Fundamentals of Electrical Practice: Identification and Measurement of Resistors

Fundamental Skills Training in Marine Electronics and Electrical Systems

I. Basic Concepts of Resistance

Resistance (R) is a physical quantity that represents the degree to which a conductor impedes the flow of electric current. The greater the resistance of a conductor, the greater its impedance to current flow. Different conductors typically exhibit different resistances, as resistance is an inherent property of the conductor itself. The unit of resistance is the ohm, abbreviated as Ω.

Figure 12-1: Resistor Shapes

II. Steps for Measuring Resistance (Analog Multimeter)

1
Select magnification:Select a resistance to block any multiplier. Standard multimeter ranges include R×1, R×10, R×100, R×1k, and R×10k.

2
Zeroing by Ohm's Law:Short-circuit the red and black test leads, then adjust the ohm-scale zero knob until the pointer indicates zero on the ohm scale.

3
Access Measurement:Connect the resistor under test to the red and black test leads to measure its resistance value. Adjust the multiplier based on the meter needle deflection to keep the needle near the center of the dial.After changing the magnification, you must reset the zero.

4
Reading Calculation:Resistance readings are generally expressed with two significant figures, read from right to left.
Actual resistance measurement = Reading × Multiplier。

12-2 Multimeter Resistance Measurement Diagram

III. Precautions for Resistance Measurement

Key Points on Safety and Accuracy

(1) Scale Characteristics:The resistance scale's markings are reversed relative to other scales, with its “0” point positioned to the right of the scale and markings applied unevenly.

(2) Strictly prohibited from carrying electricity:Do not measure resistance with power on. Large capacitors in the circuit require discharge after power is disconnected; if parallel branches exist, at least one end of the resistor must be disconnected from the circuit.

(3) Zero Adjustment Specifications:Zero the instrument before each measurement. Re-zero after switching each range.

(4) Reading position:Try to keep the hands pointing near the center of the dial.

(5) Avoid human parallel connection:Avoid incorporating body resistance (do not touch the metal parts of both test probes with your fingers simultaneously).