MadSci Network: Engineering

Re: How do variables and fixed resistors work?

Date: Sun Apr 11 11:26:10 2004
Posted By: Lawrence Skarin, Rochester Museum and Science Center Technical Assistance Group
Area of science: Engineering
ID: 1081356954.Eg

Resistance is measured in ohms.  The most common resistor is called "carbon 
composition" where a material much like pencil lead carries the electric 

A fixed resistor has only one value -- like 500 ohms.

A variable resistor (also called a potentiometer) can vary the resistance 
from 0 to a maximum -- like 500 ohms.  See:

You can make a demonstration resistor yourself.  You will need:

 an ohmmeter (VOM or DMM)
 a #2 pencil
 a piece of paper.  Heavy bond is good.

1.  Draw a rectangle, 3 inches long and 1/8 inch wide.  Blacken it in 
completely so the surface looks shiny.  You want the pencil lead to build 
up thickly on the paper.  This is so it can carry electric current.

2.  Set ohmmeter to a high range (2M on DMM; Rx1M on VOM).

3.  Check ohmmeter lead wires by touching tips together.  Ohmmeter should 
read 0.

4.  Place ohmmeter tips at ends of rectangle.  (Don't get your fingers on 
the tip -- it's not dangerous, but will cause incorrect readings)  Expect 
to read about 500,000 ohms.  This is a fixed resistor.

5.  Keep one tip in place at rectangle end.  Slide other tip along the 
black rectangle.  Watch resistance reading change to lower.  You have made 
a variable resistor.  The moving tip represents a "wiper."

When you buy a variable resistor, one of the terminals is built to move 
along the resistance material.  This is the wiper terminal.

One question -- If you wanted to change the value of your fixed resistor 
to a lower value, would you blacken in more?  Or would you try to lightly 
rub some of the pencil lead off the rectangle with an eraser?

And if you like competition, see which of you and your colleagues can make
a pencil lead resistor closest to 1,000,000 ohms.


Larry Skarin

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