1. A rheostat comprising a resistive element, a resilient plastics sheet encapsulating the resistive element, a conductive inner layer carried by the plastics sheet, first contact means extending from the resistive element to the exterior of the encapsulating plastics sheet, second contact means extending from the conductive inner layer to the exterior of the encapsulating plastics sheet, and a contact pin arranged to press on the encapsulating plastics sheet and adjustable to establish different values of electrical resistance between the first and second contact means.

2. A rheostat comprising a resistive element, a resilient plastics sheet encapsulating the resistive element, a conductive inner layer carried by the plastics sheet, first contact means extending from the resistive element to the exterior of the encapsulating plastics sheet, second contact means extending from the conductive inner layer to the exterior of the encapsulating plastics sheet, fluid interposed between the resistive element and the conductive inner layer and tending to separate same, and a contact pin that is arranged to press on the encapsulating plastics sheet to thereby establish localised contact between the conductive inner layer and the resistive element, and is adjustable to establish different value of electrical resistance between the first and second contact means.

3. A rheostat comprising a resistive element comprised of a flat insulating plate carrying a winding of resistance wire, a resilient plastics sheet encapsulating the resistive element, said plastics sheet being comprised of first and second sheet portions welded together at their edges, a conductive inner layer carried by the plastics sheet, and a contact pin that is arranged to press on the plastics sheet to thereby establish localised contact between the conductive inner layer and the resistive element, and is adjustable to establish different values of electrical resistance.

4. A rheostat comprising an encapsulating resilient plastics sheet comprised of first and second sheet portions folded against one another about a common edge and welded together at their other edges, a resistive element comprised of a resistive inner layer carried by the first sheet portion of the encapsulating plastics sheet, a conductive inner layer carried by the second sheet portion of the plastics sheet and arranged to overlie the resistive element when the first and second sheet portions are folded against one another, fluid interposed between the resistive element and the conductive inner layer and tending to separate same, and a contact pin that is arranged to press on the encapsulating plastics sheet to thereby establish localised contact between the conductive inner layer and the resistive element, and is adjustable to establish different values of electrical resistance.

5. A rheostat comprising an encapsulating resilient plastics sheet comprised of first and second sheet portions hermetically secured together at their edges, a resistive element comprised of a resistive inner layer carried by the first sheet portion of the encapsulating plastics sheet, a first conductive inner layer carried by the first sheet portion of the encapsulating plastics sheet at a location alongside the resistive inner layer thereof, a second conductive inner layer carried by the second sheet portion of the encapsulating plastics sheet anD overlying both the resistive element and the first conductive inner layer, and a contact pin that is arranged to press on the encapsulating plastics sheet to thereby establish localised contact between the second conductive inner layer and both the resistive element and the first conductive inner layer, and is adjustable to establish different values of electrical resistance.

7. A rheostat comprising an encapsulating resilient plastics sheet comprised of first and second sheet portions sealed together at their edges, a resistive element comprised of a resistive inner layer carried by the first sheet portion of the encapsulating plastics sheet, a first conductive inner layer carried by the first sheet portion of the encapsulating plastics sheet at a location alongside the resistive inner layer thereof, a second conductive inner layer carried by the second sheet portion of the encapsulating plastics sheet and overlying the resistive element, first contact means extending from the first conductive inner layer of the encapsulating plastics sheet to the exterior of the encapsulating plastics sheet, and second and third contact means extending respectively from opposite ends of the resistive e element to the exterior of the encapsulating plastics sheet, the second contact means also being connected to the second conductive inner layer, a contact pin that is arranged to press on the encapsulating plastics sheet to thereby establish localised contact between the second conductive inner layer and both the resistive element and the first conductive inner layer, and is adjustable to establish different values of electrical resistance, and a pilot light connected to, and controlled by, the first contact means.