In 1827, a German physicist named Georg Ohm published a paper describing the interrelationship between current, voltage, and resistance in circuits. The mathematical form of this relationship became known as Ohm's Law, which states that the voltage applied across a circuit is equal to the current flowing through the circuit times the resistance within the circuit, or:
Voltage = Current x Resistance
You can use this relationship to calculate the voltage across a resistor.
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Consider the resistor that you want to calculate the voltage across. Suppose, as an example, that you are considering a 4 Ohm resistor.
Measure the current passing through the wire in the circuit immediately after the resistor. Use a multimeter or ammeter to measure the current. Wire the multimeter or ammeter in series with the resistor in the circuit by cutting the circuit wire immediately after the resistor, then connecting the cut ends to the electrodes of the measuring device. For the example, suppose that the instrument indicated a current of 0.5 amps passing through the circuit after the resistor.
Plug the resistance and current values into the Ohm's Law equation to calculate the voltage across the resistor. The calculation for the example would look like this:
Voltage = 0.5 A x 4 Ohms = 2 V
There are 2 volts of voltage across the resistor in this example.