Vol. DIY Electronics Projects
Chapter 2 Basic Projects and Test Equipment

# Intro Lab - How to Use an Ohmmeter to Measure Resistance

## In this hands-on introduction to electronics, you will experiment with a multimeter and learn how to measure resistance using the ohmmeter function.

### Project Overview

In this project, you will learn how to use an ohmmeter to measure resistance. Typically, the ohmmeter is one of the functions of a multimeter, which is an electrical instrument capable of measuring voltage, current, and resistance (Figure 1).

##### Figure 1. Digital and analog multimeters with test probes connected for measuring resistance.

This experiment describes how to measure the electrical resistance of different objects. Resistance is the measure of electrical “friction” as charges move through a conductor. It is measured in the unit of ohm, that unit symbolized by the capital Greek letter omega (Ω).

### Parts and Materials

You do not need to have all of the items listed above in order to effectively learn about resistance. Conversely, you need not limit your experiments to these items (Figure 2).

### Learning Objectives

• Determination and comprehension of “electrical continuity”
• Determination and comprehension of “electrically common points”
• How to measure resistance
• Characteristics of resistance: existing between two points
• Selection of proper meter range
• Relative conductivity of various components and materials

### Instructions

WARNING: Never measure the resistance of any electrically “live” object or circuit. In other words, do not attempt to measure the resistance of a battery or any other source of substantial voltage using a multimeter set to the resistance (“ohms”) function. Failing to heed this warning will likely result in meter damage and, possibly, even personal injury.

#### Calibrating an Ohmmeter for 0 Ω of Resistance

Step 1: Set your multimeter to the highest resistance range available. As previously mentioned, the resistance function is usually denoted by the unit symbol for resistance: the Greek letter omega (Ω) or sometimes by the word ohms.

Step 2: Next, touch the two test probes of your meter together. When you do, the meter should register 0 Ω of resistance.

If you are using an analog meter, you will notice the needle deflect full-scale when the probes are touched together and return to its resting position when the probes are pulled apart. The resistance scale on an analog multimeter is reverse-printed from the other scales. Zero resistance is indicated at the far right-hand side of the scale, and infinite resistance is indicated at the far left-hand side.

Step 3: If you use an analog ohmmeter, there should also be a small adjustment knob or wheel on the analog multimeter to calibrate it for 0 Ω of resistance. Touch the test probes together and move this adjustment until the needle points exactly to zero at the right-hand end of the scale.

#### Using an Ohmmeter to Test Circuit Continuity

Although your multimeter can provide quantitative values of measured resistance, it is also useful for qualitative tests of continuity: whether or not there is a continuous electrical connection from one point to another.

Step 4: Test the continuity of a piece of wire by connecting the meter probes to opposite ends of the wire and check to see if the needle moves full-scale. What would we say about a piece of wire if the ohmmeter needle didn’t move at all when the probes were connected to the opposite ends?

Digital multimeters set to the resistance mode indicate non-continuity by displaying some non-numerical indication on the display. Some models say “OL” (open-loop), while others display dashed lines.

Step 5: Use your meter to determine the continuity between the holes on a breadboard, as illustrated in Figures 3 and 4.

##### Figure 4. Using an ohmmeter to test for continuity between horizontal sets of holes on a breadboard.

A breadboard is a device used for the temporary construction of circuits, where component terminals are inserted into holes on a plastic grid. Metal spring clips underneath each hole connect certain holes to others. Use small pieces of 22-gauge solid copper wire inserted into the breadboard's holes to connect the meter to these spring clips so that you can test for continuity.

#### Continuity and Commonality

An important concept in electricity, closely related to electrical continuity, is that of points being electrically common to each other. Electrically common points are points of contact on a device or in a circuit that has negligible (extremely small) resistance between them.

Based upon the test results from Step 5, we could say, then, that points within a breadboard column (vertical in the illustrations) are electrically common to each other because there is electrical continuity between them. Conversely, breadboard points within a row (horizontal in the illustrations) are not electrically common because there is no continuity between them.

Continuity describes what is between points of contact, while commonality describes how the points themselves relate to each other. Like continuity, the commonality is a qualitative assessment based on comparing resistance between other points in a circuit. It is an important concept to grasp because there are certain facts regarding voltage in relation to electrically common points that are valuable in circuit analysis and troubleshooting, the first one being that there will never be substantial voltage dropped between points that are electrically common to each other.

#### How to Measure a Resistor’s Resistance

Step 6: Select a 10,000 Ω (10 kΩ) resistor from your parts assortment. This resistance value is indicated by a series of color bands.

For a 10 kΩ resistor, the color bands will be brown, black, orange, and then another color representing the precision of the resistor, gold (+/- 5%) or silver (+/- 10%). Some resistors have no color for precision, which mark them as +/- 20%. Other resistors use five color bands to denote their value and precision, in which case the colors for a 10 kΩ resistor will be brown, black, black, red, and a fifth color for precision.

The All About Circuits Resistor Color Code Calculator is an extremely helpful tool to bookmark for determining the value of a resistor based on its color code.

Step 7: Connect the meter’s test probes across the resistor, as shown in Figure 5, and note its indication on the resistance scale.

##### Figure 5. Using an ohmmeter to measure the resistance of a resistor.

If the needle points very close to zero, you need to select a lower resistance range on the meter, just as you need to select an appropriate voltage range when reading the voltage of a battery.

If you are using a digital multimeter, you should see a numerical figure close to 10 shown on the display, with a small “k” symbol on the right-hand side denoting the metric prefix for “kilo” (thousand). Some digital meters are manually-ranged and require appropriate range selection, just like the analog meter. If yours is like this, experiment with different range switch positions and see which one gives you the best indication.

Step 8: Try reversing the test probe connections on the resistor. Does this change the meter’s indication at all? What does this tell us about the resistance of a resistor?

Step 9: What happens when you only touch one probe to the resistor? What does this tell us about the nature of resistance and how it is measured? How does this compare with voltage measurement, and what happened when we tried to measure battery voltage by touching only one probe to the battery?

When you touch the meter probes to the resistor terminals, try not to touch both probe tips to your fingers. If you do, you will be measuring the parallel combination of the resistor and your own body, which will tend to make the meter indication lower than it should be! When measuring a 10 kΩ resistor, this error will be minimal, but it may be more severe when measuring other resistor values.

Step 10: You may safely measure the resistance of your own body by holding one probe tip with the fingers of one hand and the other probe tip with the fingers of the other hand.

Note: be very careful with the probes, as they are often sharpened to a needle-point. Hold the probe tips along their length, not at the very points! You may need to adjust the meter range again after measuring the 10 kΩ resistor, as your body resistance tends to be greater than 10,000 ohms hand-to-hand.

Step 11: Try wetting your fingers with water and re-measuring resistance with the meter. What impact does this have on the indication?

Step 12: Try wetting your fingers with saltwater prepared using a glass of water and table salt and re-measuring resistance. What impact does this have on your body’s resistance as measured by the meter?

Resistance is the measure of friction to charge flow through an object. The more resistance there is between two points, the harder it is for charges to move (flow) between those two points. Given that electric shock is caused by a large flow of charges through a person’s body, and increased body resistance acts as a safeguard by making it more difficult for charges to flow through us, what can we ascertain about electrical safety from the resistance readings obtained with wet fingers? Does water increase or decrease shock hazards to people?

#### Measuring the Resistance of other Materials

Step 12: Measure the resistance of a rectifying diode with an analog meter. Try reversing the test probe connections to the diode and re-measure resistance. What strikes you as being remarkable about the diode, especially in contrast to the resistor?

Step 13: Take a piece of paper and draw a very heavy black mark on it with a pencil (not a pen!). Measure resistance on the black strip with your meter, placing the probe tips at each end of the mark, as illustrated in Figure 6.

##### Figure 6. Using an ohmmeter to measure the resistance of a pencil mark.

Step 14: Move the probe tips closer together on the black mark and note the change in resistance value. Does it increase or decrease with decreased probe spacing?

If the results are inconsistent, you need to redraw the mark with more and heavier pencil strokes so that it is consistent in its density. What does this teach you about resistance versus the length of a conductive material?

Step 15: Connect your meter to the terminals of a cadmium-sulfide (CdS) photocell, as illustrated in Figure 7, and measure the change in resistance created by differences in light exposure.

##### Figure 7. Using an ohmmeter to measure the changing resistance of a photocell

Just as with the LED of the voltmeter experiment, you may want to use alligator-clip jumper wires to make a connection with the component, leaving your hands free to hold the photocell to a light source and/or change meter ranges.

Step 16: Experiment with measuring the resistance of several different types of materials, be sure not to try to measure anything that produces substantial voltage, like a battery. Suggestions for materials to measure are:

• Fabric
• Plastic
• Wood
• Metal
• Clean water
• Dirty water
• Salt water
• Glass
• Diamond (on a diamond ring or other piece of jewelry)
• Paper
• Rubber
• Oil

### Related Content

Textbook:

Worksheets:

#### Lessons in Electric Circuits

Published under the terms and conditions of the Design Science License