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In the field of electronics theory, the direct current, or DC circuit is usually the first circuit studied. A DC circuit consists of only a few of the most basic components: a source of electromotive force, a method of connecting components together and providing a path for electron flow, and a device that the electromotive force and electrons act upon. In a simple analysis such as this, this device is anything that resists the flow of electrons and is referred to as a "resistor."

The function of the battery is to provide a difference of electrical potential between two points. The electrical difference of potential, or electromotive force, is measured in terms of a Volt, named after an Italian physicist that first invented the chemical battery.

When the opposite poles of a battery are connected together by a conducting mechanism, such as a strand of copper wire, electrons will flow from the negative pole of the battery to the positive pole in an attempt to create electrical equilibrium. The rate of flow of the electrons through the conductor is termed current and is represented by the symbol "I".

According to Ohm's law, the rate of current flow "I" is proportional to the amount of electro-motive force "E", divided by the total resistance connected across its terminals. This is the most basic explanation of the behavior of electron flow, expressed in formula notation as I=E/R. Current flow is measured in Amperes, and is commonly referred to as "Amps."

The most familiar source of the electromotive force is the battery. Batteries come in different shapes and sizes ranging from the typical dry cell battery used in flashlights, cameras, portable radios, and calculators, to the 12-volt wet cell battery that starts up your car on demand. There are other types of batteries as well, such and the Lithium-ion battery and the Nickel Cadmium battery. These are typically used to provide low voltages in a low current situation.

Resistors are passive devices, meaning that they do not change their characteristics if the electromotive force or the current flow changes. Devices that do change are referred to as reactive devices and exhibit a property referred to as "reactance". Two such devices are capacitors and inductors and are most useful in alternating current (AC) devices. These devices store energy in electrostatic or electromagnetic fields, respectively.

A typical resistor is constructed from a small bar of carbon with a wire attachment at each end. The behavior of carbon

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