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I want to measure the voltage(potential difference) acorss the motor A. Where should I connect the voltmeter? I am confused between two possibilities:

  1. between X and Y (between two ends of the motor A) OR
  2. Z and K (between ammeter terminal and variable resistance terminal)

And is there any difference between them? Or would the voltmeter give the same reading?

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If the resistance of the wires between Z/X and Y/K is small, there will be a small voltage drop across them so the voltage across Z/K will be larger than across X/Y.

Depending on the wires, motor, and voltmeter, this difference might well be too small to measure.

The correct way to measure the voltage across the motor is to connect the voltmeter as close to the motor as possible, so X/Y.

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  • $\begingroup$ so voltmeter should be connected between X and Y to ensure that this is the right p.d acorss the motor only, right? $\endgroup$ – Rami ki Feb 2 at 9:10
  • $\begingroup$ Yes, I included this in the answer $\endgroup$ – Jasper Feb 2 at 9:42
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Suppose the resistance of the wire between Z and X is $R_{zx}$ and the resistance of the wire between K and Y is $R_{yk}$ then if a current $I$ is flowing through the motor there will be voltage drops of $IR_{zx}$ and $IR_{ky}$ along the two wires. If you measure the voltage between Z and K what you are going to measure is:

$$ V_{zk} = IR_{zx} + V_{motor} + IR_{ky} $$

So the voltage $V_{zk}$ that you measure between Z and K will be slightly greater than the real motor voltage $V_{motor}$. However in most cases it is likely that the resistance of the motor is much higher than the resistances of the wires. That means the error in measuring the voltage between Z and K will be negligibly small.

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