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Unformatted text preview: Experiment 31 must flow through the instrument. For the proper way to measure the current in a simple circuit look carefully at the schematic Fig. 31.6. You will note that the ammeter is in series with the other elements of the circuit. hi order not to change the current in the circuit, the ammeter has to have negligible resistance. Typical ammeters have a resistance of a fraction of an Ohm. There are two problems with this: 1. The more important one is that the very low resistance may cause 3. huge current flow if the meter is inserted in the wrong place. Don't ever connect an ammeter (or a DMM in the ammeter mode) across the terminals of a batten or power supply. If you do so accidentally, you may blow a fuse. For a large curren: overload, you may also fry the instrument, wreck the power supply, hurt yourself, or do all four, because the ammeter effectively becomes a short circuit. If your meter works fine ~ the DVM mode but not in the DAM mode, the fuse is most likely blown. Ask your TA fee help. 2. The second problem is the possibility of a measurement error called burden volt.:.. error. When a DMM is used in the ammeter mode and placed in a circuit, there will be i small voltage drop across the meter caused by fuse and meter shunt resistance, hi otber words, the circuit itself will be changed slightly by inserting the meter. For mar/. measurements, this effect is insignificant. However, in some cases, for pre^ measurements, it must be taken into account. The Ohmmeter A DMM can be used to measure resistance. When a resistor is connected to the two leads of the DMM switched to the Ohm mode, the terminals of the DMM become output terminals for a small current that flows through the resistor. The meter measures that current, which depends on the magnitude of the resistor, and converts it to a resistance value, which is then displayed. The meter can be switched to be sensitive over a wide range of resistor values. Investigation 1 Electromotive Force of a Single Battery and Two Batteries Connected in Series Procedure 3. Take some time to inspect your DMM's. Study all the possible settings for volu current, etc. Also study the ranges indicated on the instruments. The notations nexi the range buttons indicate maximum values of the range at that setting. Note different inputs for different measurement conditions (currents, voltages). When ;• : are done, make sure the AC/DC button is in the DC position (out), which is the con position for all the following measurements. 4. Measure the voltage across a battery. Draw a circuit diagram that indicates the pos::x»| of the meter. Make sure that the DMM is set to measure voltage (V button pushec and that the meter range is at the highest setting. This protects the meter in case make a mistake. Connect the meter across the battery with the positive (+) terminal connected to the positive (red) meter input and the negative (-) terminal connected to the negative (black) meter input. Now push the light col;-: 31-8...
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- Spring '08
- Resistor, power supply, Electrical resistance, Series and parallel circuits