7

My peers and I started an experiment to test whether we could use the conductivity of a piece of pencil lead to determine its grade (that is, whether it is 2H, HB, 3B etc.). The paper Observational Models of Graphite Pencil Materials has a helpful list of the composition of various grade pencils, while Doğa Gürgünoğlu has made some relevant measurements of conductivities in the essay Electrical conductivities of different grade lead pencil graphite.

enter image description here

I had assumed conductivity would be linear in graphite composition, since the other constituents like clay and wax have negligible conductivity. However, when I graphed out the data, it seems more like an exponential growth.

enter image description here

Anisotropy of graphite means the conductivity changes drastically depending on its orientation, as discussed here. What else could influence the conductivity of pencil lead? Is there a theoretical explanantion of this trend?

References:

  • M. C. Sousa & J. W. Buchanan - Observational Models of Graphite Pencil Materials
  • Doğa Gürgünoğlu - Electrical conductivities of different grade lead pencil graphite
Jono94
  • 531
  • 1
  • 9
  • 7
    Guessing based on percolation theory, I suspect that at low graphite content there are few "good" pathways available for the current to be conducted through, and it looks relatively insulating. After you reach some critical graphite concentration, the conduction pathways start to connect en masse, and you see rapid increase in conductivity, until it saturates. This gives a roughly sigmoidal curve. A slight issue is that it's somewhat unexpected for the rapid growth to happen at such high graphite concentrations, but that could be a consequence of many real-world parameters. – Nicolau Saker Neto Mar 16 '24 at 12:01
  • 1
    It's not linear (or any specific function like exp) because it's not some true solution. If you cover some chunk of conductor in insulator, it won't conduct at all. Here's some mixing, though, so situation is intermediate. – Mithoron Mar 16 '24 at 14:32
  • Nicely written question and experimental data. BTW, now you can make some hand-drawn electronic circuits, and even draw the current-limiting resistor for an LED (if your instructor appreciates humor, draw it on the lab report., with a coin cell). – DrMoishe Pippik Mar 17 '24 at 01:49
  • @NicolauSakerNeto this sounds very interesting. Any chance you could write it into an answer with more details/additional readings? I'm a physics student, so theory is also welcome. – Jono94 Mar 17 '24 at 07:10
  • @DrMoishePippik by the way, the conductivity data used is not by me, but from a high school essay I found online. – Jono94 Mar 17 '24 at 07:11
  • I'm afraid I can't commit the time to writing an answer, sorry. Hopefully that will get you going in the right direction, though. – Nicolau Saker Neto Mar 17 '24 at 11:15

0 Answers0