MMT. Tutorial on Using a Multimeter (version 1)
Use this version of the tutorial if you have the meter shown below. Otherwise, click here.
Record your responses in the MMT WebAssign form. Follow the instructions below.
Read textbook sections 21.1-3 and complete E.21.1 in advance of doing this tutorial.
A multimeter is a device for measuring currents and potential differences (sometimes termed voltages for short), both AC and DC, and for measuring resistance. A photo of your digital multimeter is shown to the right (Click on the photo if you want to see greater detail.) There are two probes. The red one is conventionally taken to be positive and the black one negative. The black probe always connects to the port labeled COM. The red probe can connect to 2 or 3 different ports depending on what you're measuring. We'll discuss those later.
Note the two white, oblong buttons. The one on the left should be out. This is the position for measuring direct currents and potential differences. That's the only kind you'll be measuring. The button to the right is the power button. Always turn off the meter when not in use. This will save the batteries.
The dial is divided into sections for measuring current (A), potential difference (V), and resistance (Ω). Each section has several ranges which we'll discuss. There's also a symbol, , near the top of the dial. This is a continuity checker. It's used to determine if there's a break in a wire. You'll try using that setting next.
Part A. Using the continuity checker
Your meter should be turned off. If it's on, turn it off now. Always start with the meter off. Make sure your connections and settings are right before you turn it on.
Make sure the black probe is connected to COM. Check that the red probe is connected to the V, Ω port as in the photo above. Turn the dial to the continuity checker setting . Turn the meter on. Now touch the metal ends of the probes together. After a few seconds, the reading should drop to 0.00. If it doesn't, you may not have good contact between probes. Whenever you make measurements in the future, remember that you need to make establish good contact in order to get good readings.
Test one of your alligator clip wires in the same way. Clip one end of the wire to the positive probe and the other end to the negative probe. (In the future, we'll call them red and black, assuming that you're using the convention.) You should have continuity.
Turn your meter off and read on.
Part B. Measuring resistance
You'll be using resistors in the circuits labs. These are in your lab kit in the envelope. Take a close look at one of them. They have several colored bands. The colors and order of the bands can be read in order to determine the resistance in the ohms. Here's a link where you can find out what colors correspond to what numbers. The first two colored bands give the first two digits of the resistance. The 3rd band tells you the power of 10 to multiply by. For example, brown-orange-red translates to 1 for brown, 3 for orange, and 102 for red. So the resistance is 13x102 or 1300 ohm. Note that the value that you read from the color code has a percentage uncertainty (or tolerance) that is read from the 4th band. Gold represents a tolerance of 5%, silver represents a tolerance of 10%. In your experiments, you won't pay attention to this tolerance, because you'll measure resistances accurately to the nearest ohm or better with the multimeter. The main reason to use the color code is so that you can quickly select a resistor close to the value that you want.
Note that when measuring resistance, you didn't place a battery in the circuit. It's important to realize that if you place a battery in the circuit with the resistance, the meter will give the wrong reading. That is the most common mistake that people make in measuring resistance.
Part C. Measuring potential differences
When you measure potential difference, the reading is the difference between the potential at the red probe and the potential at the black probe. Any measurement of voltage is actually a difference of potential between two points of a circuit. When you measure potential difference, you simply touch the probes (or connect alligator clips) to the two points of the circuit in question. More about that in a moment.
A diagram of a simple circuit with a battery and a resistor is shown to the right. Four points are marked for reference. The long side of the battery symbol is the positive terminal of the battery. Conventional (positive) current goes from positive to negative in the circuit. That is, current goes from A to B to C to D to A. In the future, when we talk about current, we mean positive current (even though we know that the actual charge carriers are electrons.)
One type of battery holder--we'll call this the Type I holder--that you may be using for the circuits labs is shown below. You can connect any number of batteries up to four. Note that metal tabs separate the batteries. Make sure all batteries face the same way in the holder; that is, the positive end of one battery connects to the negative terminal of the next one. The photo shows that only one battery is connected to the circuit with the clip leads. If you moved the black clip to the middle tab, then you would have 2 batteries in the circuit. For this tutorial, you'll just use one battery as shown in the photo. (Click on the photo for a larger version.)
The Type II battery holder consists of 4 blue or yellow plastic boxes. You can interlock boxes in order to have 1, 2, 3, or 4 batteries in series. They interlock either by sliding or snapping. Snapping is easier, if your boxes have snaps on the ends. Otherwise, see the figures below for the interlocking method. Figure 1 shows two boxes aligned with the ends that will slide together. Figure 2 shows the boxes being connected. When you slide the battery holders together, be careful of the sharp edges of the plastic! It takes quite a bit of force to slide them together. Figure 3 shows the boxes connected. Note the metal terminals in the gap between the boxes that touch each other. (Click on any photo for a larger version.)
Connect two boxes together for the next set of measurements. If you want to use just one battery, you don't have to disconnect the boxes. You just clip wires to the metal tabs on one of the boxes. If you want to use two batteries in series, you connect wires to the two outermost tabs.
Turn your meter off and go on to the next part.
Part D. Measuring current
The multimeter is used in a very different way to measure current. For measuring potential differences, the multimeter is placed in the circuit in parallel. When measuring current, the multimeter must be placed in the circuit in series. Suppose that we want to measure the current at point P shown in Figure 1. To do so, the circuit must be opened at P as shown in Figure 2. Then the meter is inserted in the circuit at the point at which the circuit was opened.
Now do the following to prepare the meter and circuit.
Part E. Reflection
Submitting your work
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