Amperage limit of lead pencil resistor
2B or not 2B? 

The 'lead' is a mix of graphite and clay. The harder pencils will have more clay and will probably be higher resistance. If you've got 25 ohms then 1 A will generate \$P = I^2 \cdot R = 1^2 \cdot 25 = 25 W\$ and on a 15 cm pencil that will be \$\frac {25}{15} = 1.6 W/cm\$. You should notice a rise in temperature.

Beware of any funny effects where the resistivity changes with temperature. I can't think of any but there may be some weird temperature coefficient and if it's negative the current may escalate ...

Report back!