fig1 schematically shows a cooking hob 1 with a glass ceramic surface 2 below which there is positioned at least one electrical heating element 3 ( such as a halogen lamp , a resistance element etc .) powered by the usual electric mains ( not shown ). the heating element 3 is controlled by a unit 5 , preferably a microprocessor , which controls the element 3 via an electrical circuit 6 of known type . in particular , the element 3 is controlled by usual known means for frequency - regulating the power fed to the element 3 ( these defining a known circuit configuration of the cycle skipping type ) which are present in the circuit 6 . the control unit for the element 3 is connected to a usual activation member for this element , for example a knob 7 located in a suitable position relative to the hob 1 . by means of this knob , the user activates ( or deactivates ) the heating element to obtain consequent heating of a corresponding region 8 on the surface 2 of the hob 1 . this region ( or cooking region ) 8 is arranged to receive a food container 10 of any known type . in proximity to the surface 2 , between this surface and the heating element 3 , there is positioned a usual temperature sensor 11 for measuring the temperature of the surface 2 . this sensor is connected to the unit 5 which , as described hereinafter , activates / deactivates the heating element 3 on the basis of the data obtained by said sensor , to prevent the container 10 from overheating should it be empty or should its contents have evaporated ( in the case of a liquid ) or be undergoing carbonization ( in the case of a solid ). more particularly , by frequency - controlling the power fed to the heating element 3 and from the temperature data measured by the sensor 11 , the unit 5 is able to “ construct ” a time / temperature curve such as that shown in fig3 . in other words , this unit controls power feed to the element 3 such that a succession of different - power levels enables the temperature of the surface 2 to be controlled with very close temperature variations ( for example ± 5 ° c .) in order to maintain it substantially constant . consequently , by continuously monitoring the temperature of the hob 1 , the unit 5 controls the temperature of the surface 2 with an accuracy such as to be able to precisely identify the temperature of the region 8 of the hob 1 at every moment of operation of the element 3 . in this manner , an optimum temperature ( t 1 ) can be maintained for preparing the food contained in the container 10 . by means of this power control and the measurement provided by the sensor 11 , the unit 5 is also able to sense if , such as at the moment k of fig3 the temperature of the region 8 undergoes a sudden undesirable increase . it has been surprisingly discovered that the temperature of the region 8 is related to the temperature of the container 10 . hence , if the container 10 overheats during food preparation ( for the aforesaid reasons ), this is indicated by a sudden unscheduled increase in the temperature of the region 8 , also indicating for example that the liquid contained in it has completely evaporated . in other words , if at the moment k the temperature / time curve of the surface 2 shows a positive slope change , this indicates an increase in the temperature of the container 10 related to its overheating . in this case , the unit 5 acts on the electrical feed to the heating element 3 to interrupt it , and hence halt said overheating . the method of operation of the unit 5 is shown in fig2 . the block 20 of this diagram represents activation of a heating element 3 by the user and selection of the heating power level . following this setting , the unit 5 chooses the feed power level to the element 3 ( block 21 ) and selects the corresponding temperature level of the region 8 ( relative to the element 3 ) of the hob 1 on which the container 10 is positioned ( block 22 ). the unit 5 then evaluates whether the temperature reached by the region 8 is the correct one corresponding to the heating power level selected by the user . if incorrect , it effects a new temperature selection cycle , whereas if correct it initiates frequency control of the power fed to the element 3 ( block 24 ) so as to maintain the set ( cooking ) temperature reached by the heating element . in this case it commences cycle skipping of this power feed by modifying the power in such a manner as to maintain the set temperature of the heating element region 8 . during use of the hob , ie during power feed to the heating element 3 , the unit 5 continuously evaluates ( block 25 ) whether the temperature curve ( fig3 ) presents a discontinuity , ie a sudden temperature rise in the region 8 of the heating element representative of overheating of the container 10 . if there is not , the unit 5 continues to maintain the already attained power level of the element 3 . if however there is a positive temperature rise ( ie the aforesaid event occurs at point k of fig3 ) and this continues to a first preset limit ( for example 600 ° c . ), the unit acts ( block 26 ) on the power to the element 3 to deactivate it and maintain this element deactivated until the temperature of the hob region 8 falls to a second temperature limit , for example less than 400 ° c . these values ( and in particular the first temperature limit ) are chosen on the basis of the components of the cooking hob ( burner and surface 2 ), such as to prevent damage to them . the unit 5 acts to halt overheating of the container 10 . subsequently , after the temperature of the region 8 reaches the second temperature limit ( lower than the first ), the unit 5 reactivates power to the element 3 and maintains temperature control ( blocks 23 and 24 ) of the hob region 8 . if this again approaches the first limit and exceeds it even slightly , the unit 5 permanently deactivates the heating element ( block 28 ) to prevent any further heating of the container 10 . otherwise , it maintains power to the heating element 3 until the user switches it off via the control member 7 ( knob ). it should be noted that during the time in which the unit 5 maintains the element 3 deactivated after evaluation by the block 26 , the user can always reactivate this element by operating the knob and again setting it to its activation position . an acoustic and / or visual warning device can also be provided to display that power to the heating element has been interrupted . by virtue of the invention and the surprising discovery that an unequivocal relationship exists between the temperature of the cooking hob and the temperature of a container positioned on it , any overheating thereof can be prevented , so protecting both the container and the cooking hob against damage which could compromise their reuse or intactness with time .