fig2 and 3 diagrammatically disclose a basic form of the humidifier apparatus embodying the invention and which will maintain the electrode current and therefore the amount of steam that is generated therein at substantially preselected levels . the tank 5 comprises a receptacle for water w to be vaporized . the tank 5 is open , preferably at the top thereof , for releasing water vapor , in the form of steam . a pair of electrodes 7 , preferably formed as conductive plates , extend downwardly into the tank 5 in spaced side - by - side relation and are preferably spaced somewhat above the bottom wall of the tank 5 . the tank 5 is preferably made of an insulating material such as a synthetic resin , such as polypropylene molding resin . operation of the apparatus is controlled by an electronic control unit 3 . the electrodes 7 and control unit 3 receive electric power from a suitable fixed voltage source 9 , conveniently a standard ac power circuit to which connection may be made by any convenient means not shown , for example a conventional ac plug and / or switch . more particularly , a pair of conductors c1 and c2 connect to opposite sides of the electric power source 9 , are provided with fuses f1 and f2 , and are connected to terminals t1 and t2 of the electronic control unit 3 . if desired , a fan can be connected across the supply conductor c1 and c2 for dissipating , to the local atmosphere , water vapor produced in cell 5 . the fan f is optional and can be omitted if desired . one of the conductive electrodes 7 connects to the fixed voltage source line c2 through a conductor c3 and a paralleled fuse f3 and indicator lamp l . the other conductive electrode 7 connects through a line c4 to a terminal t3 of the electronic control unit 3 . the fuse f3 prevents excessive current flow through the electrodes should the latter be shorted . the lamp l lights when the fuse f3 opens to indicate that a fault condition has occurred . line c4 preferably includes an ammeter m . a water supply conduit 11 connects the tank 5 to a conventional water supply 10 , for example a tap connected to a city water supply , or the like , and flow through the conduit 11 is controlled by an electrically operated valve 12 , conveniently a solenoid valve . the valve 12 is electrically controlled by a connection to output terminals t4 and t5 of the control unit 3 . a drain conduit 15 is connected to the bottom of the tank 5 and it has an electrically controlled valve 16 , conveniently a solenoid valve , whose operation is controlled by an adjustable timer 17 . the timer will be set to open valve 16 at timed intervals , independent of the electrode current , to discharge some of the water in the tank when the mineral concentration therein builds up , whereby to maintain the mineral concentration of the water in the tank below a selected level . fig3 diagrammatically discloses primary circuitry of the control unit 3 . the electronic control unit 3 comprises a current sensing transformer 21 , a variable voltage reference source 22 , a decision making comparator circuit 25 , a valve drive circuit 31 and a dc power supply 41 . the primary winding of current transformer 21 connects through terminals t1 and t3 , in series loop with one of the cell electrodes 7 and one side of the ac voltage source 9 . the secondary winding of the current transformer 21 connects at point 75 to the input of comparator circuit 25 and to circuit ground . the particular voltage reference circuit 22 , shown in fig3 provides a substantially constant , preselected vapor output regardless of the water level and the conductivity of the water in the tank 5 . reference circuit 22 thus comprises a series resistance 24 and potentiometer 23 coupled between the regulated positive supply line 52 of power supply 41 and ground , and provides a preset constant reference signal at intermediate point 73 . the slider of variable resistor 23 is manually adjustable by a suitable knob 14 or the like accessible from inside of the apparatus cover hereafter discussed . the comparator circuit 25 comprises an operational amplifier 27 connected as a level comparator . the reference signal line 73 connects to the inverting (-) input of the comparator 27 and the current transformer secondary output line 75 connects through resistor 26 to the noninverting (+) input of such comparator . the comparator output is coupled through a diode 28 to the valve circuit 31 . the diode 28 and a capacitor 30 coupled therefrom to ground act as a detector for converting the alternating current signal from the comparator 27 to a dc voltage , for application to the valve circuit 31 . a regenerative feedback network , including a resistor 29 , couples the output of diode 28 to the noninverting input of the comparator 27 , which input also connects through a capacitor 39 to ground . the valve drive circuit 31 comprises an inverting amplifier transistor 34 having a collector and emitter respectively connected through a resistor 33 to a nonregulated positive power supply line 54 of power supply 41 , and to ground . the base of transistor 34 connects through a limiting resistor 32 to the output side of diode 28 . output is taken from the collector of transistor 34 and applied directly to a darlington connected power switch comprising transistors 35 and 37 and pull - down resistor 36 . a protective diode 38 between the collector of darlington transistor 37 and ground prevents reverse voltage spikes caused by operating the inductive solenoid load of valve 12 from damaging the darlington transistors 35 and 37 . the nonregulated positive potential line 54 of the power supply 41 connects through terminal t4 , the solenoid of water fill valve 12 and terminal t5 to the collector of darlington transistor 37 , the emitter of which connects to the ground side of the power supply . thus , conduction of transistor 37 flows current from the power supply 41 through the solenoid of valve 12 , energizing the latter . the power supply 41 comprises a step - down transformer 43 having a primary winding connected through terminals t1 and t2 to the fixed ac voltage supply 9 . the secondary winding of step - down transformer 43 is center tapped to ground and drives a full wave bridge rectifier comprising diode pairs 44 and 45 . the positive output of rectifier unit 44 , 45 is applied directly to unregulated positive supply line 54 and is also applied to regulated positive supply line 52 through a filter comprising a series current limiting resistor 48 and a grounded parallel filter capacitor 46 . the negative rectifier unit output is applied to negative potential line 53 through a filter comprising a series current limiting resistor 49 and parallel grounded filter capacitor 47 . voltage stabilizing zener diodes 50 and 51 , paralleled by bypass capacitors 55 and 60 , respectively , connect between respective positive and negative potential lines 52 and 53 and ground . thus , potential lines 52 and 53 provide stabilized positive and negative dc voltages , respectively . considering the operation of the apparatus of fig2 and 3 , same provides a substantially constant vapor generation rate which rate is selectable by setting of the manual adjustor 14 . with the tank 5 empty , no tank water conductively couples electrodes 7 . the electrodes 7 then act as an open switch in the ac loop comprising the electrodes , lines c3 and c2 , ac voltage source 9 , line c1 , the primary of current transformer 21 ( fig3 ) and line c4 ( fig2 ). thus , no current flows in such heating loop and no heating takes place . the same effect exists with a water level le in the tank below the bottom of either electrode 7 . with a higher water level in the tank , sufficient to wet and form a conductive path between the plates 7 , ac current flows in the ac heating loop comprising lines c3 , c2 , ac source 9 , line c1 , the primary of current sensing transformer 21 and line c4 , thus through plates 7 and the intervening water in contact therewith . such water acts as a resistance heating element and is heated by such current flow therethrough . at increasing water levels , increasing areas of conductive plates 7 are wetted and an increasing cross section of tank water flows electric current between the conductive plates 7 . consequently , the ac current flow through current sensing transformer 21 , the heating of the water in tank 5 , and the rate of vapor ( steam ) generation all increase as the water level in tank 5 rises above level le . the control unit 3 responds to the ac heating current level sensed by current transformer 21 and to the set position of manual adjustor 14 , to actuate and deactuate water supply valve 12 so as to maintain electrode current , heating , and the rate of vapor generation each substantially at the desired level . assuming for purposes of illustration an idealized condition in which the electrical conductivity of the water is constant , the water level would fluctuate within a relatively narrow range lr ( shown enlarged in fig2 with upper and lower ends marked lmax and lmin , respectively ) and a virtually constant vapor generation rate would be achieved . however , in actual practice , because the electrical conductivity of the water is not constant owing to the variability of the mineral content thereof , the lr range shifts up or down relative to the tank depending on the electrical conductivity of the water . it is to be noted that the invention does not contemplate sensing water level directly , but rather senses electrode current . thus the electrode current is maintained constant regardless of the water level in the tank . in this way variations in the water conductivity are automatically compensated for by shifting lr up or down so as to maintain substantially constant electrode current . when the water conductivity is high owing to a high mineral content , then lr shifts down , when water conductivity is low owing to low mineral content , then lr shifts up . thus , water is initially allowed to flow into the tank 5 through the solenoid valve 12 , thereby starting and increasing the ac heating current through the tank 5 . when a predetermined heating current level is reached and sensed by the control unit 3 , the latter closes the valve 12 and water supply to the tank is stopped ( as for example at lmax in fig2 ). as water heated by ac current flowing therethrough , vaporizes and leaves the cell 5 , the tank water level gradually decreases . the resultant decrease in current flow , to a specified portion of the current level preset by adjustor 14 , causes the control unit 3 to open valve 12 ( as at lmin in fig2 ) and again permit water to enter the tank 5 until the former predetermined current level is reached and the valve 12 again closes . a safety feature is that heating current will automatically shut off upon failure of the water supply system 10 - 12 to add fresh water to the tank 5 , given sufficient vaporization of tank water to drop the tank water level to below at least one of the conductive plates 7 . considering the internal operation of the control unit 3 , the respective inputs of the comparator 27 ( fig3 ) receive the stable dc reference signal on line 73 at a level set by the adjustor 14 , and the ac voltage , representing the cell heating current , through the resistor 26 from the secondary of current sensing transformer 21 . when the peak positive value of the voltage on the secondary of sensing transformer 21 exceeds the level of the reference voltage at 73 , the comparator 27 amplifies the difference and causes a greatly amplified difference signal to appear at the anode of diode 28 . the diode 28 and capacitor 30 act as a detector , converting the alternating current signal from the comparator 27 to dc voltage , which is applied through limiting resistor 32 to the inverting input transistor 34 of the valve driving circuit 31 . the resistors 26 , 29 control regenerative feedback to toggle the comparator 27 into a firm &# 34 ; on &# 34 ; state once the output of the current transformer 21 has exceeded the reference voltage on line 73 . a controlled hysteresis is provided between the &# 34 ; on &# 34 ; state of the comparator and the point at which the comparator will toggle back to its &# 34 ; off &# 34 ; state . the amount of hysteresis is fixed by the values of resistors 26 and 29 and the dc resistance of the secondary winding of current sensing transformer 21 . the filter capacitor 30 of the detector 28 , 30 provides a sustaining time delay . more particularly , capacitor 30 is charged very rapidly by the low impedance output of the comparator amplifier 27 through diode 28 , but its rate of discharge is caused to be less rapid by the relatively high discharge resistance presented by the diode 28 , the limiting resistor 32 and the large value of feedback resistor 29 . this sustaining time delay prevents nuisance toggling of the comparator 27 by brief current dropouts , caused by bubbling or sloshing of water in the tank 5 . capacitor 39 provides a short delay in toggling the comparator 27 to the &# 34 ; on &# 34 ; state , thereby preventing nuisance toggling due to noise spikes on the ac power line . with insufficient water in the boiler tank 5 to permit maintaining the preselected electrode current magnitude and corresponding desired steam generation rate , the output of current sensing transformer 21 will be lower than the value of the reference voltage from reference circuit 22 . thus , the inverting (-) input of the comparator 27 will be more positive than the noninverting (+) input and the comparator output will be &# 34 ; low &# 34 ;, or a negative dc voltage . the detector diode 28 thus will not conduct and no current will flow to the base of transistor 34 . transistor 34 is thus in the &# 34 ; off &# 34 ;, or nonconducting , state . this allows current to flow through resistor 33 into the darlington power switch 35 , 37 at the base of its transistor 35 , rendering the power switch transistors 35 , 37 conductive . thus current flows from the unstabilized positive supply line 54 through the solenoid of water supply valve 12 and conductive darlington switch transistor 37 to ground , actuating the solenoid valve 12 and causing same to admit water from supply 10 to the tank 5 . when the water in the cell 5 has risen sufficiently , the heating current causes the output signal of sensing transformer 21 to exceed the reference voltage on line 73 , the peaks of the positive half cycles of the output signal of sensing transformer 21 will be amplified by the comparator amplifier 27 and appear on the anode of the diode 28 as a positive signal , well above zero volts . such signal causes the diode 28 to conduct , charging the capacitor 30 . the comparator 27 will then toggle &# 34 ; on &# 34 ; ( become fully conductive ) because a portion of the voltage across the capacitor 30 is applied through resistor 29 to the noninverting (+) input of the comparator amplifier 27 . once the comparator amplifier is thus toggled &# 34 ; on &# 34 ;, the voltage at the anode of diode 28 will be nearly the full positive supply voltage on supply line 52 , providing sufficient current through resistor 32 to cause the transistor 34 to conduct . the conductive transistor 34 clamps the base of darlington switch input transistor 35 substantially to ground , turning off darlington switch transistors 35 and 37 , closing the solenoid valve 12 and preventing more water from entering the tank 5 . as the water level drops ( as through range lr ), the comparator amplifier 27 remains toggled in its &# 34 ; on &# 34 ; condition due to the regenerative dc feedback from output to input through resistor 29 , despite gradual lowering of the magnitude of the ac current peaks at the secondary of sensing transformer 21 with the decreasing water level . ultimately however , the water level and sensing transformer signal drop sufficiently that the regenerative feedback path can no longer maintain the comparator 27 &# 34 ; on &# 34 ;. at that point , comparator amplifier 27 again toggles &# 34 ; off &# 34 ;, and as above indicated the valve 12 again opens to admit more water to the boiler tank 5 . the above cycle repeats to maintain the preset vaporization rate , and until such time as the ac source 9 or water supply 10 is disconnected from the apparatus . the present system can steam at a preselected substantially constant rate despite considerable change ( e . g . build up ) in the mineral concentration in the water in the tank 5 . however , to slow the growth of mineral deposits on the electrodes 7 and surfaces of the tank 5 and maintain mineral concentration in the tank water below a limit , the continuously operating timer 17 periodically opens and closes the drain valve 16 in accord with a preset time cycle and independent of electrode current . the open and closed intervals may be set as desired but the drain valve 16 may be opened for example every two hours and held open for a time sufficient to drain the tank 5 , thereby carrying away flaked - off mineral deposits and tank water having a high suspended solids content . an open time of four minutes is typical . fig4 and 5 disclose an accessory usable where desired , with the above - described apparatus of fig2 and 3 . such accessory apparatus is directed to a phenomenon which may occur during start up of the apparatus of fig2 and 3 . more particularly , cold water admitted to the empty tank 5 rises along the plates 7 . the conductivity of this incoming cold water is less than if the same water were at a higher temperature , e . g . at boil . in a given instance , water temperature may still be below boiling , hence with conductivity abnormally low , as water continues to enter the tank 5 and raises the water level therein to some level lmax at which the fill valve 12 normally would shut off for the same water at boil . the tank then tends to continue to fill above normal level lmax before electrode current reaches its preselected operating level , toggles the fig3 comparator 27 , and turns off the water supply valve 12 . however , continued heating would increase water temperature and hence conductivity , and thereby cause an overshoot in heating current , in view of the abnormally high level of water in the tank . whether this effect is significant in a given instance , depends for example on initial water temperature , the conductivity - temperature coefficient of the water and the tolerable overshoot in heating current . heating current overshoot toleration may depend on the current ratings of the ac supply 9 and / or components including the several fuses f1 - f3 and current transformer 21 . again , it must be noted that the water level lmax indicated in fig2 is not a permanently fixed height on the tank wall , but rather will vary with changing conductivity of the water and is merely used as a convenient designation for any level to which the tank water has risen when electrode current grows large enough to shut off fill valve 12 , under stable operating temperature ( boil ) conditions . the labels lmin and lr are similarly variables and , with lmax , will vary as water conductivity at boil changes , due to increase or reduction in mineral content . upon boiling , water temperature and level and heating current level stabilize at normal operating values , and the above phenomenon disappears . the accessory arrangement of fig4 and 5 is directed to controlling such overshoot phenomenon during start up and may be employed where desired . the accessory apparatus comprises a water removal means , preferably an electrically controlled drain valve 60 ( fig4 ), connected in a drain conduit 61 communicating with the cell 5 . the electrical actuator ( e . g . solenoid ) of drain valve 60 is here shown as coupled through suitable conductors to terminals t6 and t7 in an accessory portion 3 &# 39 ; of the control unit 3 for control thereby . the accessory circuit portion 3 &# 39 ; ( fig5 ) is an overshoot compensation circuit which includes a further decision making comparator circuit 25a , the output of which connects to a drain valve drive circuit 63 . the circuits 25a and 63 are preferably identical to comparator circuit 25 and the supply valve drive circuit 31 , respectively , of fig3 except as hereafter noted . similar parts in fig5 carry the same reference numerals , with the suffix &# 34 ; a &# 34 ; added , as corresponding parts of fig3 circuit , and require no further description . the sensing (+) input of comparator 27a connects through resistor 26a to the secondary signal line 75 of the sensing current transformer 21 of fig3 . a reference circuit comprises series voltage divider resistors 70 and 71 . resistor 71 is connected to the stabilized positive supply line 52 of power supply 41 and divider resistor 70 connects to the reference signal line 73 of fig3 . the intermediate point 72 of the voltage divider 70 , 71 connects to the reference (-) input of the comparator 27a . the drain valve drive circuit 63 differs from the supply valve drive circuit 31 of fig3 in having an input amplifying transistor 77 which is noninverting . the base of transistor 77 connects through current limiting resistor 32a to the output of comparator circuit 25a , its collector connects to the stabilized positive supply line 52 , and its emitter connects through series dividing resistors 78 and 79 to ground . output is taken from the emitter through resistor 78 and applied to the base of darlington switch transistor 35a . drain valve terminals t6 and t7 connect in series with the positive supply line 54 and darlington switch transistor 37a . when the tank 5 has filled with cold water sufficiently that the water supply valve 12 has closed , the heating current through the tank 5 tends to increase as water temperature rises and is additionally monitored by the accessory current sensing circuitry of fig5 . such circuitry operates the drain valve 60 when the heating current overshoots its desired level by a given magnitude . water is drained from the tank 5 until the current has dropped to a tolerable value , which is above the level at which the water supply valve 12 would reopen . thereafter depending on the conductivity - temperature coefficient of the water , and its temperature , heating current is held near its desired level by subsequent openings of the drain valve 60 , if necessary . when , eventually , reduction of water level due to vaporization , ( as above discussed with respect to fig2 and 3 ) sufficiently reduces heating current , overcoming the current increase due to water temperature increase , the comparator 27 of control unit 3 again turns on the water supply valve 12 to again raise the water level , continuing the described fig3 cycle of operation as the water heats . thus , eventually the water in the tank 5 reaches its normal maximum operating temperature at boil , eliminating the initial current overshoot phenomenon , and heating current then stabilizes at the proper level . thereafter , the fig5 overshoot compensation circuit normally will remain deactuated with the drain valve 60 closed . the internal operation of the overshoot compensation circuit 3 &# 39 ; of fig5 is essentially similar to the fig3 circuit , with the following exceptions . the further reference voltage on line 72 is a somewhat higher potential than the reference voltage applied to comparator 27 of fig3 . thus a somewhat higher heating current level ( i . e . some overshoot ) must be sensed at point 75 to toggle &# 34 ; on &# 34 ; the further comparator 27a . thus , the drive circuit 63 will normally open the drain valve 60 in response to an overshoot in heating current following the cold water filling of the cell 5 and the shutting off of the water supply valve 12 . by the same token , the comparator amplifier 27a will tend to toggle &# 34 ; off &# 34 ; and shut off drain valve 60 before comparator amplifier 27a toggles off to reopen water supply valve 12 . the drain valve 60 operates in a manner complementary to the supply valve 12 , which is satisfied by the use of a non - inverting , rather than inverting , amplifier at 77 . the desired offset of toggling points of comparator amplifiers 27a and 27 is satisfied by the connection of the voltage divider 70 , 71 to common reference line 73 . while separately numbered and described above , the drain valves 16 and 60 may actually be implemented with a single valve as hereafter discussed with respect to valve 16 of fig7 and 8 . in many instances , it is desired that a humidifier attain and maintain preselected humidity level in the local atmosphere , rather than merely continuously operate at a preselected constant vaporization rate ( as above - discussed with respect to fig2 and 3 ). to adapt the control unit 3 of fig3 from a constant vaporization rate mode to a constant humidity level mode , the humidity sensing circuit shown in fig6 may be substituted for the fixed reference circuit 22 of fig3 thus providing a humidity responsive reference signal via line 73 &# 39 ; to the inverting (-), or reference , input of the comparator 27 of fig3 . the humidity sensing circuit 22 &# 39 ; of fig6 comprises a stabilized ac reference source including back - to - back zener diodes 85 connected in series with a current limiting resistor 81 across an ac reference source . the ac reference source may be conveniently the center tap and one end of the secondary of transformer 43 of fig3 . a potentiometer 83 , or a tapped resistance network ( not shown ) connects from circuit ground across the back - to - back zener diodes 85 . its slider , or tap selector , is adjustor by a manual adjuster 14 &# 39 ; to set the desired humidity level . a humidity sensor 88 , here having a resistance which decreases with increases in humidity , is connected to the ground line and , preferably by a series temperature compensating thermistor 87 , to the slider of the humidity control potentiometer 83 . a detector diode 89 and capacitor 91 connect in series across the humidity sensor 88 and the sensor output is taken from the cathode of diode 89 through a filter network comprising a series resistor 97 followed by a resistor 95 and parallel capacitor 96 connected to circuit ground . the output of such filter is applied through reference signal line 73 &# 39 ; to the reference input (-) of comparator amplifier 27 of fig3 in place of the fixed reference on line 73 of fig3 . the humidity sensor 88 , and components 87 , 89 and 91 directly connected thereto , may be housed with the remainder of the control unit 3 . alternately , the humidity sensing components 87 - 89 and 91 may be conventionally housed , as diagrammatically indicated in broken lines at 92 , and remotely connected to the humidity control potentiometer 83 , ground line and filter 95 - 97 through an intervening three conductor cable schematically indicated at 93 , the corresponding conductors of which extend between terminals t8 , t9 , and t10 on the humidity reference portion 3 &# 39 ; of the control unit 3 and terminals t8 &# 39 ;, t9 &# 39 ; and t10 &# 39 ; on the remote humidity sensor housing 92 . instead of being located in the humidity reference portion 3 &# 34 ; of control unit 3 , the humidity control potentiometer 83 and its manual adjustor 14 &# 39 ; may instead , if desired for convenience , be remotely located at remote housing 92 . in that instance , resistor 81 and the upper one of zener diodes 85 connect directly to terminal t8 , the resistive element of potentiometer 83 is coupled across remote terminals t8 &# 39 ; and t9 &# 39 ; and the connection of terminal t8 &# 39 ; to the upper side of thermistor 87 is through the slider of potentiometer 83 . the thermistor 87 and humidity sensor 88 function as a voltage divider across the ac reference source voltage supply through the slider of potentiometer 83 . the potentiometer 83 serves as an adjustable humidity control . as the humidity seen by sensor 88 increases , its resistance decreases , decreasing the voltage at the anode of diode 89 . the thermistor 87 has a temperature characteristic that matches that of the humidity sensor 88 and compensates same for variations in temperature . the detector diode 89 and capacitor 91 convert the ac voltage at the junction of thermistor 87 and humidity sensor 88 into a dc voltage . this dc signal , the amplitude of which represents the humidity status , is fed through the filter and voltage divider network 95 , 96 , 97 , which removes any unwanted ac components from the signal and reduces the signal to a level compatible with the output of the current transformer 21 of fig3 . the humidity reference signal on line 73 &# 39 ; ( fig6 ) will thus increase with an increase in the desired humidity level ( as reflected by setting of potentiometer 83 to increase the ac level applied across series thermistor 87 and humidity sensor 88 ) and with a decrease in humidity in the local environment ( as reflected an increase in the resistance of humidity sensor 88 ). thus , an increase in humidity reference signal level on line 73 &# 39 ; is a call for an increase in the rate of vapor output by the apparatus of fig2 and 3 . the apparatus of fig2 and 3 responds to an increase in the reference signal level , applied to the inverting (-) input of comparator 27 , by increasing the level to which electrode current must rise to cause comparator 27 to toggle &# 34 ; on &# 34 ; and thus shut off the water supply at valve 12 . the result is net upward shift of the operating water level range lr , an increase in electrode wetted surface , an increase in heating current conducted through the tank water , and a consequent increase in vaporization rate , as called for by the humidity sensing apparatus of fig6 . vaporization of water in the tank 5 will gradually increase the environmental humidity level toward the desired level set by humidity control potentiometer 83 of fig6 . during this time , the control unit 3 may cycle several times in the manner above - described with respect to fig2 and 3 . also as environmental humidity level rises , the humidity reference signal on line 73 &# 39 ; ( fig6 ) correspondingly decreases . thus the required heating current diminishes . meanwhile control unit 3 may cycle , periodically opening fill valve 12 to the tank to make up for vaporization losses , filling to successively reduced levels as environmental humidity increases toward the desired level . this operation continues until the humidity in the environment reaches the desired , or set , level . this ( or a manual reduction in the setting of humidity set potentiometer 83 to below the existing humidity level in the controlled area ) stops cycling of the comparator 27 with the water supply valve 12 closed and the water level in the tank 5 below plates 7 , and hence stops heating current flow and establishes an off condition . when the humidity again falls to a point at which an operative humidity reference signal level appears on line 73 &# 39 ; ( or when a manual increase in the setting of the humidity set potentiometer 83 above the existing humidity level achieves the same result ), the control unit 3 again , and in the manner above described , opens fill valve 12 ( fig3 ), raising the water level in the tank , and permitting electric current again to flow between the electrodes 7 to generate vapor and hence raise the humidity in the monitored environment . if a further decline in the humidity in the local environment , or room , occurs ( or if the setting of humidity potentiometer 83 is further manually increased ), the fig6 circuit will continue to increase the signal on reference lines 73 &# 39 ;, increasing the vapor generation capacity until the humidity requirements for the controlled environment are satisfied , or until the maximum capacity of the apparatus has been reached . in fig6 an ac voltage reference is used in order not to chemically polarize the particular humidity sensor used . also , high signal and reference voltage levels are preferably employed , to increase the signal to noise ratio , when , as shown in fig6 portions of the humidity sensing circuitry are located remotely from the control unit 3 . to incorporate the circuits of fig3 and 6 in a common control , the fig6 reference output line 73 &# 39 ; is connected both to the reference (-) input of comparator 27 of fig3 and to the reference input line 73 of fig5 . fig8 is an interconnection diagram for an embodiment of the invention and shows the way in which portions of the electrical circuitry above - discussed with respect to fig2 - 5 may interconnect with each other to provide the desired apparatus operation . for convenience , the electronic circuitry portion of the fig3 - 5 circuits may be accommodated on one or more printed circuit boards and such in fig8 is represented merely by a printed circuit board block 101 . in fig8 then , connection to the conventional ac electrical source 9 is made through a conventional terminal block 102 which connects , as through leads c1 and c2 to a main line contactor , of conventional type generally indicated at 103 and which provides a convenient source of ac power to remaining ac - fed components as indicated in fig8 . interposed in line c1 is an overload protector 104 which may be identified with fuse f1 ( and for that matter here includes a further portion corresponding to fuse f3 ) of fig2 fuse f2 of fig2 here being omitted . during apparatus operation , the contactor 103 supplies ac operating potential to various components as above - described , including the circuit loop incorporating electrodes 7 , 7 , ammeter m and the primary of current transformer 21 . the contactor also supplies ac operating potential to the transformer 43 whose secondary ends and center tap directly connect , as shown , to the printed circuit board block 101 at which is located the rest of the power supply 41 of fig3 as well as the fig3 circuitry driven by the secondary of current transformer 21 . also , the overload indicator light l is here coupled across the portion of overload protector 104 indicated at f3 . fig8 introduces several additional features not discussed above with respect to fig2 - 6 . a stop - start switch 106 of the conventional type having a built - in light to indicate the &# 34 ; on &# 34 ; condition of the switch , has its normally open , manually closable contacts connected in series loop with the secondary winding of a control transformer 107 ( and with the indicating lamp built into the stop - start switch 106 ). connected across said stop - start switch lamp is a series path including a pair of input terminals for the solenoid 103a of the contactor 103 , the portion labeled f3 of the overload protector 104 ( shunted by overload indicator light l ), a cover switch 111 closed when the cover ( hereafter discussed ) of the apparatus is properly in place , and , if desired , a terminal block 112 . in the embodiment shown , the terminal block 112 normally provides a straightthrough electrical connection between cover switch 111 and start - stop switch 106 , to flow current through solenoid 103a when switch 106 is closed . the purpose of the block 112 is to prevent circuit operation and hence continued vapor generation under specified conditions . for example , where the apparatus supplies vapor to a remote location through an overhead duct , a conventional humidity sensor switch hs in the duct may be set to open at a preset maximum humidity level in the duct ( e . g . 90 %). also , the duct may be provided with a fan ( as in fig2 ) to move vapor from the apparatus through the duct to such location , and a suitable fan motor responsive or air flow responsive switch f may be arranged to open should such fan fail . by connection of such a switch hs or f ( or both in series ) across the block 112 , opening of either , in response to excessive duct humidity or duct fan failure , guards against vapor condensation in and water leakage from such duct , by blocking current flow to the terminals 108 and 109 of the solenoid 103a . when such protective measures are not needed , the block 112 and switches hs and f can be replaced by a wired connection between switches 106 and 111 . the humidity switch hs , or additional such switches in the series path across the terminal block 112 can be located in a room for limiting the humidity therein to the desired level by opening the line contactor 103 to interrupt the heating current . thus , either an on - off humidity sensor switch , like switch hs , or the fig6 variable output humidity sensing circuit , can be used to control humidity in such room . in the preferred embodiment shown , the primary winding of control transformer 107 is ac energized from the ac supply terminals of the contactor 103 which terminals are in turn energized from ac power lines c1 and c2 . the secondary of control transformer 107 , upon closure of start - stop switch 106 ( and with a closed path through elements 104 , 111 and 113 ) thus energizes main contactor solenoid 103a , which applies ac potential from lines c1 , c2 to terminals t1 , t2 and 115a , 114a , such that portions of the apparatus connected to such terminals are controlled by the start - stop switch 106 . in the embodiment shown , the drain valve 16 is arranged to serve the functions above - described of both timer operated drain valve 16 of fig3 and cold start drain valve 60 of fig5 . accordingly , the fig8 drain valve 16 is controlled from a double - pole - double throw drain switch 117 , here for example through a voltage step - down transformer 118 . in its manual position , manual - automatic drain switch 117 supplies ac potential from terminals 114 and 115 through transformer 118 to place drain valve 116 in its open condition for draining water from tank 5 . on the other hand , when the rightward or automatic position of drain switch 117 is selected , it establishes a series connection from ac terminal 114a through the transformer 118 primary , and paralleled normally open contacts 120 and 121 of drain timer 17 and a drain relay 122 , which in turn connect to the corresponding ac terminal 115a , permitting either the drain relay 122 or drain timer 17 to open the drain valve 16 . the ac input terminals 124 and 125 of the timer 17 connect respectively to ac supply terminals 114a and 115a so that the drain timer 17 continuously times while the start - stop switch 106 is in its operating mode . the drain timer 17 may be of any convenient type capable of timing for a preselected interval , e . g . two operating hours , opening the drain to flush mineral laden water from the tank for a preselected short interval , e . g . four minutes , and repeats this cycle as long as the start switch 106 and contacts hs , f , and 111 remain &# 34 ; on &# 34 ;, substantially operating in the manner above described with respect to fig3 . in the fig8 embodiment , it is the dc input terminals 127 and 128 of drain relay 122 which connect to the fig5 terminals t6 and t7 , rather than the drain valve solenoid directly , such that conduction of the fig5 transistor 37a produces a dc current flow through the drain relay terminal 127 and 128 , closing contact 121 thereof and therethrough closing the ac connection to the automatic side of drain switch 117 for actuation of the drain valve 16 in the manner above - described . as shown in fig8 the fill valve 12 , as in fig3 is connected across dc path terminals t4 and t5 , though for convenience in fig8 positive potential terminals t4 and t6 appear as a single terminal on the output side of the printed circuit terminal block 101 . in fig8 a drain indicator light 130 and a fill indicator light 131 are respectively connected across the dc input terminals 127 and 128 of the drain relay and the dc input terminals of the fill valve 12 , by lines 132 and 133 , respectively , along with common line 134 , such that actuation of the drain relay actuates the drain indicator light 130 and actuation of the fill valve 12 actuates the fill indicator light 131 . it will be noted that the fill valve operates in the manner above - described with respect to fig3 . on the other hand , the drain valve 16 operates in the manner above - described with respect to fig3 and 5 , though in the fig5 mode through drain relay 122 , and in both modes through the automatic position of the drain switch 117 and , if desired , transformer 118 , so as to provide both periodic draining and draining on a cold start to avoid excessive heating current ( as well as to provide manually controlled draining when the manual , leftward position of switch 117 is selected ). if desired , a lapse timer 136 may be provided to monitor the total number of apparatus operating hours and may be used in conjunction with any convenient indicating or alarm means to inform the system operator that routine maintenance ( e . g ., replacement of the steam generator tank 5 or electrodes 7 therein ) should be considered . in some instances it may be desired to admit some fresh cool water from the fill valve 12 when the drain valve 16 is periodically opened by drain timer 17 , so as to dilute and reduce the temperature of water draining from the tank 5 . such is here accomplished in a convenient manner , since as water is drained from the tank , by the opened drain valve 16 , the water level and heating current fall . a sufficient drop in heating current flow through transformer 21 causes the fig3 circuit to open the fill valve 12 , as above described , thus automatically mixing cool fresh water with the hot draining tank water , at tee 159 , on the way to drain . fig1 and in more detail fig7 - 7b , show mechanical aspects of a preferred embodiment of the invention . the apparatus includes a chassis 150 ( fig7 and 7b ) preferably wall mountable , including a shelf 151 , upstanding bulkheads 152 and 153 between which the tank 5 is disposed , and a component plate 154 , the major electrical components being carried by bulkhead 152 and adjacent plate 154 , as shown in fig7 . the tank 5 is preferably a sealed , disposable unit which may be covered with insulation as indicated at 156 and has an upward opening vapor outlet 157 , here coupled to a flexible vapor distribution conduit 158 which may lead to suitable duct work and a distribution fan or the like not shown . the drain conduit 15 communicating through the bottom of the tank 5 and extending to the drain valve 16 ( fig7 and 7a ) incorporates a tee 159 flanked by suitable conduit means 161 and 162 . the water fill valve 12 is fixed to bulkhead 152 above , and empties into , the open upper end of a funnel cup 164 ( fig7 and 7b ) having a downward extending water supply tube 165 connected to the tee 159 . an overflow conduit 167 tapped into the funnel cup 164 below the top thereof extends downward , eventually connecting , along with the outlet side of the drain valve 16 , at a tee 168 with further conduit means 169 to drain . the outlet from the water fill valve 12 terminates above the funnel cup 164 and accordingly the water supply to the fill valve 12 is isolated from the water in tank 5 even should same rise to the level of overflow 167 or even the top of funnel cup 164 ( which the presence of overflow 167 would normally preclude ). opening of fill valve 12 causes water to flow into the tank 5 through the path 164 , 165 , 159 and 162 . on the other hand , opening of drain valve 16 causes water to exit the tank 5 through the path 162 , 159 , 161 , 16 and 169 . if desired , such draining of water from the tank 5 may be accompanied by opening of the fill valve 12 so that cool fill water entering the tee 159 through the path 164 , 165 mingles with and reduces the temperature of hot water exiting the tank through the path 162 , the mixture of hot and cold water passing then through the path 161 , 16 , and 169 to drain . in the preferred embodiment shown , the electrodes 7 extend substantially the length of the tank 5 and are each of inverted u - shaped cross section . one electrode is narrower than , and situated between the legs of , the other as generally indicated in fig7 b . in the embodiment shown , relatively narrow u - shaped strips 172 supported on the bottom wall of the tank contact and steady the depending legs of the outer electrode 7 . the central webs of the channel - like electrode 7 are each provided with electric current terminals , shown at 173 and 174 , respectively , to which the ac supply lines , as at c3 , c4 in fig2 may connect . the top surface of the outer electrode is provided with an aperture through which downwardly extends a boss 301 ( fig7 b ) in the top wall of tank 5 , such boss supporting the inner electrode by means of the the electric current terminal 173 , and insulating the inner electrode from the outer electrode . fig1 discloses the apparatus above - discussed with respect to fig7 - 7b mounted on a wall w with the vapor conduit 158 extending upward therefrom , and with a decorative cover 176 disposed thereover . the meter m and lamp end switch units 106 , 130 , 131 and l are disposed at an opening in the lower left corner of the cover for ready access and visibility . the cover 176 may be supported on the chassis 150 by any convenient means such as screws 177 ( fig7 b ). although particular preferred embodiments of the invention have been disclosed in detail for illustrative purposes , it will be recognized that variations or modifications of the disclosed apparatus , including the rearrangement of parts , lie within the scope of the present invention .