A measuring instrument joins the circuit it is measuring. If it draws too much current or adds too much resistance, it changes the very quantity you hoped to observe. Meter design is therefore part of the physics, not a detail to add after solving the circuit.
This chapter follows the measurement from component design to loading and uncertainty. The bridge arms are explicitly labelled so that every ratio can be derived from node potentials rather than remembered from a particular drawing.
What you will learn to do
- Use coil resistance and full-scale current to design meter ranges.
- Distinguish internal shunt/series connections from external meter placement.
- Calculate finite-meter loading of a voltage divider.
- Derive a Wheatstone balance from named midpoint potentials.
- Use post-office-box ratios to set range and resolution.
- Bracket a null and report the resulting resistance interval.
- Account for lead, contact, temperature and ratio uncertainties.
Study with a tutor prompt
Copy this into your AI assistant, then describe where you got stuck. It asks the tutor to coach you one step at a time. Check its reasoning against the lesson.
Topic: Electrical instruments: meter ranges and bridge balances Learning goals: - Use coil resistance and full-scale current to design meter ranges. - Distinguish internal shunt/series connections from external meter placement. - Calculate finite-meter loading of a voltage divider. - Derive a Wheatstone balance from named midpoint potentials. - Use post-office-box ratios to set range and resolution. - Bracket a null and report the resulting resistance interval. - Account for lead, contact, temperature and ratio uncertainties. Start by checking the prerequisite idea I need most. Use a simple example, then change one condition to test my understanding. Tutoring approach: Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.
BEFORE YOU READ
Bring back one idea
Try a sentence or a sketch before opening the comparison. If this is new, use the explanation to get started.
Why can adding a resistor in parallel protect a sensitive current-measuring coil?
Compare your reasoning
The parallel branch carries part of the total current while sharing the coil’s voltage. The coil current can remain within its limit even when the total current is larger.
Revisit the explanation →01 / Electrical instruments: meter ranges and bridge balances
A galvanometer responds to its own coil current
A galvanometer movement is a sensitive current detector. In the ideal linear range used here, its deflection is proportional to the current through its coil. Let G be the coil resistance and I_g the current that produces full-scale deflection.
Full scale is a calibration limit for the movement, not the current that every circuit can safely send through it. Converting the movement into an ammeter or voltmeter means adding resistors so that the desired external measurement corresponds to I_g in the coil.
The coil’s full-scale voltage is I_gG. That small voltage is the shared voltage needed for a parallel shunt in an ammeter. For a voltmeter, the coil current is limited by a large series resistance.
A real movement has calibration, mechanical response and temperature limitations. The calculations in this chapter use an ideal linear coil resistance and a stated full-scale current. They teach the circuit conversion, without assuming that every practical meter behaves as an ideal instrument.
G is the galvanometer coil resistance and I_g its full-scale current. Both are properties of the movement at the stated calibration conditions.
STOP & THINK
What quantity directly determines the deflection of the ideal current-sensitive movement used here?
- The total current in every external branch
- The current through its own coil
- Only the voltage rating printed on a scale
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Topic: Electrical instruments: meter ranges and bridge balances Focus: A galvanometer responds to its own coil current Learning goals: - Use coil resistance and full-scale current to design meter ranges. - Distinguish internal shunt/series connections from external meter placement. - Calculate finite-meter loading of a voltage divider. - Derive a Wheatstone balance from named midpoint potentials. - Use post-office-box ratios to set range and resolution. - Bracket a null and report the resulting resistance interval. - Account for lead, contact, temperature and ratio uncertainties. Equations in this section (LaTeX): V_g=I_gG Meaning: G is the galvanometer coil resistance and I_g its full-scale current. Both are properties of the movement at the stated calibration conditions. Start by checking the prerequisite idea I need most. Use a simple example, then change one condition to test my understanding. Tutoring approach: Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.
02 / Electrical instruments: meter ranges and bridge balances
A parallel shunt diverts most of the measured current
To extend the current range from I_g to a larger total I, connect a shunt resistance S in parallel with the coil. At full scale, the coil carries I_g and the shunt carries I − I_g. Their voltages are equal, giving I_gG = (I − I_g)S.
The resulting ammeter is the parallel combination of G and S. It is inserted in series with the branch whose current is measured. Its effective resistance is I_gG/I at full scale in the ideal linear model, which is much smaller than G when the range is greatly extended.
The coil and shunt are parallel internally, while the complete instrument is in series externally. These two different connections are easy to confuse. Putting the complete low-resistance ammeter directly across a voltage source changes the circuit into a low-resistance path instead of measuring the intended branch current.
The shunt’s temperature coefficient and power rating matter in a real instrument because it carries nearly all the current. A nominal resistance value alone does not guarantee that the range remains accurate when the shunt heats.
I is the desired total full-scale current. The galvanometer and shunt share voltage.
WORKED EXAMPLE
Design a current range and check the burden voltage
A galvanometer has G = 50 Ω and I_g = 2.0 mA. Convert it to a 0.20 A full-scale ammeter. Find the shunt, effective meter resistance and full-scale voltage drop across the instrument.
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Copy this into your AI assistant, then describe where you got stuck. It asks the tutor to coach you one step at a time. Check its reasoning against the lesson.
Topic: Electrical instruments: meter ranges and bridge balances
Focus: A parallel shunt diverts most of the measured current
Learning goals:
- Use coil resistance and full-scale current to design meter ranges.
- Distinguish internal shunt/series connections from external meter placement.
- Calculate finite-meter loading of a voltage divider.
- Derive a Wheatstone balance from named midpoint potentials.
- Use post-office-box ratios to set range and resolution.
- Bracket a null and report the resulting resistance interval.
- Account for lead, contact, temperature and ratio uncertainties.
Equations in this section (LaTeX):
S=\frac{I_gG}{I-I_g},\qquad R_A=\frac{GS}{G+S}=\frac{I_gG}{I}
Meaning: I is the desired total full-scale current. The galvanometer and shunt share voltage.
Start by checking the prerequisite idea I need most. Use a simple example, then change one condition to test my understanding.
Tutoring approach:
Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.03 / Electrical instruments: meter ranges and bridge balances
A series multiplier limits the coil current at a larger voltage
To use the movement as a voltmeter of full-scale voltage V, connect a series resistance R_s. At full scale, the same current I_g passes through the resistor and coil, so V = I_g(R_s + G). Therefore R_s = V/I_g − G.
The complete voltmeter resistance is R_V = V/I_g. The instrument connects across the two nodes whose potential difference is measured. A larger input resistance draws less current from the circuit and usually reduces loading.
Sensitivity in ohms per volt is 1/I_g for this simple movement. A movement requiring a smaller full-scale current can make a larger input resistance for the same voltage range. This is a statement about the complete range resistance, not just the added series resistor.
For a given movement, a larger voltage range increases R_V in this simple model. A real meter may use switched networks or electronics, so use its actual input specification when analysing loading rather than assuming every range follows this passive-coil construction.
R_s is the added series resistance; R_V is the total voltmeter resistance.
WORKED EXAMPLE
Distinguish the added resistance from the total input resistance
Use the same G = 50 Ω, I_g = 2.0 mA movement to make a 10 V full-scale voltmeter. Find the added resistance and total input resistance.
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Study with a tutor prompt
Copy this into your AI assistant, then describe where you got stuck. It asks the tutor to coach you one step at a time. Check its reasoning against the lesson.
Topic: Electrical instruments: meter ranges and bridge balances
Focus: A series multiplier limits the coil current at a larger voltage
Learning goals:
- Use coil resistance and full-scale current to design meter ranges.
- Distinguish internal shunt/series connections from external meter placement.
- Calculate finite-meter loading of a voltage divider.
- Derive a Wheatstone balance from named midpoint potentials.
- Use post-office-box ratios to set range and resolution.
- Bracket a null and report the resulting resistance interval.
- Account for lead, contact, temperature and ratio uncertainties.
Equations in this section (LaTeX):
R_s=\frac{V}{I_g}-G,\qquad R_V=R_s+G=\frac{V}{I_g}
Meaning: R_s is the added series resistance; R_V is the total voltmeter resistance.
Start by checking the prerequisite idea I need most. Use a simple example, then change one condition to test my understanding.
Tutoring approach:
Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.04 / Electrical instruments: meter ranges and bridge balances
A meter changes the circuit it joins
An ideal ammeter has zero resistance, but a converted movement has a finite series burden. Inserting it can reduce the branch current. The reading is the current in the circuit with the meter present, which may differ from the original undisturbed current.
An ideal voltmeter has infinite resistance, but a real voltmeter creates an additional parallel path. Its current can change node voltages, especially when the circuit’s own resistances are comparable to the meter’s input resistance.
To calculate loading, include the instrument as an ordinary resistor with its calibrated response. Do not first calculate the unloaded voltage and then assume the meter reads it unchanged. Solve the modified network.
A null measurement, such as a balanced bridge, can avoid a detector-current drop at the balance point. That advantage does not remove errors from lead resistance, imperfect ratios, heating or uncertainty in the standards.
WORKED EXAMPLE
Calculate the loaded reading of a divider
An ideal 12 V source drives two 5 kΩ resistors in series. A 5 kΩ input-resistance voltmeter is connected across the lower resistor. What does it read, and what was the unloaded divider voltage?
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Study with a tutor prompt
Copy this into your AI assistant, then describe where you got stuck. It asks the tutor to coach you one step at a time. Check its reasoning against the lesson.
Topic: Electrical instruments: meter ranges and bridge balances Focus: A meter changes the circuit it joins Learning goals: - Use coil resistance and full-scale current to design meter ranges. - Distinguish internal shunt/series connections from external meter placement. - Calculate finite-meter loading of a voltage divider. - Derive a Wheatstone balance from named midpoint potentials. - Use post-office-box ratios to set range and resolution. - Bracket a null and report the resulting resistance interval. - Account for lead, contact, temperature and ratio uncertainties. Equations in this section (LaTeX): Build the physical reasoning before choosing an equation. Start by checking the prerequisite idea I need most. Use a simple example, then change one condition to test my understanding. Tutoring approach: Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.
05 / Electrical instruments: meter ranges and bridge balances
Define the bridge connections before writing a ratio
A Wheatstone bridge has two resistor dividers connected across the same source. Define the source nodes as A at voltage V_s and D at zero. The left branch contains P from A to B and Q from B to D. The right branch contains R from A to C and unknown X from C to D. A galvanometer connects the midpoints B and C.
At balance, the galvanometer current is zero and B and C are at the same potential. Each side can then be treated as an unloaded divider: V_B = V_sQ/(P + Q) and V_C = V_sX/(R + X). Equating them gives PX = QR, or X = QR/P.
The ratio is tied to these labels. A differently labelled drawing may produce a visually different formula with the same physics. Deriving it from equal midpoint potentials prevents inverting the ratio by memory.
Balance means zero current in the detector branch, not zero current in the two source-fed resistor branches. Their currents can differ because their total branch resistances differ, even while their midpoint voltages agree.
| Element | Connected nodes |
|---|---|
| P | A to B |
| Q | B to D |
| R | A to C |
| X | C to D |
| Source | A to D |
| Null detector | B to C |
This ratio follows from the explicitly defined two-divider bridge. Do not transfer the symbol order to a differently labelled circuit without checking.
WORKED EXAMPLE
Verify a bridge balance by calculating both midpoint voltages
In the defined bridge, P = 100 Ω, Q = 200 Ω and the adjustable arm balances at R = 75 Ω. Find X. Then check both midpoint voltages for a 6 V source.
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Copy this into your AI assistant, then describe where you got stuck. It asks the tutor to coach you one step at a time. Check its reasoning against the lesson.
Topic: Electrical instruments: meter ranges and bridge balances
Focus: Define the bridge connections before writing a ratio
Learning goals:
- Use coil resistance and full-scale current to design meter ranges.
- Distinguish internal shunt/series connections from external meter placement.
- Calculate finite-meter loading of a voltage divider.
- Derive a Wheatstone balance from named midpoint potentials.
- Use post-office-box ratios to set range and resolution.
- Bracket a null and report the resulting resistance interval.
- Account for lead, contact, temperature and ratio uncertainties.
Equations in this section (LaTeX):
\frac{Q}{P+Q}=\frac{X}{R+X}\quad\Longrightarrow\quad PX=QR\quad\Longrightarrow\quad X=\frac QP R
Meaning: This ratio follows from the explicitly defined two-divider bridge. Do not transfer the symbol order to a differently labelled circuit without checking.
Start by checking the prerequisite idea I need most. Use a simple example, then change one condition to test my understanding.
Tutoring approach:
Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.06 / Electrical instruments: meter ranges and bridge balances
The post-office box implements selectable ratio arms and a resistance standard
A post-office box is a practical resistance bridge with selectable ratio arms P and Q and a known adjustable resistance R. The unknown X is connected in the fourth arm. With the connection convention above, a null gives X = (Q/P)R.
Changing Q/P changes the range and resolution referred to the unknown. If the resistance box changes in steps δR, the corresponding ideal step in X is (Q/P)δR. A small ratio can resolve a smaller change in X, but only if the balancing R remains within the available range and detector sensitivity is adequate.
The stated resistance of the box comes from its actual plug or switch convention. Some designs insert a resistance when a plug is removed; others use different arrangements. Read the instrument markings and circuit rather than assuming that every visible plug adds its printed number.
Begin with a configuration that limits detector current away from balance, then approach a null by controlled adjustments. In apparatus with separate source and detector keys, the source is commonly established before briefly connecting the sensitive detector, following the instrument’s procedure. The quantitative result is the reproducible null and the known arm values, not the size of a large off-balance deflection.
WORKED EXAMPLE
Translate resistance-box steps into unknown-resistance resolution
For the defined bridge, choose Q/P = 0.10 and suppose the known resistance R can be set in 1 Ω increments. A balance is found at R = 237 Ω. Find X and the unknown-resistance interval represented by one box step.
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Copy this into your AI assistant, then describe where you got stuck. It asks the tutor to coach you one step at a time. Check its reasoning against the lesson.
Topic: Electrical instruments: meter ranges and bridge balances Focus: The post-office box implements selectable ratio arms and a resistance standard Learning goals: - Use coil resistance and full-scale current to design meter ranges. - Distinguish internal shunt/series connections from external meter placement. - Calculate finite-meter loading of a voltage divider. - Derive a Wheatstone balance from named midpoint potentials. - Use post-office-box ratios to set range and resolution. - Bracket a null and report the resulting resistance interval. - Account for lead, contact, temperature and ratio uncertainties. Equations in this section (LaTeX): Build the physical reasoning before choosing an equation. Start by checking the prerequisite idea I need most. Use a simple example, then change one condition to test my understanding. Tutoring approach: Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.
07 / Electrical instruments: meter ranges and bridge balances
Bracket a null and distinguish a detector voltage from a loaded current
If an exact null falls between available settings, find neighbouring settings that give opposite detector directions. Under a monotonic local response and stable connections, the balance lies between them. Report the corresponding interval in X rather than inventing more digits than the adjustment supports.
The open-detector midpoint difference V_B − V_C is useful for predicting the sign of imbalance. With a finite-resistance galvanometer connected, the exact off-balance midpoint voltages and current require solving the loaded bridge. The null location remains the same because detector current vanishes there.
The graph below uses the open-detector voltage for the same P = 100 Ω, Q = 200 Ω, X = 150 Ω and V_s = 6 V example. It crosses zero at R = 75 Ω. It is not labelled as galvanometer current because that would additionally require the detector resistance and loaded-circuit calculation.
Near balance, repeat the adjustment from both directions and check whether the inferred null shifts. A persistent shift can indicate contact variation, source instability, heating or detector zero offset. A null method removes one loading mechanism at balance, not every experimental imperfection.
Bracket the bridge null
- Open-detector difference
- -1.1429
WORKED EXAMPLE
Report a balance interval instead of false precision
A post-office bridge uses Q/P = 0.10. Detector deflection changes sign between R = 237 Ω and R = 238 Ω, with no finer adjustment available. What interval is established for X under the ideal bridge model?
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Study with a tutor prompt
Copy this into your AI assistant, then describe where you got stuck. It asks the tutor to coach you one step at a time. Check its reasoning against the lesson.
Topic: Electrical instruments: meter ranges and bridge balances Focus: Bracket a null and distinguish a detector voltage from a loaded current Learning goals: - Use coil resistance and full-scale current to design meter ranges. - Distinguish internal shunt/series connections from external meter placement. - Calculate finite-meter loading of a voltage divider. - Derive a Wheatstone balance from named midpoint potentials. - Use post-office-box ratios to set range and resolution. - Bracket a null and report the resulting resistance interval. - Account for lead, contact, temperature and ratio uncertainties. Equations in this section (LaTeX): Build the physical reasoning before choosing an equation. Start by checking the prerequisite idea I need most. Use a simple example, then change one condition to test my understanding. Tutoring approach: Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.
08 / Electrical instruments: meter ranges and bridge balances
A balance removes detector current, not resistance errors elsewhere
Lead and contact resistance can add to the unknown arm in a two-terminal measurement. This matters most when X is small. A bridge may balance perfectly while measuring the unknown plus its leads. Appropriate lead compensation or a different measurement arrangement is needed if that contribution is significant.
Temperature can change the unknown and standard arms during a measurement. Keep test currents low enough for the intended thermal condition, make repeated readings and distinguish random detector uncertainty from a systematic resistance change.
For small worst-case relative bounds in X = QR/P, the conservative relative bound is δX/X ≈ δQ/Q + δR/R + δP/P. Independent statistical uncertainties would instead be combined through an appropriate quadrature model. State which interpretation the input values have.
A useful measurement record includes arm labels, selected ratio, balancing or bracketing resistance, detector behaviour, source condition and any lead or temperature assumptions. This is enough to reconstruct the inference and identify what the result actually measures.
WORKED EXAMPLE
Propagate a ratio measurement’s stated bounds
In X = QR/P, the stated small worst-case relative bounds are 0.2% in Q, 0.3% in P and 0.1% in R. Estimate the worst-case relative bound in X.
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Study with a tutor prompt
Copy this into your AI assistant, then describe where you got stuck. It asks the tutor to coach you one step at a time. Check its reasoning against the lesson.
Topic: Electrical instruments: meter ranges and bridge balances Focus: A balance removes detector current, not resistance errors elsewhere Learning goals: - Use coil resistance and full-scale current to design meter ranges. - Distinguish internal shunt/series connections from external meter placement. - Calculate finite-meter loading of a voltage divider. - Derive a Wheatstone balance from named midpoint potentials. - Use post-office-box ratios to set range and resolution. - Bracket a null and report the resulting resistance interval. - Account for lead, contact, temperature and ratio uncertainties. Equations in this section (LaTeX): Build the physical reasoning before choosing an equation. Start by checking the prerequisite idea I need most. Use a simple example, then change one condition to test my understanding. Tutoring approach: Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.
BRING THE IDEAS TOGETHER
What should stay with you
- The movement responds to its own coil current.
- An ammeter uses a parallel shunt internally and joins the measured branch in series.
- A voltmeter uses a series multiplier internally and joins the measured nodes in parallel.
- Finite input resistance can alter the quantity being measured.
- A bridge ratio is meaningful only with its arm labels and connections.
- A detector null means equal midpoint potentials, not zero current in all arms.
- Adjustment resolution and overall measurement accuracy are different.
- Lead and thermal errors can remain even at a perfect null.
RETRIEVE, THEN EXPLAIN
Try it without the examples
Work these out before checking. If an answer surprises you, identify which assumption or step needs another look. A correct choice is most useful when you can explain why the alternatives fail.
SELF CHECK 1
A movement has I_g = 1 mA and G = 100 Ω. What added series resistance gives a 5 V full-scale voltmeter?
Answer in Ω.
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SELF CHECK 2
For the defined bridge X = QR/P, use P = 200 Ω, Q = 100 Ω and balancing R = 80 Ω. Find X.
Answer in Ω.
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SELF CHECK 3
Why is the shunt connected in parallel with the galvanometer coil?
- To give the coil all of the external current
- To provide another current path at the same voltage
- To make the total ammeter resistance infinite
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SELF CHECK 4
At a balanced resistance bridge, which current is zero?
- Every branch current
- The detector current between the midpoints
- The source current necessarily
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SELF CHECK 5
Does a detector null automatically remove lead resistance in the unknown arm?
- Yes, every parasitic effect cancels at null
- No; the bridge can balance the unknown plus its leads
- Only if the detector is sensitive
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APPLY THE CHAPTER
Take the ideas into a problem
Try a complete problem next. Commit to a method before revealing a hint, then compare your reasoning with the worked solution.
Read the four bridge armsIntermediate · Hints and guided reasoningLeave the coil only its rated currentIntermediate · Hints and guided reasoningCOME BACK TO THE IDEA
What can you explain later?
Try these from memory at your next study session, then again a few days later. Say your reasoning aloud or sketch it before looking. Adjust the gap to what you can explain.
Next study session
Why should an ideal voltmeter have very large resistance and an ideal ammeter very small resistance?
Compare your reasoning
A voltmeter should draw negligible branch current; an ammeter should introduce negligible voltage drop in series. Both aims reduce disturbance of the original circuit.
Revisit the explanation →A few days later: change the situation
At balance in a bridge, does zero detector current mean that every branch carries zero current?
Compare your reasoning
No. It means the detector’s two terminals are at equal potential. Other branches can still carry current. A null result constrains a difference rather than declaring the whole circuit inactive.
Revisit the explanation →Compare the physical reason, the assumptions and the conclusion. Your wording can differ. If one part is missing, revisit that section and try explaining it again after a break.
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