## Resistivity Fundamentals
Electrical resistivity (rho) measures opposition to current flow, in microohm-cm. Conductivity is the reciprocal. The IACS standard defines annealed copper as 100%.
## Values Across Engineering Metals
| Metal/Alloy | Resistivity (microohm-cm) | % IACS | Application |
|------------|--------------------------|--------|-------------|
| Silver | 1.59 | 108 | Contacts |
| Copper (C11000) | 1.72 | 100 | Wiring |
| Aluminum (1350) | 2.83 | 62 | Transmission lines |
| Carbon steel | 14-17 | 10-12 | Magnetic circuits |
| 304 Stainless | 72 | 2.4 | Structural |
| Nichrome (Ni80Cr20) | 108 | 1.6 | Heating elements |
| Kanthal A1 (FeCrAl) | 139 | 1.2 | Furnace elements |
## Temperature Coefficient of Resistance (TCR)
Pure metals have high TCR (copper: +3,930 ppm/degree C). Heavily alloyed materials have near-zero TCR (Manganin: +/-15 ppm/degree C), making them ideal for precision resistors.
## Resistance Heating Alloys
**Nichrome**: Maximum element temperature 1150 degrees C. The standard for toasters, kilns, and industrial furnaces.
**Kanthal A1**: Maximum 1400 degrees C. Higher resistivity and better oxidation resistance than nichrome due to alumina scale.
## Conductor Alloys
**C11000 ETP Copper**: 101% IACS. Standard for wiring and bus bars. Excellent conductivity but soft.
**C17200 Beryllium Copper**: 22% IACS but 1280 MPa strength. For contact springs and connectors requiring conductivity plus spring force.
## Superconductivity
NbTi wire (Tc = 9.8 K) is the workhorse of MRI magnets and particle accelerators, drawn into fine filaments embedded in a copper stabilizing matrix.
Electrical Resistivity of Alloys
Electrical resistivity determines suitability for conductors, resistors, or heating elements. It spans three orders of magnitude across engineering metals, from copper at 1.7 microohm-cm to nichrome at 108 microohm-cm.
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