Thus, as early as the 19th century, a question arose, "How do we make an ideal voltmeter?" And much later, the dual question arose, "How do we make an ideal ammeter?" Why did it happen in this sequence? Here, the problem arises with the impact of real measuring instruments on the measured value - the voltmeter diverts (subtracts) current, and the ammeter introduces (subtracts) voltage drop. But these are only assumptions that should be verified in practice by measurement. The problemĪs in the OP question, so in most answers here, the current through the closed switch and the voltage across the open switch are determined by calculation or common sense. While for junior students the goal was the electrical ideas and circuits themselves, for seniors the goal was more the applications of these ideas in the ubiquitous non-inverting and inverting op-amp circuits with negative feedback. I figuratively called them "Ideal" voltmeter and "Ideal" ammeter. It is a happy coincidence for me that, over the past two weeks, I have decided to conduct with my students, in the subjects in Electrical Engineering (1st year) and Basic Electronic Circuits (3rd year), two unusual laboratory exercises dedicated to this topic. But it is much more meaningful and useful for the OP and all of us to reveal the basic ideas behind these specific circuit solutions.
It can be answered with banal clichés such as "op-amp follower" or "transimpedance amplifier" as can be seen in millions of web pages. The OP question is seemingly naive but as can be seen from the many posts, it concerns important circuit concepts.
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