Abstract: 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.