Practical Skills and Laboratory Applications
The practical domain checks whether you can build a circuit from a diagram, take readings correctly, and interpret them. It rewards careful method more than rote formulas.
Core experiments
- Verification of Ohm's law: vary the supply, record V and I, plot V against I; the slope gives R.
- Series and parallel networks: measure the equivalent resistance and compare with calculation.
- Measurement of resistance by the ammeter–voltmeter method: R = V/I.
- Power measurement with a wattmeter; energy measurement and calibration check of an energy meter by counting disc revolutions.
- Load test of a DC shunt generator to obtain its characteristics; speed control of a DC shunt motor by field and armature control.
- Insulation resistance test of a wiring installation with a megger.
Good laboratory method
Select a range above the expected value, check the zero of analog meters, connect with the supply off, have the circuit checked before switching on, and record every reading with its unit. Read analog scales with the eye directly above the pointer to avoid parallax error.
Worked example
In an ammeter–voltmeter test, the voltmeter reads 2.4 V across a coil and the ammeter reads 0.4 A. R = V/I = 2.4/0.4 = 6 Ω. If the true value is 5.9 Ω, the percentage error is (6 − 5.9)/5.9 × 100 ≈ 1.7%, which is acceptable for this method; connecting the voltmeter across both the coil and the ammeter would add the ammeter's small resistance and inflate the result slightly. Note that this method suits medium resistances; for very high resistances the voltmeter's own resistance causes serious error.
Traps
- Selecting a range below the expected reading and damaging the meter.
- Reversing the ammeter's polarity or ignoring the marked terminals.
- Omitting units or not mentioning the least count of the instrument.
- Recording a single reading and calling it verified; repeat and average.
- Forgetting that the wattmeter has four terminals and two coils.