I.   MEASURING RESISTANCE

Purpose:  In this lab you will learn to construct a circuit and measure voltage, current, and resistance using a VOM and a DMM. When using meters to measure values, you want to learn to use the meter with confidence. You will study the relation called Ohms' Law and you will draw a graph and interpret it.  

Equipment List:     
                Simpson VOM
                hand-held DMM
                HP-DMM
                DC power supply (for part 2 only)
                Five carbon-type resistors:
                    from about 50 to 100,000 ohms
                Assorted banana cables

Introduction:  Typical carbon type resistors have a color code on their body that you must learn to interpret. Learn how to read the resistor's value and its "tolerance" from the code below.

0    Black            Tolerance (i.e. nominal uncertainty)
1    Brown              Nothing  ± 20% (+ or - nominal value)
2    Red                  Silver   ± 10%
3    Orange             Gold     ±  5%
4    Yellow              Red      ±  2%
5    Green
6    Blue
7    Violet
8    Grey
9    White

Hold the resistor so the bands of color are located on the left side of the resistor; the first and second colors give the two significant figures of the resistor's value (no decimal point between them). The third color from the left gives the power of ten. The fourth color (if it exists) is the tolerance value.

Procedure:  
1.  Determining resistance by direct measurement:  There are no voltages from power supplies and no currents to be measured in this part. Measure the resistance of all five resistors with the VOM, hand-held DMM, and HP DMM (use 2 wire mode). Perform a discrepancy test against each measurement compared to the nominal value stamped with the color code on the resistor. State whether the measured value lies within the most probable range determined from the tolerance stamped on the resistor. Comment on the accuracy of the three measuring instruments used. What range setting on your instrument gives the most precise reading?

    Just for curiosity, measure your own resistance, from one hand to the other; examine how your resistance can change as you change the tightness of your grip or as your hands become moist. Record the values and variations of your body's resistance in your lab book.

2.  Determining resistance by calculating a slope from measured data:  Take one of your resistors and construct a circuit based on the schematic and pictorial diagram below. Construct the circuit first and then connect your meters. Use the HP DMM for the current readings and the hand-held DMM for the voltage. Prepare your lab book so that you can record your data and draw your graph at the same time, point by point.  




Take your data by starting at a low voltage value (e.g. less than five volts) and record the voltage across the resistor and the current through the resistor. Increasing the voltage each time, repeat your measurements four more times for a total of five different pairs of voltage and current readings. Each successive data point will represent an increase in voltage and current readings. Plot voltage on the horizontal and current on the vertical axis; this graph is called a "characteristic curve" of the resistor.

Draw an "eyeballed" straight line using your ruler that best represents the line formed by the data points.

Don't use the origin as a data point. Don't connect one point to the next point, draw one straight line. Don't "force" the line to intersect as many points as possible; the best fit line may not intersect any data points.

Analysis:

Use the data points to determine the line, then use the line, not the data points, to determine the slope. Choose two points on your drawn line (not data points and outside the range of the data points) that span a large part of your graph. Calculate the resistor's value by applying Ohms' law and understanding how the slope of the graph is related to the resistance.  

Also, have a computer graph the data and calculate a least squares best slope value to yield a more accurate value for the resistor. Compare the mathematical best slope from the computer (using linear regression) to your eyeball best slope from the hand drawn graph in your lab book. Compare your resistor's value from the computer graph to the resistor's value from the three measurements of the same resistor as performed in part 1. Neatly tape the computer graph in your lab book.

Conclusion:
Discuss the agreement of all your resistor's values with one another.