A number of improvements to electrical devices, particularly sheet heaters, comprising conductive polymer compositions, are provided. The preferred heater has the following features (a) it comprises a laminar resistive element and a plurality of electrodes which are so positioned that the predominant direction of current flow is parallel to the faces of the laminar element, (b) it comprises a laminar insulating element adjacent to but not secured to the electrodes and the resistive element; (c) it comprises a metallic foil, which acts as a ground plane and is positioned adjacent the insulating element but is not secured thereto; (d) it comprises a dielectric layer intimately bonded to the resistive element and to the electrodes.The invention also provides an electrical device comprising first and second members having different resistivities, and a thin contact layer of intermediate resistivity positioned between the first and second members.