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I can't seem to understand the problem with connecting a voltmeter in series. All explanations say that voltmeters have extremely high resistance so connecting it in series will effectively stop the current. But aren't they designed this way? What if we made the voltmeter low resistance? Would it not work normally.

The only explanation I can come up with is, connecting a voltmeter in series is essentially finding the potential difference of a point, hence its 0. Is this right? Or why must we connect a voltmeter in parallel.

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  • $\begingroup$ If you make a voltmeter with a low internal resistance then what you have is an ammeter. $\endgroup$
    – gandalf61
    Commented Aug 23, 2022 at 9:52

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You measure a voltage between two nodes by connecting the voltmeter between the two nodes $A$ and $B$, ie in parallel with the component across which you wish to measure the voltage.

enter image description here

If you connect a voltmeter in series then all you are doing is measuring the voltage across the voltmeter, ie across nodes $B$ and $C$ not across nodes $A$ and $B$.

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First of all, you're free to use a voltmeter in any way you see fit. However its most common use case is to measure the voltage between two points. To do that, you have to plug each connector of the voltmeter to one of those points.

Here's the example with a simple resistor:

voltmeter

As most measuring instruments, the voltmeter is not supposed to modify the quantity that it is measuring. As you can see on the image, the current that you want to measure ($i$) is split in two ($i'$ and $i-i'$) before it reaches the resistance, because of the voltmeter.

You can't avoid this problem, but you can minimize it. If the voltmeter has a very high resistance, $i'$ will be negligible, and the current actually going through the resistor will be very close to $i$.

Because of this construction choice, putting a voltmeter in series will usually be equivalent to opening the circuit, so it isn't usually useful (unless you do want to open the circuit, see current clamp or voltage filter with open output).

Another use case for a voltmeter in series: if you want to measure its precise internal impedance, you'll plug it in series with another resistor (voltage divider).

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