This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
Furnace electrodes frequently consist of carbon rods, and if there is a short gap between them, forming a break in the circuit, the current jumps across that gap, forming an "arc." The intense heat of the arc is used in fusing and melting metals. As large electrodes are necessary for use in furnaces where great masses of metal are melted, so small electrodes are adapted to finer or more delicate work, says Popular Electricity.
Ill: Pencil Electrodes Which Furnish Intense Heat
As the lead or graphite in a lead pencil is a form of carbon, it will make an excellent electrode for small work. Two ordinary lead pencils, costing only one cent each, may be used. They are first sharpened as if they were to be used for the usual purpose of writing. Then a small notch is cut in one side of each pencil, laying the lead bare at a point about 3 in. from the sharpened end.
A small copper wire is wound around the pencil and into this notch, thereby making contact with the exposed lead or graphite. By means of these small wires the pencils are connected to larger wires, which in turn are connected to a switchboard or source of electric-current supply.
At some place in the circuit there should be a resistance to prevent short-circuiting and also to control the strength of the current. As the wood sheath on the pencils offers sufficient insulation, they may be picked up, one in either hand, and no electrical effect will be felt by the person so doing. If the pointed tips are touched together, a fine little arc, not much larger than the tips of the pencils, will be formed. The temperature of this arc, however, is such that fine wires or small quantities of metal may be melted readily.
These little lead-pencil arcs may be used to fuse very small gold or silver wires, or platinum thermometers, or wires for tungsten or tantalum lamps. The bead or globule of molten metal formed on the end of a fine wire need be no longer than a small-sized grain of sand.