Multimeter and Circuit Lab Notes (Parts 1 & 2)

Lab Logistics and Policy

• You must obtain a lab manual from the bookstore for data analysis and lab work.

• Policy changed: manuals are now required purchases from the bookstore (even if the physics lab last year was department-provided).

• If you don’t have the manual today, you can write results on paper for this session.

• Group size: each group should have 3–4 students; reorganize the existing setup to form groups of three or four.

• There may be tables with different equipment configurations (some with two multimeters and a resistor, some with one). Start with whatever part is available, then switch as needed.

Measurement devices and basic terminology

• The two most important measurement devices introduced:

  • Multimeter (general instrument, can be configured to measure multiple quantities)

  • It can function as three different meters depending on configuration:

  • Ohmmeter for resistance measurement

  • Voltmeter for voltage measurement

  • Ammeter for current measurement

• Units:

  • Resistance: $R\,\text{in}\,\Omega$

  • Voltage: $V\,\text{in}\,\text{V}$

  • Current: $I\,\text{in}\,\text{A}$

• Names derived from the unit names:

  • Ohmmeter (for resistance)

  • Voltmeter (for voltage)

  • Ammeter (for current)

• Part identification: you will switch the meter’s function to measure R, V, or I during the lab.

Wiring conventions and safety notes

• Always start connections from the ground line, marked as COM. This is the reference point and never changes.

• For resistance measurements (ohmmeter): do not power the resistor; the resistor must be isolated from the power supply before measuring resistance.

• For voltage measurements (voltmeter): you measure across the component (in parallel with the component in the circuit).

• For current measurements (ammeter): you measure in series by breaking the circuit and inserting the ammeter between the two open points (A and B).

• Important wiring concepts:

  • There are dedicated jacks/leads:

  • COM (ground, common)

  • A jack for current (usually labeled A) used when measuring current (ampere range, e.g., 10 A or 20 A)

  • A jack for voltage/ resistance (often labeled V or Ω, depending on the range)

  • In practice, you usually use the large-current path (10 A range) and ignore the milliamp (mA) path for this course.

• Power supply considerations:

  • The lab power supply has three output pairs; use one pair at a time (red is positive, black is ground). The three pairs cannot be adjusted simultaneously.

  • The dial settings and the meter’s scale must match the measurement you are performing (voltage vs. current vs. resistance).

  • The meter’s dial has an orange portion for current (A, mA ranges); voltage-related scales (V, mV, etc.) span the portion labeled with V or mV; resistances use the resistance scales (Ω, kΩ).

• DC vs AC awareness:

  • Some scales are labeled for DC or AC; for this experiment, the power supply is DC and the measurement setup should use the DC scale unless otherwise instructed.

  • If you see a negative sign on voltage or current readings, it indicates reverse polarity; you can swap the output leads or reverse the connection to make it positive.

• Sequence of measurement on the bench:

  • For each measurement series, follow the V–I order: V1, I1, V2, I2, V3, I3. Do not mix orders (for example, V1, V2, V3, I1, I2, I3) because keeping I1 with V1 ensures the same voltage for the current measurement.

Part 1: Resistance measurement (ohmmeter)

• Objective: measure the resistance of a resistor using the ohmmeter setting.

• Setup prerequisites:

  • Isolate the resistor from the power supply (no power connected).

  • Connect the ohmmeter leads directly across the two ends of the resistor (not across any power source).

• Steps:

  • Set multimeter to resistance mode (Ω range).

  • Connect the black lead to COM and the red lead to the resistance measurement jack (as per your manual).

  • Place the leads on the two terminals of the resistor.

  • Read the resistance value; expected to be around the order of tens of ohms in this example (e.g., around 10 Ω as mentioned by the instructor).

• Notes:

  • If the value is unstable or out of range, switch to a higher or lower Ω range (e.g., 200 Ω or 2000 Ω) depending on the expected magnitude.

• Important caution: do not apply power to the resistor while measuring resistance.

Part 1: Voltage measurement (voltmeter)

• Objective: measure the voltage across a resistor in a powered circuit.

• Setup prerequisites:

  • Power supply is turned on; connect the resistor to the power supply circuit.

  • The voltmeter is connected in parallel with the resistor (across the two nodes of the resistor).

• Steps:

  • Turn on the DC power supply and adjust voltage output. The display in the lab shows the supply's voltage on a dial and scale.

  • Choose the appropriate voltage range on the voltmeter based on the expected voltage (range indicators show which scale to use; ranges may include small millivolt to several volt ranges).

  • If the voltage reads negative, switch the polarity of the output to obtain a positive reading.

  • Record the voltage reading V1 corresponding to the first voltage setting.

• Important notes on range and DC/AC:

  • Confirm you are using the DC measurement scale for this DC power supply setup. The AC scale is separate and should not be used here.

  • The power supply can be set so that the output becomes DC after regulation; the meter will reflect the DC value.

• Data handling:

  • Keep track of which voltage range you used to obtain V1, and relate the reading to the current measurement I1 (see Part 1: Current for the sequence).

Part 1: Current measurement (ammeter)

• Objective: measure current through the resistor by inserting the ammeter in series.

• Procedure (two volunteers recommended):

  • First, turn off or disconnect the voltmeter so you can safely modify the circuit.

  • Break the circuit at the resistor connection (two open points A and B).

  • Attach the ammeter in series by connecting one lead to point A and the other lead to point B, forming a single loop that includes the resistor, the ammeter, and the power supply.

  • Ensure the ammeter is set to the large-current range (e.g., 10 A or 20 A) to match the expected current. Use the DC current range for this experiment.

  • Reconnect the circuit and observe the ammeter reading I1. The current path must be continuous through the ammeter; you should not bypass or short out the ammeter.

• Data collection sequence:

  • With V1 established, record I1 (I1 is measured with voltage V1 held constant).

  • After completing I1, adjust the voltage to V2 and record I2, then adjust to V3 and record I3.

  • Maintain the same voltage value (V1) while taking I1, then change to V2 for I2, and V3 for I3 as needed.

• Practical tips:

  • If a measurement seems off, re-check the series connection and ensure the ammeter is in series (not in parallel) for current measurements.

  • The orange portion of the dial is associated with current ranges; ensure you select the appropriate scale for the current you expect.

• Data reporting:

  • Record V1, I1, V2, I2, V3, I3 in order as part of your experiment data sheet.

Part 2: Charging and discharging with a galvanometer (galvanometer-based readout)

• Objective: observe charging and discharging behavior and read the galvanometer (D value) and associated voltage (V value).

• Setup:

  • Use the charging side of the circuit; then discharge through the galvanometer.

  • Observe the galvanometer’s deflection (D value) as the device charges and discharges.

• Procedure and ranges:

  • Use the three ranges on the instrument. For range 1, aim for a deflection D in the interval roughly between 15 and 25 (range-specific). Record D1 and V1 (the corresponding voltage reading).

  • Move to the second range to see the voltage corresponding to the same or a different deflection (D2 and V2). For example, you may switch to a higher range (e.g., up to the 200 range) and observe D and V readings. If the meter reads near zero on a higher range, switching ranges can help obtain a measurable value.

  • Repeat for the third range (D3 and V3) as needed.

• Practical notes:

  • If the reading is zero on a higher range, switch to a more suitable range to obtain a measurable D value.

  • As with Part 1, ensure you do not exceed the scale limits of the galvanometer when discharging or charging.

• Data interpretation:

  • For each range, record the corresponding D and V values (D1, V1; D2, V2; D3, V3) to analyze how the galvanometer deflection relates to the charging/discharging process.

Data recording and sequence rules

• Always record in the order: V1, I1, V2, I2, V3, I3 for Part 1 (to keep the same voltage when measuring current).

• For Part 2, record D and V for each range as described above (D1 with V1, D2 with V2, D3 with V3).

• If readings are negative due to polarity, switch the output leads to obtain positive values where required.

• In all parts, cross-check measurements against the expected ranges and ensure the readings fall within the meter’s scale.

Practical considerations, troubleshooting, and ethics

• Safety and lab conduct:

  • Do not force connections; break circuits gently when inserting the ammeter.

  • Do not power a resistor while measuring resistance.

  • Always verify that the ground/COM lead is properly connected before making measurements.

  • Seek two volunteers for the ammeter insertion step to minimize risk of miswiring and to ensure proper handling.

  • Keep your work neat and labeled; sign your own data entries; the instructor will review and sign the work.

• Real-world relevance:

  • Understanding how multimeters are configured for resistance, voltage, and current is foundational for any electronics work.

  • Correct wiring practices prevent component damage and ensure accurate measurements.

  • DC vs AC considerations are important when selecting ranges and interpreting readings in real experiments.

Notation and quick reference formulas

• Ohm's law (core relationship for this lab):

  • $V = IR$

  • $R = rac{V}{I}$

• Typical measurement relationships in this lab (conceptual):

  • Ohmmeter measures $R$ with the circuit unpowered: that is, $V = 0$ across the resistor during measurement.

  • Voltmeter measures across a component in parallel: $V<em>{ ext{across}} = V</em>{ ext{source}}$ in the measured branch.

  • Ammeter measures in series: current is the same through the series path, $I<em>{ ext{through}} = I</em>{ ext{resistor}}$.

• Range conventions (example values mentioned):

  • Resistance ranges: $200\,\Omega\quad \text{and} \quad 2000\,\Omega$ (illustrative from the session).

  • Current range: up to $10\,\text{A}$ (large-current setting) for the ammeter.

  • Voltage ranges: scale indicators mention millivolts and volts (e.g., $200\,\text{mV}$, $200\,\text{V}$, etc.), with DC vs AC distinctions.

Appendix: Key terms to remember

• COM: common ground reference for the multimeter leads.

• Ohmmeter: multimeter setting used to measure resistance.

• Voltmeter: multimeter setting used to measure voltage.

• Ammeter: multimeter setting used to measure current (must be in series).

• DC vs AC: Direct current vs alternating current; ensure correct scale is used for the lab’s power supply.

• Galvanometer: device used to measure small electrical currents, used in Part 2 to observe charging/discharging behavior.

Quick glossary of procedures you should be able to perform

• Identify and set the multimeter to the correct mode (Ω, V, I) for the task.

• Connect COM and the appropriate leads for the chosen measurement.

• Wire a circuit for resistance measurement by disconnecting power and placing the meter across the resistor.

• Wire a circuit for voltage measurement by placing the voltmeter in parallel with the resistor in a powered circuit.

• Wire a circuit for current measurement by breaking the circuit and inserting the ammeter in series.

• Interpret readings within the correct range and polarity; switch ranges if needed to obtain measurable values.

• Record data in the required order (V1, I1, V2, I2, V3, I3) and (D1, V1; D2, V2; D3, V3) for Part 2.

• Safely sign off the lab work and ensure all data are properly attributed to your group.