Voltmeter in series?

Abstract

A recent physics challenge shows a circuit, where a voltmeter is connected in series. Indeed, real voltmeters have finite input resistance, therefore, one may think that they can be used as resistors. In addition, voltmeters measure the voltage difference between their terminals, so it seems to be possible to calculate the current flowing through them. Is it okay? Are there any hidden secrets? The underlying methodology is related to high-quality physics and STEM education, which are increasingly important in the modern world. On one hand, it can be considered as an approval of an improper use that one can never see in any textbook and application, and it may also occur that suggesting and teaching such uncommon solutions can generate an undesired attitude. On the other hand, one can also say that if the students are taught to always follow the application rules, then the development of their creativity can be hindered. We do think it deserves some discussion.