**How to Use Current and Voltage Meters**
The ammeter and voltmeter are two of the most fundamental and essential measuring instruments used in junior high school electrical experiments. These devices are also commonly tested in middle school entrance exams. Many students lose points due to not mastering their correct usage, making it crucial for students to understand how to properly use these tools. Let’s go over the key points that should be emphasized when teaching students about ammeters and voltmeters.
**1. How to Read the Meter**
For example, Figure 1 shows an ammeter. The dial is labeled with the letter "A," indicating it measures current. Similarly, Figure 2 displays a voltmeter, marked with the letter "V," which measures voltage. Understanding the labels on the dials is the first step in using these instruments correctly.
**2. Correct Connection Method**
An ammeter must be connected in **series** within the circuit being tested. Its positive terminal should be connected to the positive side of the power supply, and the negative terminal to the negative side. On the other hand, a voltmeter should be connected in **parallel** across the component or circuit being measured. It's important to ensure the polarity is correct and not reversed. Never connect the ammeter directly across the power supply, as this could damage the meter due to excessive current.
**3. Choosing the Right Range**
Both ammeters and voltmeters have multiple terminals, and the range depends on how the wires are connected. For instance, if the ammeter is connected between the “+†and “0.6†terminals, the range is 0.6 A. If it’s connected between “+†and “15,†the range is 15 V. Before starting an experiment, estimate the current or voltage to choose the appropriate range. If unsure, use the **test method**: connect one terminal and briefly touch the other to the highest range. Adjust accordingly based on the result.
**4. Testing the Circuit**
After setting up the circuit according to the diagram, perform a test by observing the pointer movement on both meters. If the pointer doesn’t move, there may be a break in the circuit or incorrect placement of the meters. If it moves in the opposite direction, the polarity is reversed. If the needle swings too far, the range is too small; if it barely moves, the range is too large. Adjust as needed before proceeding with the experiment.
**5. Reading the Meter Accurately**
Reading the meter accurately is critical for solving problems in physics exams. For example, in Figure 1, each large grid represents 0.2 A, and each small grid is 0.02 A. If the pointer is at 1 large grid and 8 small grids, the reading is 0.36 A. In Figure 2, each large grid is 5 V, and each small grid is 0.5 V. If the pointer is at 2 large grids and 4 small grids, the reading is 12 V.
**6. Distinguishing Between Ammeters and Voltmeters**
Although both meters measure electrical quantities, they differ in function and connection:
- **Similarities:**
- Both have positive and negative terminals.
- Both require attention to the measurement range.
- **Differences:**
- An ammeter must be connected in **series**, while a voltmeter is connected in **parallel**.
- An ammeter cannot be directly connected to the power supply, but a voltmeter can.
**Fault Diagnosis Methods for Current and Voltage Meters**
1. **Tapping Method:** Gently tap the device to check for loose connections or poor contact.
2. **Observation Method:** Look for physical signs such as discoloration, burning, or bulging components.
3. **Exclusion Method:** Remove parts one by one to identify the faulty section.
4. **Replacement Method:** Swap suspected components with known good ones to isolate the problem.
5. **Comparison Method:** Compare readings from a working meter to a faulty one.
6. **Temperature Rise Method:** Check if the fault occurs under high temperature conditions.
7. **Parallel Method (Shoulder Riding):** Place a known good IC or component in parallel to test for faults.
8. **Capacitor Bypass Method:** Use a capacitor to bypass sections of the circuit and identify issues.
9. **State Adjustment Method:** Adjust settings only after recording initial values.
10. **Isolation Method:** Narrow down the fault by isolating sections of the circuit.
**Common Faults and Their Analysis**
- If the voltmeter shows a reading, it means it is connected to the power supply. If there is an open circuit, the fault lies in the part where the voltmeter is connected.
- If the voltmeter shows no reading, it might be due to poor contact, a shorted circuit, or an open circuit elsewhere.
- If the ammeter pointer does not move, there may be an open circuit or a shorted ammeter.
- If a bulb does not light, it could be due to a broken filament, a shorted bulb, or an open circuit in the wiring.
**Tips for Using Current and Voltage Meters**
- Handle the meters carefully to avoid damage to internal components like the bearings and hairspring.
- Always turn off the power before installing or removing the meter to prevent accidents.
- Estimate the expected current or voltage before connecting the meter to avoid overloading it.
- Ensure proper polarity when measuring DC, but no polarity is required for AC.
- Use clean and secure connections to prevent contact issues.
- Regularly clean the meter with a dry cloth to maintain accuracy and appearance.
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