IN09 · The meter changes the circuit
Why does connecting a meter change the quantity you want to measure? A meter becomes part of the circuit. A voltmeter draws some current across the component, and an ammeter adds resistance along the current path. Their input networks change the voltage and current they read; the size of that change depends on the circuit and the selected range.
Connect physical probes, compare moving-coil and digital meters, and retain calibrated DC readings to measure loading and current burden.
Grey is the ideal unconnected circuit reference; turquoise is the calibrated stored reading, or orange when invalid. These values keep their saved circuit and wiring while the apparatus previews new settings.
| Record | Channel | Raw value | Calibrated | Valid for target |
|---|
Exports retain actual input resistors, probe nodes, bridge state, raw detector value, calibration observations, circuit solution, ideal reference, model version and seed.
Physics tutorial
BackgroundA voltmeter adds a current path between two nodes. An ammeter occupies a path that previously carried current through a wire. Both change the circuit they observe.
Why it mattersA precise detector can report a changed circuit very accurately. Calibration of the detector and disturbance of the measured circuit are separate questions.
Start with the essentials
The shaped poles give an ideal radial field at the two active coil sides. Positive coil current produces positive shaft torque. The back emf returns mechanical work to the electrical model; it vanishes when the pointer is stationary.
A shunt carries the part of the terminal current that bypasses the coil. A series multiplier limits coil current in voltage mode. The current range exceeds the coil full-scale current; each displayed range uses its actual resistor values.
With the bridge closed, the voltage input and the load connect to the same two circuit nodes. The source resistance then sees a different parallel combination. Digital mode has a large but finite input resistance.
The raw channel is pointer angle or ADC count. The zero and known-span signals come from separate stored calibration observations. The inverse does not use the hidden coil constant, spring stiffness or ADC gain.
Typical misconceptionA digital reading must be the voltage that existed before the meter was connected.
Better mental modelThe ADC measures the circuit with its input connected. A high input resistance makes its disturbance smaller, and the finite resistance still belongs in the circuit.
Typical misconceptionA negative reading means the passive load acquired negative resistance.
Better mental modelThe indication follows the red-to-black orientation. Reversing the probes reverses the detector signal; the resistor and its positive power dissipation are unchanged.
Choose The meter pulls voltage down, then Digital, with finite input.
What to observe: The input network changes, the loaded node moves and the raw sensing mechanism changes from a rotating coil to an ADC display.Drag a probe between the feed, load and return contacts. Use the bridge to open or close the feed-to-load gap.
What to observe: A snapped probe touches the selected physical post. The circuit is solved with those actual nodes, so a wrong connection can bypass the meter, disconnect the load or overload the input.Compare A sensitive current range and Less current burden.
What to observe: The more sensitive range inserts more resistance. Its burden is the voltage across the whole meter input path, including a digital shunt and the extra series path.Keep this reading, move either probe and export CSV or JSON.
What to observe: The saved reading retains its original raw channel, circuit, range resistors and zero/span calibration. A transient or overrange record carries an explicit invalid-target flag.How physics instruments measure · Reading path
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