If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Course: Physics library>Unit 18

Lesson 6: Discovery of resistors

# Variable resistor (pencil)

We use the inside of a pencil (graphite) to apply different "lengths of pencil" to the circuit. What relationship do you observe between length of graphite and deflection angle? Created by Brit Cruise.

## Want to join the conversation?

• That's pretty cool. What other materials could you use to get the same effect?
• in general, most of the conductors possess a linear variation of resistance with length at particular physical conditions..however the proportionality constant varies with any changes made in the physical conditions like temperature, material, etc.
• Okay, so now I know that resistance is proportional to Resistivity*length/area. I was recently reading a Science Olympiad question paper. There was a question that baffled me: If the length of the copper wire is doubled, what will be the effect on its resistance. I should think the answer is 2R. But apparently the answer is 4R. Why?
• This is an interesting question, but it's a trick. This question assumes you stretch the wire, rather than simply double the amount of wire. This decreases the cross sectional area as well. (will need to do these experiments next)
• Why does he use the water cup ? what is the use of it ?
• If you watch previous videos, you'll see why. As a short explanation, he uses the cup of water because it's the water and the zinc of the battery that cause a reaction, which is what is being measured.
(1 vote)
• For how long can you discharge one of those home made batteries?
• From what I observe, the metal clips are the best conductors and provide the highest deflection. As the length of the graphite increases, the deflection decreases. Does this prove a linear relationship between length of conductor and current. Maybe more of the conductor opposes the flow of current?
• yes ! resistance is equal to resistivity ( characteristic property of a material ) times length divided by cross sectional area . here area and resistivity being constant..as we alter the length..due to the linear variation of resistance..we end up getting lesser deflection !
• is this pencilly apparatus also called a potentiometer? and if so are potentiometers just resistors with a variable output?#
(1 vote)
• Is there any other ways you can use graphite for besides using it for the inside of a pencil?
(1 vote)
• I'm sure there are, most likely in factories somewhere. If you want details you could try looking it up on Google.
(1 vote)
• What materials did you use?
(1 vote)
• what is in graphite that makes it do that
(1 vote)
• Why do you use hot water here?
(1 vote)

## Video transcript

(pencil scratching) (water splashing in cup)