Ohm's Law

The unit of resistance, the ohm, is named after the German scientist Georg Ohm (1789–1854). It was Ohm who first determined how resistance and current affect voltage. He discovered that voltage is not the same everywhere in a circuit. Ohm hypothesized that resistance reduces the voltage. He published his research in 1826, but his findings were so controversial that he lost his job. Eventually his work became widely accepted.

Ohm found a mathematical relationship between voltage, current, and resistance. This relationship became known as Ohm's law. According to Ohm's law, the voltage (V) in a circuit equals the product of the current (I) and the resistance (R).

Figure 10 A multimeter can be used to measure current, voltage, or resistance. Here the voltage of a 9-volt battery is measured.

When the current is in amperes and the resistance is in ohms, the voltage is in volts. What is the voltage if the resistance is 3 ohms and the current is 3 amps?

$V =\mathit{ }I\mathit{ }\times R\mathit{ }= 3 \mathrm{amps} \times 3 \mathrm{ohms} = 9 \mathrm{volts}$

Increasing the voltage increases the current. Keeping the same voltage and increasing the resistance decreases the current. A multimeter, shown in Figure 10, is a device used to measure current, voltage, and resistance.