Resistivity of pencils

[houlahound]

forgive me if this is too chemistry.

I am playing around with pencils, writing and art pencils, to teach some kids about resistance and resistivity in a fun way, or so I believe.

I get about 200 Ohms for a standard full length HB (whatever that means) pencil.

I have some charcoal art pencils and a range of "HX" or "YH" writing pencils where X & Y are a number or letters.

my DMM is dead right now but hope to see a measurable difference when "H" is varied.

anyone know;

a) how the _H_ labelling system works and what it means. I figured which ones are dark and light and observe a different hardness but what is the logic of the labelling system.

b) how _H_ value relates to the atomic structure of the pencil in order to explain the different resistivity's. obviously theory describing conduction electrons in metals and semi-conductors is useless here.

c) are there other systems than _H_

how is graphite related to charcoal properties wise.

I did get a data sheet from a pencil manufacturer but it zero info on micro-structure.

thanks any ideas.

 

[andrewkirk]

The H vs B system indicates where the pencil sits on a hardness dimension. Softer pencils write more easily, draw darker, wider lines, and need sharpening more often.

The medium designation is HB.
Softer (harder) pencils are labelled nB (nH) where n is a positive integer that increases with the softness (hardness). 1B (1H) is just written as B (H).

 

[houlahound]

Gotcha, thanks. What in the composition caused the hardness to change, more carbon?

 

[andrewkirk]

You'd need a chemist to answer that, and my chemistry is thin and very old. All I know is that graphite is a form of carbon in which the atoms are bonded in sheets, so they slide off one another easily. Maybe the sheets have some bonds between them, and the harder forms of graphite have more such bonds. And maybe there is some preparation process that controls the profusion of such bonds.

But this is all guesswork. We need somebody to turn up that knows what they're talking about. I know a lot about pencils (I love pencils. I'm very anti-pen!) but almost nothing about their chemistry or construction.

 

[anorlunda]

Pure graphite is very conductive, so it surprised me when you measured 200 ohms. I think the explanation is that the pencil core is not pure. I found this:

https://en.wikipedia.org/wiki/Pencil

Most pencil cores are made of graphite mixed with a clay binder which leaves grey or black marks that can be easily erased. Graphite pencils are used for both writing and drawing and result in durable markings: though writing is easily removable with an eraser, it is otherwise resistant to moisture, most chemicals, ultraviolet radiation, and natural aging. Other types of pencil core are less widely used, such as charcoal pencils, which are mainly used by artists for drawing and sketching.

https://en.wikipedia.org/wiki/Pencil#Grading_and_classification
Explains the H scale.

In many cases, it is easier to find answers to simple questions on Wikipedia than by posting questions on forums.

 

[andrewkirk]

In many cases, it is easier to find answers to simple questions on Wikipedia than by posting questions on forums.

Yes, but then one misses out on the delightful opportunity for human contact (with people like US!) that asking questions on physicsforums provides.

 

[houlahound]

the above post may have been posted in jest but it is actually true for me.

Socially I have some really nice people around me but to be honest I have nothing really to talk to them about. this weekend i went on a wine tasting tour of vineyards and everyone was all about posting pics of their meal to their facebook page and commenting on the wine while I am thinking about how conductivity is determined by the micro-structure in the H/B pencil series and what wins out of emissivity of paint versus thermal conduction over the paint layer on a heat sink.

so sadly I concede that the PF plays a valid and important social and intellectual role in my life.

 

[robphy]

I'm actually introducing this as one activity in my intro lab for next week.

Possibly useful:
https://www.qcaa.qld.edu.au/downloads/senior/snr_physics_07_sai_electric_conduct_graphite.pdf
https://www.efxkits.us/pencil-resistors-working-with-simple-project/
http://iopscience.iop.org/article/10.1088/0031-9120/38/2/401
http://www.pencilpages.com/articles/thickness.doc

 

[houlahound]

thanks links, will check them out.

 

[Merlin3189]

Funnily enough I was recently having the conversation with my grandson about lead pencils not being lead, but a mixture of clay and graphite and graphite being called that because it can be used to draw pictures. (We were soldering and when I said it contained lead, he thought it was the same as a pencil.)
I haven't tested a range of pencils myself, but I would expect the resistance to increase from 7B to 7H with HB somewhere in the middle. The conductive element is graphite and the binder is probably an insulator. The more B you have, the more graphite in the mix. The more H, the less graphite. Since mine smoke when they get hot, I suspect there is something other than pure graphite and clay in the mix - maybe some organic glue.
Since this is more or less the same recipe as used for carbon composition resistors (apparently not so common now, but I still have loads of them) I'd expect you could find a wide range of resistance with the varying composition pencils.

I've never tested charcoal nor charcoal pencils and thinking about it, I realize I don't even know what charcoal is! I think I'll have to investigate that myself. (According to some web pages I've found, charcoal is a good conductor.)