---
title: "High resistivity materials and their uses"
subject: "Electric Machine Design"
chapter: 1
key: "semester_5/electric_machine_design:high-resistivity-materials"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/high-resistivity-materials
site: BCT Engineering
---

# High resistivity materials and their uses

Electric Machine Design · Chapter 1: Materials used in electrical equipment

- Asked in: 2075 Chaitra, 5 marks; 2073 Chaitra, 5 marks; 2070 Asar, 5 marks (asked 3 times)
- Video: [Electrical Conducting Materials used in Machine - Introduction to Machine Design](https://www.youtube.com/watch?v=brjpOF54rw8) — Ekeeda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/high-resistivity-materials
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

Where are the conductors of high resistance used? Classify them according to their purpose.

## 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.

## Answer

**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).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which material is the standard heating-element material and why?
   A. Manganin, because of low temperature coefficient
   B. Nichrome, since it forms a protective chromium oxide layer at red heat
   C. Tungsten, since it is cheap
   D. Constantan, since it is a thermocouple metal
   Answer: B. Nichrome's oxide layer protects it at 1000-1200 °C. (Key point: Heating elements)

2. Which property is most needed for standard resistances?
   A. High melting point
   B. Very low temperature coefficient and stability with time
   C. High arc resistance
   D. Self-lubrication
   Answer: B. Manganin has near-zero temperature coefficient so standards do not drift. (Key point: Precision resistors)

3. Why is tungsten used for lamp filaments?
   A. Very high melting point (about 3400 °C) and low vapour pressure
   B. Lowest resistivity
   C. Forms an oxide layer
   D. Self-lubricating
   Answer: A. The high melting point permits high efficiency in lumen per watt. (Key point: Lamp filaments)

## Related in Electric Machine Design

- [High conducting materials: requirements and classification](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ahigh-conducting-materials) — Electric Machine Design, chapter 1, asked 3×
- [Soft magnetic materials and commercial applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Asoft-magnetic-applications) — Electric Machine Design, chapter 1, asked 2×
- [Insulating materials for different machines](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulator-materials-machines) — Electric Machine Design, chapter 1

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