deposition thereon and hence the background smearing . the above described operation is graphically illustrated in fig9 where the potential of the developing electrode is indicated in a solid line and maintains a constant average positive potential v 1 during a period a corresponding when the latent image is being developed during one copying cycle and maintains a constant negative potential v 2 during a period b when the developing of the latent image does not take place or when the primary corona discharge is turned off . in this figure , a superimposed a . c . component is indicated at c . the broken line shows the potential of the photosensitive member 31 under the condition that an image having an increased non - image area is projected thereon . a negative potential on the photosensitive member subsequent to the completion of the developing is caused by triboelectricity which is produced by the cleaning unit to charge the member to the negative polarity as well as by the application of the a . c . discharge which is primarily of the negative polarity , after the primary corona discharge is turned off . in this manner , the photosensitive member 31 is periodically charged to the positive and the negative polarity in an alternate fashion . fig1 shows another embodiment of the present invention . the embodiment shown in fig1 is adapted for use with a developing process employing a floating electrode which has its potential varied in accordance with that of the latent image , between an upper and a lower limit of the potential . components which function in a manner similar to those mentioned above are designated by like numerals . reference numeral 57 represents a rectifier , and 58 an element such as a varistor which maintains a constant voltage in both polarities . with this circuit arrangement , it is necessary to use a small electrode plate 47 which supplies the a . c . component . while in the embodiments described above , a negative component has been forcedly applied to the developing electrode to remove the toner therefrom , it is found that this is not essential . this is because the deposition of the toner onto the developing electrode is unlikely to occur if the potential is primarily comprised of the a . c . component , even through the average potential remains in the vicinity of zero . fig1 shows an example of the electrode which is adapted to reduce the negative component from the a . c . corona discharger , which primarily produces a negative component , and to increase the pure a . c . component . as in the previous embodiments , the electrode comprises an electrode plate 59 formed of a foil of copper or aluminium on which an insulating film 60 such as a mylar film is applied so as to expose a portion of the electrode plate 59 . a wire electrode is shown at 61 . if the insulating film 60 entirely covers the electrode plate 59 , a substantially pure a . c . component can be derived . fig1 shows a further embodiment of the present invention which derives , from the a . c . corona discharge designed primarily for the negative component , a potential including an a . c . component which is biased to the positive polarity for supply to the developing electrode as a bias potential . a plurality of electrodes 62 , 63 and 64 are disposed inside a shield casing 65 and insulated therefrom , and an insulating film 66 is applied to the electrode plate 63 . a wire electrode 67 is connected with an a . c . source for producing an a . c . corona discharge . another wire electrode 68 is connected with a d . c . source through a timing relay for producing the primary corona discharge . the developing electrode is shown at 69 . the circuit shown includes a pair of rectifiers 70 , 71 and a capacitor 72 having a small capacitance . in this embodiment , the discharge current produced by the a . c . corona discharge is biased to the positive polarity , so that the potential of the developing electrode varies as indicated in fig1 . it should be understood that a d . c . voltage rather than a voltage including an a . c . component may be applied to the developing electrode during the developing process . to this end , a relay or a pair of switch contacts may be connected in the circuit including the electrode plate 47 and the developing electrode 38 , and operated in synchronism with the primary corona discharge in a manner such that a d . c . bias potential can be applied to the developing electrode during the developing process , as indicated in fig1 . from the foregoing description , it will be appreciated that , according to the present invention , the magnitude and the polarity of the bias potential applied to the developing electrode can be chosen so as to avoid the deposition of the toner on the developing electrode , thus assuring that a high quality copy free from the background smearing can to be obtained .