an electric resistance humidifier which increases atmospheric humidity by boiling water in a tank . spaced conductive plates , or electrodes , are fixed in the tank . as the tank water level rises , the immersed area of the electrodes increases . an electric supply causes the electrodes to pass electric current through the tank water therebetween for heating and vaporizing such water . electric current and heating cease automatically when the tank water level falls below the electrodes . a control includes comparator circuitry responsive to a reference signal and a signal related to electrode current for actuating and deactuating a water supply to the tank . in one embodiment , the control includes further comparator circuitry responsive to electrode current and a further reference signal for controlling a tank drain , to compensate for rising conductivity of the tank water as it warms toward boil and thus prevent substantial overshoot in heating current flow . in one embodiment , humidity sensing circuitry varies the reference signal level , to increase the upper water level limit in the tank , and hence heating current flow and speed of vaporization , in response to decreasing atmospheric humidity , thereby to increase atmospheric humidity to a desired level . the aforementioned apparatus , if desired with a fan for distributing vapor boiled from the tank water , is housed in a cabinet . a manual adjustor for selecting a fixed reference or humidity level is conveniently located on a panel of the cabinet .