Electrical resistance is measured in Ohms (Ω), named after the German physicist Georg Simon Ohm. In practical electronics and electrical troubleshooting, you will rarely see just a bare Ohm value. Depending on the circuit, you will measure in milliohms (mΩ), kilohms (kΩ), or megohms (MΩ). Understanding these prefixes and knowing how to interpret the numerical readout on your digital multimeter (DMM) is the difference between diagnosing a faulty heating element in five minutes and chasing a ghost for three hours.
The Short Answer: Units of Electrical Resistance
The base unit of resistance is the Ohm. One Ohm is defined as the resistance between two points of a conductor when a constant potential difference of one volt applied to these points produces a current of one ampere. Think of it like water flowing through a pipe: voltage is the water pressure, current is the flow rate, and resistance is the diameter of the pipe or the debris clogging it. A narrower, clogged pipe (higher resistance) restricts flow.
Because real-world components span massive scales of resistance, we use metric prefixes. According to standard circuit theory fundamentals, here is how the units break down on your meter's display:
- Milliohms (mΩ): 1/1,000 of an Ohm. Used for measuring thick copper busbars, grounding straps, and current-sense shunts. (e.g., 0.005 Ω is displayed as 5.0 mΩ).
- Ohms (Ω): The base unit. Used for standard wire runs, heating elements, motor windings, and low-value resistors. (e.g., 220 Ω).
- Kilohms (kΩ): 1,000 Ohms. Used for standard electronic resistors, potentiometers, and thermistors. (e.g., 10,000 Ω is displayed as 10.0 kΩ).
- Megohms (MΩ): 1,000,000 Ohms. Used for measuring insulation integrity, open switches, and high-impedance sensor circuits. (e.g., 2,000,000 Ω is displayed as 2.0 MΩ).
Meter Setup and Probe Placement for Accurate Readings
Measuring resistance requires your multimeter to act as both a power source and a measuring device. The meter sends a small known current out of the red probe, through the component, and back into the black probe, calculating resistance using Ohm's Law (R = V/I). Because the meter supplies the voltage, the circuit under test must be completely de-energized.
Never measure resistance on a live circuit. Doing so can destroy your multimeter's internal fuse or blow the shunt resistor. However, accidents happen. If you accidentally touch live mains voltage while the dial is set to Ohms, your meter's Safety Category (CAT) rating is your last line of defense against an arc flash. For any residential or light commercial troubleshooting, use a meter rated at least CAT III 600V or CAT IV 600V (such as the Fluke 87V or Brymen BM235). Always verify the circuit is dead with the AC Voltage setting before switching the dial to Ohms.
Meter Setup Block
- Dial Position: Turn the rotary switch to the Ohms symbol (Ω). If your meter has a dedicated continuity mode (sound wave symbol), use that only for checking if a connection exists, not for measuring specific values.
- Lead Jacks: Plug the black lead into the COM (common) jack. Plug the red lead into the VΩ (or VΩmA) jack. Never use the high-current 'A' or '10A' jack for resistance measurements.
- Range Selection: If using an auto-ranging meter, simply touch the probes to the component. If using a manual-ranging meter, start at the highest range (e.g., 20MΩ) and step down until you get the most significant digits without the display showing 'OL' (Over Limit).
Probe Placement Technique
Isolate the component. If measuring a resistor on a PCB, desolder at least one leg; otherwise, you will measure the parallel resistance of the entire surrounding circuit. For a standalone component like a fuse or heating element, place one probe tip firmly on each metal terminal or leg. Polarity does not matter for standard resistance; swapping the red and black probes will yield the exact same numerical reading.
Expected Readings: Good vs. Bad Values by Component
Knowing what electrical resistance is measured in is only half the battle; you must know what the numbers actually mean for specific components. A reading of 15 Ω might indicate a perfectly healthy incandescent bulb, but a dead short in a control board trace. According to Fluke's official testing guidelines, context is everything.
| Component | Expected Good Reading | Bad Reading (Fail State) | Technical Notes |
|---|---|---|---|
| 120V 60W Incandescent Bulb | 15 Ω to 25 Ω (Cold) | OL (Open) or < 2 Ω | Tungsten has a high positive temperature coefficient. Cold resistance is low; operating (hot) resistance is ~240 Ω. |
| 240V Dryer Heating Element | 10 Ω to 20 Ω | OL (Burnt open) or 0.0 Ω | Calculated via P = V²/R. A 5000W element at 240V should read roughly 11.5 Ω. |
| Standard 5A Glass Fuse | 0.1 Ω to 0.5 Ω | OL (Blown) | Use the REL (Relative) mode to zero out your test lead resistance before checking low-ohm fuses. |
| Small AC Motor Winding | 2 Ω to 15 Ω | 0.0 Ω (Short) or OL (Open) | Check between the run and common terminals. Also check winding-to-ground (should be OL / >1 MΩ). |
| Copper Grounding Conductor | < 0.5 Ω (End-to-End) | > 1.0 Ω | High resistance in a ground path prevents breakers from tripping during a fault. Clean the lugs if reading is high. |
Common Mistakes That Give Misleading Resistance Readings
When you are trying to determine if a component is faulty, a misleading reading can send you down the wrong path. Here are the most frequent bench and jobsite errors that skew Ohm measurements:
1. The 'Finger Parallel' Error on High-Resistance Components
When measuring components in the kilohm or megohm range (like a 100kΩ pull-up resistor or an insulation barrier), do not touch the metal probe tips or the bare component leads with your bare fingers. The human body has a resistance ranging from 10kΩ (wet skin) to 100kΩ+ (dry skin). By holding both leads, you place your body's resistance in parallel with the component, artificially lowering the meter's reading. Hold the insulated probe shafts or use alligator clips.
2. Ignoring Test Lead Resistance on Low-Ohm Measurements
Standard multimeter test leads have an inherent resistance of about 0.2 Ω to 0.5 Ω, depending on wire gauge and banana plug contact quality. If you are measuring a 0.1 Ω current shunt, your meter will read 0.4 Ω, leading you to believe the shunt is faulty. To fix this, short the probe tips together, press the REL (Relative) or NULL button on your DMM to zero out the lead resistance, and then take your measurement.
3. Measuring In-Circuit (Parallel Path Skew)
Resistance in a parallel circuit is always lower than the lowest individual branch. If you measure a 1,000 Ω resistor while it is still soldered into a PCB alongside other components, the meter's test current will flow through all available parallel paths, giving you a falsely low reading. Always isolate the component by removing it or desoldering one leg.
4. Capacitor Charging Ramp-Up
If you place probes across a circuit that contains large capacitors, the resistance reading will start low and slowly climb until it maxes out at 'OL'. This is not a failing component; it is the multimeter's internal 3V battery charging the capacitor. Discharge all capacitors safely with a bleeder resistor before testing resistance in power supply circuits.
Frequently Asked Questions About Measuring Resistance
What is the difference between measuring resistance and continuity?
Continuity is simply a fast, binary resistance check. When you set your dial to the continuity symbol (usually an acoustic wave icon), the meter checks if the resistance is below a specific threshold—typically between 15 Ω and 30 Ω. If it is, the meter emits an audible beep. Use continuity to quickly verify if a trace is broken or a switch is closed, but use the Ohms (Ω) setting when you need the exact numerical value to diagnose a degraded component.
Why does my multimeter show 'OL' when measuring resistance?
'OL' stands for Over Limit (or Open Loop on some older models). It means the resistance between your two probe tips is higher than the maximum range your meter can measure, which is typically 20 MΩ or 40 MΩ on standard handheld DMMs. An 'OL' reading is the expected 'good' result when testing the insulation between a motor winding and its metal casing, or when checking a blown fuse.
Can I use a standard multimeter to measure very low resistance like a shunt?
Standard handheld multimeters struggle to accurately resolve values below 0.1 Ω due to the fluctuating contact resistance of the probe tips and banana jacks. To accurately measure milliohms (mΩ) for applications like battery pack busbars or PCB shunt resistors, you need a dedicated milliohm meter or a benchtop DMM that supports Kelvin (4-wire) measurement. Kelvin clips use two separate pairs of wires: one pair forces a known current, and the other pair measures the voltage drop, completely eliminating lead resistance from the equation.
What is electrical resistance measured in for insulation testing?
Insulation resistance is measured in Megohms (MΩ) or Gigohms (GΩ). However, you cannot test insulation reliably with a standard multimeter. A standard DMM only applies about 3V to 9V from its internal battery during an Ohms test, which is not enough voltage to reveal microscopic breakdowns in wire insulation. According to industry insulation testing standards, you must use a specialized tool called a Megger (Insulation Resistance Tester). These devices apply high DC voltages (typically 250V, 500V, or 1000V) to stress the dielectric material and measure the resulting micro-ampere leakage current, displaying the result in MΩ or GΩ.
