Sheet resistance is a measure of how easily electric current flows across a material’s surface, reported in ohms per square (Ω/□). It characterizes conductive materials without needing to know the exact thickness. This is useful because material thickness is often non-uniform or difficult to measure precisely, so sheet resistance allows direct comparison of materials and quality control even when thickness is unknown.
The four-point probe technique separates the current-carrying and voltage-sensing functions into different probe pairs. The Jandel RM3 applies a known current through the outer two probes and measures the voltage drop across the inner two. Because virtually no current flows through the voltage-sensing probes, the contact resistance between each probe tip and the sample surface drops out of the measurement entirely — only the resistance of the sample itself is captured.
| Category | Details |
|---|---|
| Instrument | Jandel RM3 Four-Point Probe |
| Technique | Four-point probe — separate current and voltage pairs eliminate contact resistance |
| Measurement | Sheet resistance (Ω/□) |
Each sample was placed on the measurement stage, the four-point probe head lowered onto the surface, and current applied through the outer probes while voltage was measured across the inner two. Multiple points were measured per sample.
| Category | Samples |
|---|---|
| Coins | quarter, penny (unpolished / semi-polished / fully polished) |
| Household metals | stainless steel spoon, aluminum foil, metal washer, house key |
| Biological | leaf |
| Other | DVD, paper cardboard |
Conductive samples (coins, household metals) produced measurable sheet resistance readings. Non-conductive samples (leaf, DVD, paper cardboard) returned Contact Limit or Out of Range at all current settings. The penny was sanded into three bands — untouched copper, lightly polished, and fully sanded to exposed zinc — to compare surface condition effects. Each sample was measured multiple times in both forward and reverse current directions.
Raw data were photographs of the instrument display taken after each measurement. These were manually transcribed into a CSV file. The raw photos are in the PHOTOS directory and the scrubbed CSV is in the OUTPUT directory.
In total 56 valid sheet resistance readings were collected at 9 µA. Three non-conductive samples (leaf, DVD, paper cardboard) returned Contact Limit at all current settings, and one metal washer reading was excluded due to an incorrect current range (20 nA) — the current had been changed while testing insulator samples to see if a different current could produce a detection, and was not reset before measuring the washer.
| Sample | Material | n | Mean (Ω/□) | Range |
|---|---|---|---|---|
| Quarter | Nickel-clad copper | 2 | 37.8 | 37.6–37.9 |
| Spoon | Stainless steel | 7 | 39.0 | 36.6–41.5 |
| Penny (unpolished) | Copper-plated zinc | 3 | 47.3 | 44.5–48.9 |
| Aluminum foil | Aluminum | 5 | 48.1 | 47.6–48.7 |
| Penny (semi-polished) | Copper-plated zinc | 10 | 48.6 | 46.1–51.8 |
| Penny (fully polished) | Copper-plated zinc | 16 | 50.1 | 47.5–55.7 |
| Aluminum foil (flipped) | Aluminum | 3 | 54.7 | 52.8–56.2 |
| Metal washer | Steel | 5 | 54.8 | 53.2–56.2 |
| House key | Brass | 5 | 57.2 | 56.1–58.3 |
The quarter and spoon were the most conductive samples, while the brass house key was the least — all non-metals were too resistive to measure. Two results stood out: sanding the penny from copper through to zinc increased sheet resistance (47.3 → 50.1 Ω/□), and flipping the aluminum foil also increased it (48.1 → 54.7 Ω/□), showing the matte and shiny sides have measurably different surface conductivity.
Readings fluctuated significantly during measurement — the display value drifted continuously and never fully stabilized, even on the same sample without moving the probes. Repeated measurements of the same item produced a wide spread of values, making it difficult to draw firm quantitative conclusions. The broad ranges in the table above reflect this instability rather than true differences between measurement points. Four-point probes are designed for flat, uniform samples with controlled contact pressure, so the irregular and curved surfaces of household objects likely contributed to the variability.
See the static notebook or run the reproducible analysis yourself.
