Choose a discriminating reading within a budget

Decide what a reading could change

A measurement is useful when its possible outcomes change the explanations you can defend. This lab allows two distinct probes per case: terminal voltage, junction voltage and loop current are the available choices. Plan before pressing the reading control. A third distinct probe is outside the case budget, even when it would resolve an inconvenient ambiguity.

The budget counts distinct simulated probe types, not time, money or physical meter connections. Reopening an existing reading neither spends another slot nor generates new evidence. Every new reading needs a prediction first. The dedicated notebook retains case histories across case changes and reloads; a generic lesson capture does not preserve the diagnostic sequence in its place.

Compare two possible first probes

Use the five supplied hypotheses from the circuit explanation. Four predict terminal voltage 6 V; the weak ideal source predicts 3 V. Therefore a first terminal reading of 3 V singles out the weak source within this set. A result of 6 V leaves healthy, open R1, open R2 and bypassed R2 compatible.

A junction reading partitions the same candidates differently: 4 V for healthy, 6 V for open R2, 2 V for the weak source, and 0 V for either open R1 or bypassed R2. It leaves a two-candidate ambiguity for one outcome. This comparison does not require assigning probabilities. The fact that four listed mechanisms share 6 V does not mean a real source has an 80% probability of producing that reading; the list supplies no prevalence distribution.

On paper, draw a decision tree. Label each branch with a possible reading and write the surviving mechanisms at its end. A tree that retains an unresolved set is more honest than one that forces every branch to a diagnosis unsupported by the measurements.

Spend the second probe on the remaining distinction

Suppose you first obtained junction voltage 0 V. The remaining explanations are open R1 and bypassed R2. A terminal-voltage probe would return 6 V for either, so it cannot separate them. A current probe predicts 0 mA for open R1 and 60 mA for the bypass. It addresses the actual unresolved distinction.

Now consider a different history: terminal voltage 6 V, followed by current 0 mA. Both open cases remain compatible. The junction would distinguish them, but the two distinct slots have already been used. Your conclusion should preserve both possibilities and identify the extra measurement you would request if resources were expanded. Do not substitute a remembered case letter for evidence or relabel a revealed answer as a third measurement.

Use the notebook without erasing familiarity

Choose a case with unused readings if one is available. Record a first-probe prediction that includes more than a guessed number: state which outcomes would distinguish which mechanisms. After taking it, update the surviving candidates and predict the second outcome or alternatives before requesting the second probe.

If all cases have been explored, use the retained records to critique your original sequence. Repeated public practice can improve a method without being an unfamiliar assessment. Revealing an answer records exposure; switching away and returning does not undo it. Explain how the evidence sequence supports the conclusion even if you already know the supplied answer.

OpenStax’s instrument discussion explains why voltages and currents involve different connections. This model uses ideal meters: infinite voltmeter resistance and zero ammeter resistance. It does not charge for reconnecting a physical meter, model loading, or supply a practical instrument-operation credential.

Transfer: choose a probe for a new ambiguity

On paper, suppose a changed candidate set contains two ideal-source possibilities, each with total resistance 300 Ω: one source is 6 V and the other 3 V. You may obtain one additional reading, either terminal voltage or current. Predict both possible readings for each probe. Must one choice be uniquely best?

Check the alternative-probe transfer

Terminal voltage would be 6 or 3 V. Current would be 20 or 10 mA because I = V/300 Ω. Either ideal reading distinguishes these two candidates. Without different probe costs, errors or access restrictions, the evidence does not require a uniquely best choice. Those extra conditions would need to be supplied, not invented.

Write a two-sentence method review: what your original next probe could distinguish, and what it could not. Continue to fault localization to connect the readings to node potentials rather than memorized case labels. A good measurement plan uses its resources to resolve a stated question, and can explicitly conclude that the resources were insufficient.