One of the most frequently applied relationships in current electricity is known as Ohm's law. This relationship, discovered by the German physicist Georg Ohm (1787-1854) is basic in the analysis of electrical circuits. Ohm's law applies to a wide range of materials used as electrical conductors. For an "ohmic" conductor, the potential difference (= voltage drop) V across the conductor is linearly proportional to the current I through the conductor: V/I = R, where R is called the resistance of the conductor and measured in units of ohms.

To understand the relationships of the quantities in Ohm's law, it is often helpful to consider a water current analogy.

In a water circuit, the force to move the water is supplied by a pump. The rate of water flow depends on the resistance to the flow (e.g. due to some obstruction in the circuit pipe) - the greater the resistance, the less water flow.

Analogously, in an electrical circuit, a voltage source (e.g. a battery or power supply) supplies the (electromotive) force for current flow and the magnitude of the current is determined by the resistance R of the circuit. For a given voltage, the greater the resistance, the less current passes through the resistor, as may be seen from Ohm's law: I = V/R. Notice that source supplies a voltage "rise" that is equal to the voltage "drop" across the resistor and is given by V = I R.

Consider the circuit diagram shown below

The applied voltage is supplied by a power supply or battery. An ammeter
measures the current in the circuit and in the resistor R,
whereas a voltmeter
registers the voltage drop across R. The switch is used for closing (activating) and opening (deactivating) the circuit.

Any component in a circuit that does not generate or supply a voltage acts as a resistance in the circuit. This is true for the connecting wires, the ammeter, and voltmeter. However, the metallic connecting wires and the ammeter have negligibly small resistances, thus they do not significantly affect the current. Also, a voltmeter has a high resistance, so very little current flows through the voltmeter. Hence, to good approximations the ammeter registers the current in the circuit and the voltmeter reads the voltage drop across the resistance. These approximations are adequate for most practical applications.