Laminar electrical heaters

An electrical heater which comprises

• (1) a laminar resistive element which (i) is composed of a conductive polymer which comprises a continuous matrix comprising a first organic polymer and a first particulate conductive filler which is distributed in the matrix and maintains its identity therein and each particle of which comprises a second organic polymer and a second particulate conductive filler which is distributed in the second polymer and (ii) has a first face and an opposite second face;
• (2) a first laminar electrode, the whole of which is secured to the first face of the resistive element;
• and
• (3) a second laminar electrode, the whole of which is secured to the second face of the resistive element;
• the first and second electrodes being connectable to a power supply, whereby current can be passed through the resistive element;
• and the first and second electrodes being so shaped and positioned that (a) when current passes between the electrodes, it does so in a direction which is substantially at right angles to the faces of the resistive element, and (b) there is a continuous marginal portion around the whole of the periphery of the resistive element in which at most one of the first and second electrodes is present.