Self-regulating electric heating element for heaters shaped as cartridges or test tubes

DETAILED DESCRIPTION I claim: 1.
An electric resistance heating element for heaters shaped as cartridges or test tubes, comprising a plurality of resistive conductors (2,8) connected to an electric supply by a system of electrodes (11, 12, 16, 17) and at least a core (3), said core being made of electric insulating material, said resistive conductors (2, 8) being applied to said core, said resistive conductors (2, 8) being formed of a mixture of a polymeric binder, an electric conductive filler, constituted by a conductive powder in the form of micrometric particles, said powder being carbon black, graphite or silver, said mixture having an electrical resistance with high positive temperature coefficient (PTC) at the operation temperatures, wherein said resistive conductors (2, 8) are formed as strips applied onto said core (3), wherein said electrodes (11, 12, 16, 17) are made of metallic materials in the form of foils, plates, wires, cables, said metallic materials including conductive coating layers consisting of inks or paints, said electrodes being disposed on said core (3), and wherein said core (3) is provided with at least a longitudinal internal hole (4) for the passage of a supply cable or electric conductors, and a longitudinal slot (5) for the entire length thereof, whereby said core (3) is caused to be expanded for being introduced into a test tube or cartridge, and wherein said resistive conductors (2, 8) are disposed on said core (3) with such an arrangement as to permit the electric current to flow axially, circumferentially or helically with respect to the core extent.
2.
The heating element according to claim 1 wherein said binder is a thermoplastic material.
3.
The heating element according to claim 1 wherein said core is expanded with the interposition of an electrical insulating material.
4.
The heating element according to claim 1, wherein said mixture of said resistive conductors comprises an additive consisting of a not-conductive filler, said non-conductive filler being a plasticizer, an inert filler, a lubricant or a stabilizer