PASCO EM-8812 Resistance Apparatus - Manual (Page 11)

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M o d e l N o . E M - 8 8 1 2 E x p e r i m e n t 2 : R e s i s t a n c e v e r s u s L e n g th
11
How to Measure Resistance
In this experiment, you will make several resistance measurements for various
lengths and diameters of wire. Use this method to measure R:
Note: These instructions assume that you have set up the galvanometers and power supply as
detailed above.
1. Set the reference and slider probes for the desired value of .
2. On the power supply, press to start the applied voltage ramp. At the same
time, click Start in DataStudio (or press on the GLX) to start data collection.
3. Watch the voltage reading on the power supply. Just before it reaches its maxi-
mum value (which you set in setup step 8), click Stop in DataStudio (or press
on the GLX) to stop data collection.
4. On the power supply, press and hold to turn off the applied voltage ramp.
5. In DataStudio (or on the GLX), open a Graph display. For the vertical axis, select
Voltage in units of mV. (Make sure that this is the voltage measured by the galva-
nometer connected to the reference and slider probes, not the current-sensing gal-
vanometer.) For the horizontal axis, select current (the calculation you defined
in setup step 7).
6. Apply a linear fit to the V versus I data. The slope equals R measured in m Ω.
To make another measurement of R (for a different length or a different wire, for
instance), repeat the steps above. However, you do not need to repeat step 5 because
the new data will appear on the graph that you set up previously.
Procedure
1. With largest brass wire, measure the resistance for lengths of 24 cm, 20 cm,
16 cm, 12 cm, 8 cm, and 4 cm. (See “How to Measure Resistance” above.)
2. Make a graph of R versus .
3. Apply a linear fit to the graph.
4. Use the slope of the line, the cross-sectional area of the wire, and Equation 2-2 to
calculate
ρ.
5. Repeat steps 1 through 4 for the other three diameters of brass wire.
Questions
How do the values of
ρ for the four brass wires compare to each other? How does
your average value of ρ compare to the accepted value?
Further Study
Repeat the procedure to find the resistivities of the copper, aluminum, nichrome, and
stainless steel wires.