dc power connections 152 a and 152 b to conductive pathways 156 a and 156 b at the outer edges of remaining opposing sides of film 88 a series of diodes 166 alternatively connect from the positive dc power supply of 152 a and the negative dc power supply of 152 b for uniform operation it is desirable for the power applied to the resistive film 88 to be substantially identical regardless of whether the power applied is ac or dc this maintains a uniform heating characteristic of the heating plate 12 regardless of the power source when ac power is supplied to the film 88 the entire resistance of the film is applied thereto and thereby establishes its heating parameters for ac similarly applying dc across the entire film 88 would however have a much lower current with corresponding lower applied power and a different heating characteristic for the dc supply in the embodiment of fig4 when dc power is supplied to the film the diodes forward bias to allow current to only have to pass through the individual subsections 154 a 154 e as each of these subsections is smaller than film 88 the resistance is lower and the resulting current through each subsection is higher than film 88 as a whole this provides the same.