Why graphite carries current better sideways
Each carbon atom in graphite bonds tightly to three neighbors, making a broad, honeycomb-like sheet. The fourth electron is not locked into one bond: it can spread across the sheet, forming a pool of mobile charge. When an electric field is applied, those electrons drift through the layer and carry current.
The sheets themselves are held together much more loosely than the atoms within them. Electrons therefore have fewer easy pathways from one sheet to the next. Graphite is not a perfect electrical wall between layers, but its conductivity is far greater along the sheets than across them. This directional difference is called electrical anisotropy.
A pencil mark shows the layered structure at work
When graphite rubs onto paper, thin flakes shear away from the larger piece because neighboring layers can slide past one another. Those flakes still conduct electricity along their carbon sheets, which is why graphite can serve as a simple electrode or form part of a conductive track. The same structure that lets graphite leave a dark mark also makes its electrical behavior depend on which way the current must travel.
