By Carlos Mano Updated Mar 24, 2022
Electricity is the movement of electrons. Voltage is the electrical pressure that drives these electrons, while current is the flow rate—how many electrons pass a point each second. Resistance is the opposition that slows this flow. Ohm’s law ties them together: V = I × R.
The most straightforward measurement in a live circuit is voltage. With a multimeter set to volts, simply place the probes on the two points you want to compare and read the difference. Because voltage can be measured without disconnecting the circuit, it often serves as a gateway to calculate other parameters.
To find resistance, you must turn off the power and remove the component from the circuit. A true ohmmeter will read zero when the leads are shorted; if it does not, adjust the zero‑adjust knob until the display reads 0 Ω.
Measuring current is more involved. It requires the meter to be inserted in series with the circuit—effectively cutting a wire to place the meter. Once connected, the reading shows how many amperes flow through that point.
In a series arrangement, the current is identical through every component because there is only one path for electrons. Voltage, however, divides across the components proportionally to their resistance. For example, a 12‑V battery powering three 100‑Ω resistors in series yields a total resistance of 300 Ω. The current through the string is therefore:
I = V / R = 12 V / 300 Ω = 0.04 A (40 mA)
Changing the resistor values changes the current proportionally. If the series includes one 80‑Ω resistor and two 40‑Ω resistors (total 160 Ω), the current becomes:
I = 12 V / 160 Ω = 0.075 A (75 mA)
Parallel networks reverse the roles of voltage and current. Every branch experiences the same voltage, but the current splits, with larger currents flowing through lower‑resistance branches. This inverse relationship can be quantified: I_branch = V / R_branch.
Because the voltage is common, you can use the voltage measurement from Step 1 to compute the current in each branch if you know its resistance.
To obtain an accurate resistance reading, always zero‑adjust your ohmmeter before measuring. With the leads together, turn the zero knob until the meter reads 0 Ω.
Resistor tolerance affects the accuracy of your calculations. Typical tolerances are:
When using Ohm’s law to compute current, remember that this tolerance propagates into your final result.
