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High resistivity materials and their uses
Short answer
High resistivity materials are used in precision resistors (manganin, near-zero temperature coefficient), rheostats (constantan, cheap), heating elements (nichrome, oxidation-resistant), and filaments (tungsten, high melting point). Classified by purpose: standards, control resistors, heaters, lamp filaments, and contacts.
Where are the conductors of high resistance used? Classify them according to their purpose.
High resistivity materials are metals and alloys with high resistivity (typically 40–140 µΩ·cm, compared with 1.72 µΩ·cm for copper). They are used wherever a definite resistance, heat or light is needed rather than low loss.
Where they are used
- Standard resistances, shunts and multipliers in measuring instruments
- Starters, rheostats, field regulators and load banks of machines
- Heating elements of furnaces, heaters, irons and ovens
- Filaments of incandescent lamps
- Brushes, arcing contacts and carbon resistors
Classification according to purpose
| Purpose | Required properties | Materials |
|---|---|---|
| 1. Precision instruments and standard resistances | Very low temperature coefficient, stable with time, low thermo-emf against copper | Manganin (Cu 84 %, Mn 12 %, Ni 4 %) |
| 2. Rheostats, starters and control resistors | High resistivity, low temperature coefficient, cheap, easily drawn | Constantan/Eureka (Cu 55–60 %, Ni 40–45 %), cast iron grids, nickel-silver |
| 3. Heating elements | Withstand 1000–1200 °C without oxidation, high melting point, high resistivity | Nichrome (Ni 80 %, Cr 20 %), Kanthal (Fe-Cr-Al) |
| 4. Lamp filaments | Very high melting point (about 3400 °C), low vapour pressure, ductile | Tungsten, earlier carbon, tantalum |
| 5. Brushes and contacts | High resistance to arcing, self-lubricating | Carbon, graphite, tungsten, silver-tungsten |
Notes
- Manganin has a temperature coefficient close to zero, so resistance standards do not drift with temperature.
- Constantan has a nearly constant resistance over a wide temperature range. It is also used in thermocouples (with copper or iron) because of its large thermo-emf.
- Nichrome forms a protective chromium oxide layer, so it lasts long at red heat. This makes it the standard heating element material.
- Tungsten is used for filaments because its high melting point allows high efficiency (lumen per watt).
✓ Answer checked by Claude Sonnet 5.5
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