Conductors and Insulators
Part of Electrical Theory
The physical and practical differences between conducting and insulating materials, and how to select the right material for each electrical role.
Why This Matters
Every electrical system requires both conductors (to carry current where it is wanted) and insulators (to prevent current from flowing where it is not wanted). The choice of materials for each role determines how efficiently the system operates, how safe it is, how long it lasts, and how much it costs in scarce materials.
In a rebuilding context, the ideal conductor (copper) and the ideal insulator (modern polymer plastics) may be scarce. Understanding what alternatives are available, and what their limitations are, allows electrical work to continue with available materials. A community that can identify local materials that serve as adequate conductors and insulators has far more flexibility in building electrical infrastructure.
This knowledge extends beyond wires: transformer cores, relay contacts, switch housings, and cable supports all involve informed material selection based on conductivity requirements.
The Scale of Conductivity
Electrical materials span an enormous range—more than 20 orders of magnitude from the best conductor to the best insulator:
Conductors (metals): Electrons move freely. Resistivity range 10⁻⁸ to 10⁻⁶ Ω·m.
Semiconductors: Intermediate conductivity, strongly temperature- and impurity-dependent. Resistivity range 10⁻³ to 10⁴ Ω·m.
Insulators: Electrons tightly bound. Resistivity range 10⁶ to 10¹⁶ Ω·m.
Note that the worst conductor (graphite) is still 10 million times better at conducting than the best insulator (quartz glass). These are not adjacent on a spectrum—they are fundamentally different in mechanism.
Electrical Conductors
What makes a good conductor:
- Many free conduction electrons (metals have typically 1 per atom)
- Low electron scattering (determined by crystal purity and regularity)
- Adequate mechanical properties (strong enough to be formed into wire, flexible enough to be routed)
- Corrosion resistance (oxide films on contacts cause high resistance)
- Workability (can be drawn into wire, bent, crimped, soldered)
Conductors ranked by resistivity:
| Material | Resistivity (nΩ·m) | Practical notes |
|---|---|---|
| Silver | 15.9 | Best, too scarce for bulk use |
| Copper | 16.8 | Standard for electrical wiring |
| Gold | 22.1 | Corrosion-resistant, scarce |
| Aluminum | 26.5 | Lighter than copper, adequate |
| Tungsten | 53 | Very high melting point |
| Iron | 97 | Available, corrodes, low conductivity |
| Steel | 100–250 | Stronger than iron, similar conductivity |
| Nichrome | 1,000–1,500 | Used for resistors and heaters |
| Graphite | 3,000–60,000 | Carbon, variable, anisotropic |
Copper: The standard for electrical wiring. Easily drawn to thin wire, easily soldered, adequate strength, modest cost (relative to silver), and excellent corrosion resistance (the green patina on copper is a surface layer, not progressive corrosion like iron rust).
Aluminum: 60% of copper’s conductivity but 30% of copper’s weight. Requires 1.6× the cross-section of copper for equal current capacity. Oxidizes more readily than copper—the aluminum oxide film has very high resistance, requiring special treatment at connections. Do not simply twist aluminum and copper together—galvanic corrosion and the oxide film will destroy the connection over months to years.
Iron and steel: 12% of copper’s conductivity. Corrodes rapidly in humid conditions. Adequate as a conductor only for rough work at short distances. Iron wire was historically used for telegraph lines where cost was more important than efficiency.
Carbon (graphite): Used as brush contacts in motors and generators (where its lubricity is also valuable), as electrodes in arc lamps and batteries, and as resistance elements. Anisotropic—conductivity along graphite planes is much better than perpendicular to them.
Electrical Insulators
What makes a good insulator:
- No free electrons (or very few—bound electrons only)
- High band gap (large energy barrier preventing electrons from reaching conduction band)
- High dielectric strength (withstands high voltage before breakdown)
- Low moisture absorption (water dramatically reduces insulation resistance)
- Adequate mechanical and thermal properties for the application
Insulators ranked by dielectric strength and practical use:
| Material | Dielectric strength (kV/mm) | Notes |
|---|---|---|
| Air (dry) | 3 | Readily available, no material cost |
| Glass | 10–30 | Brittle, excellent electrical properties |
| Mica | 40–200 | Very stable, used in capacitors |
| Natural rubber | 20–30 | Flexible, good for wire insulation |
| Ceramic | 10–20 | Hard, excellent at high temperature |
| Paper (oiled) | 8–15 | Used historically in transformers |
| Wood (dry) | 1–5 | Variable, degrades in moisture |
| Shellac | 5–15 | Varnish for coil winding |
Air: The first choice for high-voltage outdoor insulation—overhead power lines use air as the primary insulator between conductors, with ceramic or glass insulators only at the support points. Air requires distance between conductors proportional to voltage: approximately 1mm per 3kV for reliable insulation.
Glass: Excellent dielectric properties, stable over long periods, unaffected by most chemicals. Brittle and difficult to shape except in molten form. The classic material for overhead line insulators (the familiar bell-shaped ceramic or glass insulators on telegraph and power poles). Can be fabricated locally from sand and lime.
Ceramic: Similar to glass but can withstand higher temperatures and mechanical stress. Clay-fired ceramics can be made locally. Porcelain (kaolin + quartz + feldspar, fired at high temperature) is the traditional material for power-line insulators, switchgear, and spark plugs.
Natural rubber: Excellent flexible insulation for wire, historically the standard before synthetic plastics. Derived from Hevea tree latex. Vulcanized with sulfur to improve durability. Rated to several hundred volts; multiple layers to 1,000V and above. Degrades with UV exposure and ozone—must be protected from sunlight in outdoor applications.
Shellac: Insulating varnish made from lac resin dissolved in alcohol. Applied by dipping or brushing, dries to a hard, water-resistant film. Used for coil winding (each layer insulated with shellac before winding the next), for impregnating insulation on components, and for general low-voltage insulation.
Waxed paper and oiled cloth: Traditional transformer and capacitor insulation. Paper saturated with mineral oil or linseed oil has good dielectric properties. Adequate for protected, dry indoor use. Degrades if water ingress occurs.
Semiconductors
Between conductors and insulators lies a third category with unique behavior: semiconductors. Pure silicon and germanium are insulators at low temperature but conductors at room temperature, and their conductivity can be dramatically altered by controlled impurity addition (doping).
N-type doping (donor impurities): Adding phosphorus to silicon introduces extra electrons that are free to conduct. The material is now a conductor with electrons as carriers.
P-type doping (acceptor impurities): Adding boron to silicon creates “holes” (missing electrons) that move through the material when current flows. The material is now a conductor with holes as carriers.
The interface between N-type and P-type material is a diode—current flows easily in one direction (electrons to holes) but is blocked in the other. This junction is the basis of all semiconductor electronics.
For a rebuilding community without semiconductor manufacturing capability, semiconductor devices must be salvaged from existing electronics. However, selenium and copper oxide rectifiers—primitive semiconductors—can be fabricated with basic chemistry and metalworking.
Matching Material to Application
Selection criteria:
| Application | Key requirement | Best materials |
|---|---|---|
| Power wiring | Low resistivity, workable | Copper (first), aluminum (second) |
| Heating elements | High resistivity, temperature stable | Nichrome, iron-chromium alloy |
| Contact surfaces | Low contact resistance, oxidation resistant | Copper, silver, gold |
| Wire insulation | Flexible, high dielectric strength | Rubber, PVC (salvaged) |
| High-temperature insulation | Survives heat | Ceramic, glass, mica |
| Transformer cores | High permeability, low eddy losses | Laminated soft iron |
| Arc contacts | Withstands arc erosion, conducts | Carbon, tungsten, copper-tungsten |
Understanding the reasoning behind each material choice allows substitution when the ideal material is unavailable: the key is to understand what property the material is providing, then find an alternative that provides the same property adequately for the specific application.