Understanding Voltmeters: How They Work and Common Misconceptions

[pivoxa15]

Do voltmeters work by sampling current from two points in a circuit and output the voltage or potential difference between the two points? Must there be resistor in the middle of the two points? Since V=Ed. there dosen't have to be does there?

Sometimes the voltmeter gives a negative reading. Why is that?

 

[Integral]

Since voltmeters measure a potential difference, if there is no resistor (or some other voltage dropping device) there would be no potential difference between the leads resulting in a reading of zero. So there only needs to be a resistor between the leads if you want a non zero voltage reading.

Voltmeters have red and black leads, generally the black lead is connected to the common terminal of the meter and the point at a lower potential in the circuit. The red is connected to the V+ terminal of the meter and to the higher potential in the circuit, this results in a positive reading. If the black lead is at a higher potential then the red, you will get a negative reading. It really does not matter in a digital meter, however, if you ever get your hands on an old fashioned meter with a D'Arsonal meter movement, you can damage the meter if it is connected backward.

 

[astrokreng]

I can provide some clarification on the topic of voltmeters and address the common misconceptions surrounding them. Firstly, it is important to understand that voltmeters work by measuring the potential difference, or voltage, between two points in a circuit. This is achieved by sampling the current at those two points and using Ohm's law (V=IR) to calculate the voltage.

It is not necessary for there to be a resistor in the middle of the two points being measured. The voltmeter itself acts as a high resistance load, so adding a resistor in the middle would not significantly affect the measurement.

One possible reason for a negative reading on a voltmeter is if the leads are connected to the circuit in the wrong polarity. In this case, the voltmeter is essentially measuring the voltage drop across the internal resistance of the voltmeter, resulting in a negative reading. It is important to always double-check the polarity of the voltmeter leads to ensure an accurate measurement.

Other factors that can affect voltmeter readings include external magnetic fields, temperature variations, and the quality of the voltmeter itself. It is important to calibrate the voltmeter regularly and use it in a controlled environment to minimize these potential sources of error.

In conclusion, voltmeters work by sampling current from two points in a circuit and calculating the voltage difference between them. A resistor in the middle is not necessary, and negative readings can occur due to incorrect polarity or external factors. By understanding how voltmeters work and being aware of potential sources of error, we can ensure accurate measurements and avoid common misconceptions.