Equivalent circuit for the real voltage measuring device?
If you want to measure voltage, you place your voltmeter in parallel with the thing you want to measure. But if you connect your voltmeter directly to your voltage source, for example, the two ends of a battery, then the voltmeter is connected in series, right?
The ideal internal resistance of a voltmeter is infinite ohms. Since there's no such thing as infinite ohms, you simply use a very large one, like 10 MOhm. In the picture, this resistance is marked Ru. But I don't understand the equivalent circuit of a voltmeter. If you connect Ru in parallel to the voltmeter, then all the current will flow in the direction of Is, right?
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If you have only one voltage source and one resistance, it is hard to judge whether it is now in series or parallel, the term is used only when you have several consumers or sources.
The U is ideal and has infinite resistance. Is therefore 0.
The internal resistance of the voltmeter is denoted by Ru.
Thank you for your answer. So if I really understand it is the process of measuring that takes place in the voltmeter(U) so that there is an infinite resistance?
In the ideal voltmeter yes.
The ideal voltmeter (here U) has infinite internal resistance (Is=0). In order to take into account that this is not the case, it is necessary to switch a paralle resistor Ru to U. The current into the measuring device is then Is’ = U/Ru
Ri and Ru are in fact always in series and theoretically leads to a measurement error. Suppose Ri is 0.1 ohm and Ru 10 MOhm, then the measurement accuracy caused by the “serial circuit” is one hundred millionths. This is practically zero in relation to other influences at trade fairs.
Theoretical stone circuit, logical, practically totally negligible.