operation depending on a tapping being performed however the second regime of operation is preferably combined to the first in order to avoid excessive heating up of the sensor 11 in other variations of the steam generator according to the invention the reference temperature is maintained within the temperature transition range not by acting on the heating power but directly on the reference temperature itself thus according to a fourth variation an auxiliary thermosensitive unheated sensor is placed in the liquid 26 preferably close to the bottom 22 of the vessel 21 and provides the electronic circuit 33 with a signal depending on the temperature of the liquid 26 the assembly composed of the heating resistor 13 the diode 44 and the switch 45 is replaced with a simple heating resistor the electronic circuit 33 is capable of modifying the reference temperature as an increasing function of the temperature of the liquid 26 thus when steam 27 is tapped the reference temperature decreases at the same time as that of the liquid 26 and can be adjusted continuously in relation to the temperature transition range in a fifth and particularly simple variation whereas the device is similar to that of the fifth variation the circuit 33 is adequate to measure the temperature difference between the level sensor 11.