The pencil lead and the lighter

You need

• From the drawer: the multimeter on Ω, two alligator clips, a lighter, 20 cm of bare 0.25 mm copper wire.
• To buy: mechanical-pencil leads, 0.5 or 0.7 mm, B or 2B, rich in graphite. A 2 mm lead works too; there, the clips weigh as much as the graphite.
• Optional, to see the copper move: a 5 V or automotive relay coil (50 to 400 Ω) and a hair dryer; or an AA battery and a 10 Ω, 1 W resistor.

Do

BeforeThe lead glows red around 600 °C and the clips stay burning hot for a minute: wait. A tile underneath; a red-hot lead can fall. The flame away from the test leads, whose insulation melts, and from the meter. The lead smokes as its wax burns: don’t breathe over it; it sometimes cracks apart: safety glasses. The copper is heated bare, in the yellow of the flame, never the blue: at 1,000 °C, a 0.25 mm wire parts and drips. Ten seconds per pass; the lighter heats up too.

1. Test leads shorted together: 0.2 to 0.5 Ω, to subtract from everything. Both clips on the lead, 5 cm apart; wiggling a clip moves the reading by 0.5 to 2 Ω, the clip-to-graphite contact.

   expected10 to 50 Ω for a 0.5 mm, half that for a 0.7 mm, 2 to 10 Ω for a 2 mm, half of it contact; a wide spread between brands, more for a hard (H) lead, which has more clay. Read again at 1 cm apart: the difference is graphite alone, no contact. Write down R1

2. The flame under the middle of the lead, five to ten seconds, one or two centimeters out of five.

   expectedthe reading drops at once, by 5 to 15% of R1, dancing with the flame; a 12 Ω reads 10 to 11.5 Ω. Write down R2, the lowest. More flame does not go lower: graphite loses 0.05% per degree down to a floor around 600 °C; the lead, fine and disordered graphite, a little more; and only the hot centimeter counts

3. Flame out, one minute.

   expectedthe reading climbs back to R1, within 10%. Higher, a crack in the clay or oxide on the graphite; "OL", the lead has broken

4. The copper wire, 20 cm, between the clips.

   expected0.0 to 0.1 Ω with the leads subtracted, nothing. The wire is 70 mΩ; an ordinary ohmmeter resolves 0.1 Ω

5. The flame under the wire, five seconds, in the yellow.

   expectedstill 0.0 to 0.1 Ω, at best a last digit that flickers. Copper climbs 0.39% per degree; a few centimeters at red heat, and the wire gains 30 to 50 mΩ. It is not the copper that fails to move, it is the instrument

6. To see it anyway. The coil, case open, on the ohmmeter, then two minutes of hair dryer: 30 to 40 °C and 12 to 16% more, a 50 Ω reads 56 to 58 Ω. Or the wire itself, in series with an AA and the 10 Ω, the meter on mV, probes on the wire, not on the clips: 0.15 A through 70 mΩ, 10 to 11 mV; under the flame, 15 to 18 mV.