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.

• Get a resistor.  A one k-ohm resistor would be good.  You can tell it is a one k-ohm resistor by the stripes.  They should be brown-black-red in that order from the end, as shown at the right.
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.

• The first stripe is the most significant digit, X, in XY x 10Z.
• The second stripe is the next digit, Y, in XY x 10Z.
• The third stripe is the exponent in XY x 10Z.
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?

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

P3.   What is the exponent?

P4.   What is the value of the resistor?

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?

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.

• Measure all four values and check that they agree with what the color code says they should be.  Remember, you won't have great accuracy if you are using 10% resistors!  Use the Hydra to measure them.
• Connect two of them in series as shown below.  Use your circuit board as shown below.
• Click here if you need to be convinced that this is really a series combination.
• Next measure the series combination and compare to what you expect.
• Measure at least five different combinations and record in your laboratory book.  Now is the time to start establishing good habits.  Record both your measurements and the values you expected to get from the color code values.
• Compare your results and predictions remembering that you probably have 10% resistors.
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.
• Figure out how to get them in parallel on the circuit board.
• Measure and record values in your lab notebook.
Some Points to Ponder

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

• What happens it you make the center leg longer?
• What happens if you make the center leg wider?
• What happens if you make the pencil markings in the center leg darker?
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.

• Imagine that you are a resistor manufacturer.  As you manufacture resistors, measure them as the come off the line.  Say you're trying to make 1000W resistors.  You know that when you're done you are going to have a whole bunch of resistors and that they are going to range from 800W to 1200W .  That means that they run from 20% low to 20% high.  While that's not bad, you also realize that you can sell 1% resistors for a lot more money.
• So, here's the idea.  As you measure the resistors, select out the ones that are within 1% of the value you want.  Leave the rest in the bin.  Maybe while you're at it you put the ones that don't make 1%, but are still within 5% into another bin.  The result of this process is that you are left with a bunch of resistors that don't include any that are really accurate.  You sell those as ordinary 20% resistors.
• Next, you package the 1% resistors and 5% resistors separately and sell them at a higher price.
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.

• Go to your resistor supply and get twenty (20) resistors of the same size.
• Choose something from 100W to 10,000W.
• Measure and record the values of those resistors and from your measured values come to a conclusion about the claim above.
• If you need to, you can click here to go to the lesson on measuring resistance with an ohmmeter.
• Your report should evaluate the claim that resistors are sold by selecting out the accurate values and selling them separately.
• Your reasoning should be based on the experimental values you obtained from your measurements seasoned with your knowledge of statistics.

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.
• Get some resistors - 10kW, 22kW, 47kW, and connect all series combinations of two resistors (including two the same) and check individual values and series values.
• To connect two resistors in series, use two resistors in a circuit board as shown below.  Notice that the point in common to the two resistors - the node to which they are both connected - is made by connecting to  a group of five points in a single row.  Make sure you understand  the correspondence between the circuit diagram and the actual circuit.

• Is the claim true?
• Can you predict the series resistance more accurately if you take the "stripe value" for the resistance, or is it better if you actually measure the individual resistors?
•  Does the order of the resistances matter?  If you interchange the two resistors does the series resistance change?
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.

• Get some resistors - 10kW, 22kW, 47kW, and connect all parallel combinations of two resistors (including two the same) and check individual values and parallel values.
• To connect two resistors in parallel, use two resistors in a circuit board as shown below.  Notice that points in common to the two resistors - the nodes to which they are both connected - is made by connecting to  a group of five points in a single row.  (Those five points constitute a node.)  Make sure you understand  the correspondence between the circuit diagram and the actual circuit.

• Is the claim true?
• Can you predict the parallel resistance more accurately if you take the "stripe value" for the resistance, or is it better if you actually measure the individual resistors?
• Does the order of the resistances matter?  If you interchange the two resistors does the parallel resistance change?

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.

• The two light bulbs are designed to operate from the same voltage source.  Call the value of that source Vs.
• When operated from the same source voltage, the two light bulbs dissipate different amounts of power.  Call the power values P1 and P2.
• As questions are raised in this lesson, you should record your answers and any supporting experimental observations in your laboratory notebook.  You will need that data and those thoughts to write your report.  Questions that you should answer will always be colored dark blue.
There is one other requirement.
• Your analytical/paper work should be done and handed to your instructor before you begin the experimental work.
• Your analytical work should be done entirely symbolically.  You will not learn values for the bulbs until you have done the theoretical work.
• The first thing you should do is to answer the following questions.
• If the two light bulbs are wired in parallel, which light bulb will give off the most light?
• Since the two light bulbs dissipate different amounts of power when connected to the same voltage, which light bulb will give off the most light?
• Devise an experimental scheme to determine which light bulb has the higher power/wattage rating.
• Now, consider what happens when the light bulbs are wired in series.
• Predict which light bulb will give off the most light when the supply voltage is applied to the series combination.  Is it the higher power or the lower power bulb?
• Enter your argument into your lab book.  It should be written as a convincing argument - i.e. one which would convince someone who knows about power, series resistors, Ohm's Law, etc.
• Now, wire the two light bulbs in parallel.
• Apply the source voltage.
• Determine experimentally which light bulb has the higher power rating. Your instructor will give you the value of the supply voltage you should use.
• Determine experimentally which light bulb gives off the most light.
• Now, connect the bulbs in series.
• Apply the source voltage.
• Determine which bulb gives off the most light.

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.

• You will need a power supply that can produce a variable voltage - up to 25 or 30v.
• As questions are raised in this lesson, you should record your answers and any supporting experimental observations in your laboratory notebook.  You will need that data and those thoughts to write your report.
• Your analytical/paper work should be done and handed to your instructor before you begin the experimental work.
• Your analytical work should be done entirely symbolically.  You will not learn values for the bulbs until you have done the theoretical work.
Now do the following
• Imagine applying a voltage, V1 to two resistors in parallel.  Assume both resistors have the same wattage rating.
• Predict which resistor will get warmer, and record your reasoning in your lab notebook.
• Imagine applying a voltage, V2 to two resistors in series.  Assume both resistors have the same wattage rating.
• Predict which resistor will get warmer, and record your reasoning in your lab notebook.
• Now, hand in your work to your instructor and begin the experimental portion of this exercise.
Now, after you measure both resistors you use,
• Apply 5v to a 100W resistor in parallel with a 150W.  Wait for the two resistors to come to thermal equilibrium.  Wait at least two minutes.
• Do not touch the resistors during this period.  Then, see if you can determine which resistor is warmer.
• Determine if the resistors are operating within their rated range.
• Now, slowly increase the voltage above 5v.  At this point you are putting a voltage across the resistors which will eventually take them beyond their rated wattage.  As you carefully and slowly increase the voltage, the resistor which first emits smoke is the one which is warmest.  Turn off the power supply, and let that resistor cool down to room temperature and measure the resistance value to determine if it has changed value.
Now, using the other two resistors, measure both resistors.  Then
• Apply 12v to a 100W resistor in series with a 150W.  Wait for the two resistors to come to thermal equilibrium.  Wait at least two minutes.  Do not touch the resistors during this period.  Then, experimentally determine which resistor is warmer.
• Determine if the resistors are operating within their rated range.
• Now, slowly increase the voltage above 12v.  At this point you are putting a voltage across the resistors which will eventually take them beyond their rated wattage.  As you carefully and slowly increase the voltage, the resistor which first emits smoke is the one which is warmest.  Turn off the power supply, and let that resistor cool down to room temperature and measure the resistance value to determine if it has changed value.