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.

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.

        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?

        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?

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

        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.

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?