Measurement of the resistivity of the material of a wire

The resistivity (r)of a material is one of the factors that determines the resistance of that material to the flow of electric current through it. In this experiment you measure the resistance (R) of a length of nichrome wire (length l) using an ohmmeter. You then measure the diameter (d ) of the wire using a micrometer screw guage. The resistance is given by the formula: R = rl / A, where A is the cross-sectional area of the wire (A = p3r squared).

The resistivity can be calculated from the formula: r = RA / l

Procedure:

  1. Drag the moveable contact to some convenient point on the wire, eg 90 or 100 cm mark. Record the resistance and the length of the wire.
  2. Press "Get Micrometer". Before using a micrometer you must check for a zero error and add or subtract this from all subsequent readings.
  3. Press "Check Zero Error". An error between 0.00 and 0.03 will have to be subtracted from later readings.
  4. Press "Get Micrometer" and drag the bottom of the guage to the left hand side of the wire
  5. Press "View Micrometer Scale". Record the diameter of the wire. Drag the guage to a new position on the wire.
  6. Repeat step 5 until you have at least 6 readings
  7. To repeat the experiment press "Reset" and work through the procedure again.

Analysis:

Use the formula given above to calculate the resistivity of nichrome.

Precautions:

  • Ensure that the wire is straight when measuring its length. Measure only between the ohmmeter contacts.
  • Take great care with the micrometer screw guage and the zero error, if present. Take the average of at least six readings as a very small error in the diameter of the wire causes a large error in the resistivity calculated.
  • Press the ohmmeter contacts together and zero the ohmmeter, or else subtract the reading from later readings