fig1 schematically illustrates a structure having two ztc layers 1 and 2 with a ptc layer 3 in between . the layers are in full contact with each other . electrodes 4 and 5 are diagonally arranged in the ztc layers , within the layers as shown or in contact with the layers as an alternative . 2rztc is the resistance in each of the ztc layers so that the resistance of both layers in parallel is rztc . the resistance across the ptc layer is rptc1 and the resistance along the ptc layer between the electrodes is rptc2 . in fig2 is illustrated a curve showing the relationship between the ptc resistance rptc1 and the ztc resistance as a function of temperature . at room temperature the resistance across the ptc in the electrode area must be very small compared to the resistance in the ztc layers . its function is to couple the parallel ztc layers . at the same time the resistance in the ptc layer between the electrodes rptc2 , because of the ratio of thickness to width , must be substantially greater than the resistance in the ztc layers , so that heat is generated almost only in the ztc layers . at control temperature the resistivity in the ptc layer has risen so that its resistance between the ztc layers equals the resistance in the ztc layers themselves . once the heating element has reached the control temperature the wattage output remains virtually unaffected despite substantial increases in voltage . another way of expressing the relationships between the resistances , and in more detail , is as follows : the relationship between rptc1 and rptc2 is inherently taken care of by the geometry involving the ratio between the thickness of the ptc layer and the distance between the electrodes . the main requirement is that the resistances and thus power development at the control temperature are equal . again , because of the 100 % contact area of the ptc and ztc components the watt density requirements are fulfilled . the reason for the requirement that , at control temperature , the resistances in the ptc and ztc components are equal , is that below this temperature , essentially all power is developed in the ztc layers . when the resistance in the ptc layer exceeds the resistance in the ztc layer , and because of the nature of the ptc resistance temperature curve , the heat will be generated predominantly in the ptc layer , but this also means that the resistance of the whole composite rapidly increases . in other words , at temperatures below the control temperature the characteristics , power output , stability etc are only a function of the ztc component and the ptc provides only the limiting control . the calculation of the relative values of resistance and resistivity in a given heating element can then go through the following procedure . in essence this is the same as that outlined above . the effects of geometry on effective resistance in the ptc layer are as follows : the resistance through the ptc layer is greatly dependant on the direction of current flow . for example , the resistance through the thickness of the ptc layer is very small compared to the resistance from electrode to electrode through the width of the layer . and furthermore the resistance across the ptc layer in the limited area of the electrodes must also be small compared to the resistance in the parallel ztc layer . depending on the ratio of distance between the electrodes ( d ) and the thickness of the ptc layer ( t ), the resistance in the vertical direction will at all times be t 2 / d 2 times that in the horizontal direction . since at room temperature the ztc resistance must be very much less than the rptc2 and very much greater than the rptc1 , this sets limits on these resistance values in relation to the geometry of the device . but to be fully effective , at room temperature , the resistance through the ptc layer only in the area of the electrodes must also be so small compared to the ztc resistance , that it acts as a coupling short circuit between the two ztc layers , and then the watt density developed in this area is no greater than the watt density developed in the combined ztc layers . under these conditions the current will flow essentially straight across the ptc layer at each electrode , and then through the ztc layers to the opposing electrodes . now , as the temperature rises and the resistivity in the ptc layer increases , more and more of the ptc layer conducts current between the ztc layers until a temperature is reached where the resistance offered by the ptc layer equals the resistance of the ztc layers . under these conditions equal heat is developed in both components . if the temperature rises still further , the ptc component in the series starts to limit the current passing through it and hence through the whole device . at this control temperature the current is flowing in effect half through the ptc and half through the ztc and if the temperature increases further , due to for example an increased voltage , the resistivity in the ptc component continues to rise while the current flows more and more vertically through the ptc and less and less horizontally through the ztc . this then surprisingly causes a compensating reduction in element resistance , which has the effect of maintaining a controlled wattage output over the whole control temperature range . the relationship between the geometry of the heating elements and the ptc and ztc compositions used to make the elements will clearly appear from the following examples , with reference to fig3 where two ztc layers 11 and 12 are separated by a ptc layer 13 . electrodes 14 and 15 are connected diagonally to the ztc layers . the ptc layer has a thickness t , a length l and a distance d between the electrodes 14 , 15 which is equal to the length l , when the heater element is formed as a square . in the squarely formed elements tested , the size of the elements varied from d = 1 . 6 cm to d = 45 cm . the thickness of the ptc layer varied from 0 . 05 to 0 . 1 cm , and the thickness of the combined ztc layers from 0 . 0032 to 0 . 1 cm . it is a requirement of the invention that at cutoff or control temperature : where rptc1 is the electrical resistance measured across the ptc layer and where rztc is the resistance of the two ztc layers connected in parallel , each having a resistance of 2 . rztc . when the thickness of the ptc layer is 0 . 025 cm and the thickness of the combined ztc layers are 0 . 0032 cm ( using glass scrim impregnated layers ), the ohmcm ratio values for the heaters are as follows , at control temperature ( ct ) and at room temperature ( rt ): ______________________________________d ct rt______________________________________1 . 6 32 , 000 3 , 2004 . 5 250 , 000 25 , 00045 2 . 5 · 10 . sup . 7 2 . 5 · 10 . sup . 6______________________________________ the corresponding ratios for heaters made with extruded ztc layers of 0 . 025 cm thickness were : ______________________________________d ct rt______________________________________1 . 6 2 , 048 2044 . 5 16 , 200 1 , 62045 1 . 6 · 10 . sup . 6 1 . 6 · 10 . sup . 5______________________________________ a comparison with a prior art heating element ( beispiel 5 ) shows an ohmcm ratio of 20 / 7 = 2 . 85 at room temperature with a 2 . 54 times 2 . 54 size heating element , whereas one of the heating elements of the same size have an temperature . these ratios show the main difference between the prior art concepts and the concept of this invention . an example of a heating device made in accordance with the invention and compared with a heating device made in accordance with the prior art as represented by de patent no . de - 02 - 2 543 314 and u . s . pat . no . 4 , 017 , 715 are as follows : the ptc layer consisted of 45 parts of eletex carbon in 100 parts of pe ( polyethylene ) or eva ( ethylene vinyl acetate ) resin . the compound was made into a 0 . 1 cm thick film at a resistivity of 4 · 10 4 ohmcm at room temperature . the ztc layers consisted of a glass scrim of an open structured glass paper , mat or cloth impregnated with an aqueous dispersion of ketchen black . the ketchen black was run through a fluid energy machine along with 20 % by weight of 40 % colloidal silica ( dupont ludox hs - 40 ). the fluid energy machine was utilized to modify the surfaces of the carbon particles by impinging the particles against one another in a high velocity gas stream . this material was dispersed in water along with 5 % polyethyleneimine ( pei ) to effectively wet the carbon black and control the charge on the carbon particles . the coating is modified with a binder consisting of an acrylic latex , clay and colloidal silica and also pei , the binder being in a proportion to produce the desired resistance level on the coated scrim . thus with the ptc layer at a resistivity of 4 · 10 4 ohmcm and an ohm / square resistance of 400 , 000 combined with two ztc layers of 3 , 000 ohm / square each ( together in parallel 1 , 500 ohm / square ) this device had at room temperature a resistance of 1 , 520 ohm where the area between the electrodes was 6 . 3 · 6 . 3 cm . for comparison a sample was made according to the principles outlined in the prior art patents , the ohm / square resistance in the ptc layer was 1 , 600 and the ohm / square resistance in the combined ztc layers was 15 , 000 ohms . as will be seen from the above , the resistances of the components of the prior art device at room temperature , have the following characteristics : which is quite different from the resistance relationships of the components according to the invention . the room temperature resistance of the two samples , before connecting them to a power supply , was comparable , but the performance , when connected to a power supply , was very different as can be seen from the curves illustrated in fig4 especially with respect to the increasing voltage in the control temperature region .