Experiments With Resistors
Measuring Resistors
The Resistor Color Code
A Simple Resistor Experiment
Measuring Resistor Accuracy
Resistor Combinations
Series Resistors
Parallel Resistors
The Two-Light-Bulb Experiment
The Two-Resistor Experiment

Measuring Resistances

        Let's start by making one resistance measurement.

        You are now ready to measure the resistance.  Connect your resistor to an ohmmeter as shown at the right.  Don't get uptight about which lead goes on which end of the resistor.  It doesn't matter.  (The resistor is  a bilateral element and should be the same either way!)  You should see a reading in the window, and that reading should be in the vicinity of one thousand ohms.  Click here for the lesson on setting up an ohmmeter.  Here are the  connections you make.  They're simple enough.

        Take the measurement now!  Then reverse the leads to check the claim above by measuring the resistance with the opposite lead connections.


The Resistor Color Code

       You probably wondered about those stripes on the resistor.  There is a color code that lets you tell what value the resistor has.  Here's what's important.

                                       This resistor is 1000 ohms = 10x102

        Notice how cleverly we put certain  parts of the result in bold text and colored it.  The bolded text corresponds to the stripes, and the colors are shown on the bold text.  Here is the color code.
 

0 Black Black
1 Brown Brown
2 Red Red
3 Orange Orange
4 Yellow Yellow
5 Green Green
6 Blue Blue
7 Violet Violet
8 Gray Gray
9 White White

        Given a resistor, to calculate the value of the resistance you use the three stripes.  (If there are four stripes, just use the first three.  The last stripe tells you how accurate the resistance value is.)  Here is the algorithm.

Here is an example/problem.

Problems

P1.   For the resistor at the right answer the following questions.  Note, for this problem, if you do not get a grade of 100 you answered incorrectly.  Now, what is the first digit on the resistor's set of stripes?

Enter your answer in the box below, then click the button to submit your answer.

Your grade is:

P2.   What is the second digit on the resistor's set of stripes?

Enter your answer in the box below, then click the button to submit your answer.

Your grade is:

P3.   What is the exponent?

Enter your answer in the box below, then click the button to submit your answer.

Your grade is:

P4.   What is the value of the resistor?

Enter your answer in the box below, then click the button to submit your answer.

Your grade is:


A Simple Resistor Experiment

        Any conducting material can be used to make a resistor.  Any metal or metallic alloy can be used.  Other conducting materials, like carbon, germanium or silicon can be used, even if the material does not conduct as well as a metal.  In this exercise you are going to construct some resistors made of carbon.

        First, you are going to have to locate some carbon.  Do you have any on you?  Were you able to locate any carbon?  You may not have realized that you have some carbon if you are carrying a pencil.

      The carbon you will use is the carbon found in a pencil lead.  On a clean sheet of paper draw a shape like the one below.  It can be smaller but drawn to scale.  The two "pads" at the end of the structure are larger to permit you to to make connection to your resistor with the ohmmeter leads.  Be sure that your resistor is filled in completely, and when you are done measure the resistance.  Then click on the Enter Resistance button to show this value.  Record your measured value.  You will need to use it later in this lesson.

        You have just constructed and measured a resistor.  There is a special symbol for
 a resistor that you need to become familiar with.  Here it  is.

The diagonal lines are intended to suggest some resistance to the flow of current.  This symbol can be used in any orientation, and you will often see this symbol rotated as shown at the right.

1. When you measured your resistor the ohmmeter that you used actually applied a small voltage across your resistor and current flowed through it.  Often symbols are attached to the resistor symbol to indicate how the voltage is applied across the resistor and to define a positive direction for current flow.  In a case where both a current and a voltage symbol were used the situation would look like what we have shown above.  (If you need to review voltage or current, click the hotwords.

        These symbols define polarity for voltage.  When the voltage across the resistor, V, is positive as shown, then positive current will flow from through the resistorin the direction indicated by the current arrow.

V = I R

        OK!  You have measured your resistance and you've learned a little about resistors.  Next, you are going to add a second resistor in series with your original resistor.  Draw in the second resistor in a pattern like the one below.  One of the pads on the first resistor is used as part of the second.  You are going to make a few measurements, but first you need to answer a few questions.


Q1.  What will the resistance of the new resistor be?


        The second resistor should be the same physical size as the original resistor.  Since it is the same size and made of the same material it should have the same resistance.   There are physical reasons why that is so, and there is a mathematical expression that relates the resistance to length, cross sectional area and a property of the material called resistivity.  For resistors that have a constant cross section, A, and a lenght, L, the resistance is:

R = rL/A

        For now, however, you should expect the same physical structure to give the same resistance.

        Keeping in mind that R = rL/A, answer this question.


Q2.  If the length of a resistor is doubled, how is the value of the resistor changed?



OK!  Now to the next question.


Q3.  What value do you expect for the series combination of two resistors that you have made?



 Q4.  Where ( which points of X, Y and Z ) should you connect your ohmmeter leads to measure the second resistance?


Q5.  Where ( which points of X, Y and Z ) should you connect your ohmmeter leads to measure the series resistance?



        Now you're ready to measure values for your second resistor and the series combination.  Go ahead and measure your second resistor.

        Is your measured value close to what you expected?  You should have written that down so you can check that against your new value..  Make sure, at this point, that everything is going as expected.  If your second resistance measurement was not what you expected, maybe you need some art lessons.  Maybe your connection wasn't firm enough.

        Now that you have the two resistors measured, go ahead and measure the series combination.  When you have that measurement taken and you think you are close enough to the correct measured value you can go on.

        OK!  Now you're going to measure some resistors that aren't home-made. Get at least four resistors, all of different values.

Finally, as a test of what you've learned, measure the same five combinations in parallel.  When two resistors are in parallel, the same voltge appears across both resistors. Some Points to Ponder

        When you construct your resistor, you should be able to answer the following questions.

You can check the answers to these questions experimentally.  Do that now.

Resistor Accuracy - Lab Problem

        Are you a skeptical, maybe cynical, person?  Then consider this idea.

        Now, the question is "Is the above process what actually happens?".  In this laboratory problem you will test that idea and gain some practice measuring resistors in the process.  (You can find more discussion of this idea in "Electrical Engineering Uncovered" by White & Doering (Prentice Hall, 1997) in Chapter 4.)

        Here's what you should do.


Resistor Accuracy

       Here's what you're trying to get from this lab problem.

Given that you need a resistor of some nonstandard size,
To know what to expect when you use a resistor from a bin of standard resistors and use them in series and parallel combinations.
        Resistors in series should have an equivalent that has a resistance that is the sum of the values of the resistors in series.  In this lab problem you will check that theoretical claim, and you will get some practice wiring series elements.  Here are the goals for this problem
Given that you need to put two resistors in series or parallel,
Be able to predict the equivalent value with confidence,
Be able to make the physical connection in series on a circuit board.
Here is what you should do.

Your report should answer the following questions.

        Resistors in parallel should have an equivalent that has a resistance that is computed using a standard formula.  In this lab problem you will check that theoretical claim, and you will get some practice wiring parallel elements.

        Here is what you should do.

        Your report should answer the following questions.


The Two-Light-Bulb Problem - A Lab Problem

        This problem requires two dissimilar light bulbs.  Example light bulbs are shown in the photo below.  If you look at the light bulbs closely they are not the same.  Here's what you should note.

        There is one other requirement.
The Two-Resistor Problem

        This problem requires two resistors.
Use four 1/4 watt resistors.  You will have two resistors of one value, Ra, and two resistors of another value, Rb.

        Now do the following         Now, after you measure both resistors you use, Now, using the other two resistors, measure both resistors.  Then