The voltmeter is a device used to measure the voltage in an electrical circuit. It is commonly represented by the symbol "V" and typically consists of a sensitive ammeter with a permanent magnet and a coil connected in series between its terminals. This coil is placed within the magnetic field of the magnet and connected to a pointer that moves across a scale. Most voltmeters have two measurement ranges, and they usually have three terminals: one negative and two positive. The positive terminal should be connected to the positive side of the circuit, while the negative terminal connects to the negative side.
A key feature of a voltmeter is that it has a very high resistance, ideally acting like an open circuit. Therefore, it must be connected in parallel with the component being measured. In a parallel circuit, the voltmeter is placed alongside other devices, allowing it to measure the voltage without significantly affecting the current flow. If there are no other components in the branch, the power supply can be directly measured.
Understanding how to read a voltmeter and ammeter is essential for accurate measurements. The reading rules depend on the range and the smallest division on the meter’s scale. For example:
1. When using a 3 V or 3 A range, where the smallest division is 0.1 V or 0.1 A, the reading should be estimated to the nearest tenth of the division.
2. For a 0.6 A range with a minimum division of 0.02 A, the reading should be estimated to half of the smallest division—either rounding up or down based on the pointer’s position.
3. With a 15 V range and a minimum division of 0.5 V, the reading should be estimated to the nearest half of the division, resulting in one decimal place.
Examples help clarify these rules. If the selected range is 0.6 A, and the pointer is at 0.37 A, that’s the correct reading. If the range is 3 A, and the pointer shows 1.86 A, that is the accurate value. Similarly, for a 15 V range, if the pointer lands at 6.0 V, that is the measured voltage. For a 3 V range, a reading of 1.20 V would be appropriate.
Reading meters also involves identifying the correct scale and understanding the number of decimal places. For instance, a 10-division meter (like 0–3 V or 0–3 A) allows for one estimated decimal place. A 2-division meter (like 0–15 V) only permits one decimal place, while a 5-division meter (like 0–0.6 A) requires two decimal places.
These rules ensure that readings are both precise and consistent, helping students and professionals alike make reliable measurements in various electrical applications.
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