At the start the voltmeter reads a specific value (I guess it is the value of the voltage of R2, right?)


If circuit closes, what value does the voltmeter read?

My answer is: It does not change!

Is my argumentation correct?

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is my argumentation correct?


This assumes the voltmeter has infinite input impedance: -

  • When the switch is open, the voltmeter reads the voltage across R1.

R3 is irrelevant in this situation and might as well be a short circuit.

  • When the switch is closed, the voltmeter reads the voltage across R2
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  • \$\begingroup\$ Good day. thank you for replying. Now i look at the circuit and i see why it messures R1 at start (since both parts of voltemeter messure the same potential) But i somehow do not see how it messures R2 after closing. (i am not really electro Engineer so my knowledge is limited) Could you please explain how it messures R2? Thanks \$\endgroup\$ – Mad Spaces Aug 5 at 9:37
  • \$\begingroup\$ @ Andy aka, Short circuit? R3 is a constant resistor that is obviously present in both situations; so it cannot be a short circuit. \$\endgroup\$ – Circuit fantasist Aug 5 at 9:45
  • \$\begingroup\$ When the switch is open. Read what I said. \$\endgroup\$ – Andy aka Aug 5 at 9:56
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    \$\begingroup\$ @Andyaka - It would be nice if both of you could play just a wee bit more gently. Andy implied something without stating it explicitly. He could have been clearer if he wanted easy understanding by everyone. However, CF's statement that what is said "makes no sense" to speak of R3 being 0 - it makes sense in the area that Andy was speaking about - but not for the switch closed situation. Both you gentlemen know much. It would be nice if you could both be on the same team as much as you can manage and be a bit slower in being annoyed by the other guy. \$\endgroup\$ – Russell McMahon Aug 5 at 12:35
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    \$\begingroup\$ Andy - might I suggest you make the conditions under which R3 may as well be 0 clearer. I know what you mean BUT I also felt on 1st reading that it was not conveyed wholly clearly. || I note that it may be worth saying that this cct is probably trying to be tricky on purpose. It relies on Rmetter >> R3 cor meter to almost read VR1 accurately (low by R3/(R3+R_meter) . BUT it is no better than using the switch as a straight changeover to transfer the meter from one to other. In both cases the meter polarity swaps which is liable to be non useful. \$\endgroup\$ – Russell McMahon Aug 5 at 12:40

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