Every reading on a multimeter, clamp meter, insulation tester, or LCR meter traces back to one equation. If you truly understand Ohm's Law, you understand what every electrical tester is actually doing — and why each instrument exists.
What Is Ohm's Law?
Stated simply: the current through a conductor is directly proportional to the voltage across it, and inversely proportional to its resistance. It was established by German physicist Georg Simon Ohm in 1827 and became one of the bedrock laws of electrical engineering.
(Volts)
(Amperes)
(Ohms)
The OHM Triangle — Rearranging in Seconds
Engineers use a visual triangle to rearrange Ohm's Law on the fly. Cover the quantity you want to find — the remaining two show the calculation.
V = I × R
Cover V — multiply current by resistance to find voltage.
I = V ÷ R
Cover I — divide voltage by resistance to find current.
R = V ÷ I
Cover R — divide voltage by current to find resistance.
Figure — Ohm's Law triangle and rearrangements
The Physics Behind the Law
At the atomic level, voltage is the work done per unit charge to move electrons through a material. Resistance is the opposition to that movement — caused by collisions between electrons and the atoms of the conductor. The relationship is linear for ohmic materials (most metals at constant temperature): double the voltage, and the current doubles proportionally.
Ohmic vs non-ohmic devices
Diodes, transistors, and lightbulb filaments do not obey a straight-line V-I relationship — their resistance changes with temperature, current, or direction. This is why multimeters have a dedicated diode test mode that applies a controlled current rather than assuming fixed resistance.
How Ohm's Law Powers Every Electrical Tester
Multimeter — Resistance Mode
R = V ÷ IApplies a known test current (I) through the probes, measures the voltage drop (V), then calculates R = V ÷ I and displays it in ohms.
Multimeter — Current Mode
I = V ÷ RPlaces a precision shunt resistor (R_shunt) in-series. Measures the tiny voltage across it, then I = V ÷ R_shunt.
Clamp Meter
V ∝ I (transformer)The jaw acts as a current transformer. Secondary voltage is proportional to primary current via the turns ratio — Ohm's Law applied through electromagnetic coupling.
Insulation Tester (Megohmmeter)
R = V ÷ I (HV)Applies a high DC test voltage (500–2500 V), measures the tiny leakage current, then R = V ÷ I — in megohms.
Power and Ohm's Law — Watt's Law
Combine Ohm's Law with the definition of power (P = V × I) and you get three equivalent power formulas — together called Watt's Law:
P = V × I
Basic definition
P = I² × R
Substitute V = IR
P = V² ÷ R
Substitute I = V/R
Practical Examples
Checking a heater element
A 230 V heater rated at 2 kW. What current does it draw, and what resistance should it measure?
I = 2000 ÷ 230
I = 8.7 A
R = 230 ÷ 8.7
R ≈ 26.4 Ω
On resistance mode (heater disconnected), expect ~26 Ω. A reading far from this indicates a damaged element.
Diagnosing a voltage drop in a cable
Panel shows 230 V, motor terminals show 218 V — a 12 V drop at 20 A load. What is the cable resistance?
R = 12 ÷ 20
R = 0.6 Ω
Extra resistance above spec points to a loose connection or corroded joint — exactly what a milli-ohmmeter or multimeter can pinpoint.
The Limits of Ohm's Law
When Ohm's Law doesn't apply
Ohm's Law assumes constant temperature, a linear (ohmic) conductor, and DC or quasi-static AC. At high frequencies, inductance and capacitance introduce reactance (X) — you need complex impedance Z = R + jX, measured by an LCR meter rather than a simple multimeter.
Understanding which regime your circuit falls into tells you which instrument to reach for. CIE manufactures multimeters, insulation testers, LCR meters, and clamp meters — browse our full product range or contact our team to find the right instrument.