High resistive materials:
The materials which have their resistivity 10-8 to 10-6 ohm-m come under this category. These materials are used for making resistance elements for heating devices, starters for electric motors, resistance used in precision measuring instruments, loading resistances and rheostats and filaments for incandescent lamps. In fact, high resistivity materials are used n all such applications where a large value of resistance is required.
These materials should possess the following properties:
(A). Low temperature coefficient: An important property of a high resistivity conducting material should be low temperature coefficient. High resistivity material find applications as shunt in measuring instruments, in wire wound precision resistors and resistance boxes. For such precision applications an important requirement is that the material of the element should have negligible temperature coefficient of resistance otherwise the accuracy of measurements will be reduced.
(B). High melting point: In applications like loading rheostats and starters for electric motors the material of the resistance element should be able to withstand high temperature for a long time without melting. The consideration of high melting point is important also for important materials used in electrical heating devices like room heaters, furnaces etc.
(C). Oxidation resistance: Materials used as high resistance elements in heating appliance should be able to withstand high temperature for a long time without oxidation. This is because if an oxide layer is formed on the heating element the amount of heat radiation will reduced.
(D) Ductility: High resistance material are required in the shape of very thin wires in the case of precision wire-wound resistors and in the shape of thick wires in the case of elements used in ovens, heater, starters etc. High resistance materials to be used for such applications should be therefore be capable of being drawn into wires of different sizes and further be capable of being coiled.
(E). High mechanical strength: High resistivity materials to be used for applications where the wire must be very thin are required to have high tensile strength as otherwise they may break during the drawing of the wire of during the assembly and subsequent operation.