Abstract: A multi-electrode boiler, especially for use as a humidifier, comprising water-changing means arranged to allow at least some of the water in the boiler to be changed, monitoring means arranged to monitor the electrical-current which flows through at least one of the electrodes of the boiler, control means responsive to the monitoring means to control the change of at least some of the water in the boiler to maintain the electrical-current in said at least one monitored electrode within a predetermined range of values, in which switching circuitry is provided to switch in and out electrodes of the boiler to vary the boiling rate.