Resistor
A resistor is a component of an electric circuit that resists the flow of current and protects delicate electronic equipment from surges. The resistor allows a specific amount of current to pass, but inhibits a high voltage spike that could cause damage to other components. Most resistors are of a fixed value, but others, called a rheostat, are tunable.
In 1827, Georg Simon Ohm (1789-1854) experimented with resistance and found that the amount of current which was transmitted through a conductor was inversely proportional to the length and directly proportional to the thickness of the conductor. This relationship between the amount of current I, the electric potential V, and resistance R became known as Ohm's Law, famously written asV=IR. Today, in Ohm's honor, the value of resistance is measured in ohms. ( Henry Cavendish (1731-1810) discovered this same relationship nearly 50 years earlier, but he failed to publicize it and Ohm received the credit.)
As the electric current passes through a resistor, the energy is reduced and dissipated as heat, as the charge-carrying electrons collide with small imperfections in the atomic structure of the metallic conductor. Power levels are varied to prevent the heat from burning up the resistor. They range from fractions of a watt, for use in transistor radios, to kilowatts, needed in electrical power plants.
In 1850 temperature sensitive nonlinear resistors, known as thermistors, were invented. In thermistors, changes in resistance are induced by a change in temperature. Today, because of the high demand for resistors in electronic circuits, they are mass produced from simple ingredients at very low cost. Carbon composition resistors, made of a mixture of powdered carbon black and an inert filler, are among the most common. The ratio of the two ingredients determine the value of the resistor. A liquid resin is added and the mixture is pressed into rods and molded with a copper conductor at each end. After the resistor is tested, bands of paint, corresponding to a standard color code indicating value, are applied.
While carbon composition resistors are adequate for most tasks, they have a poor tolerance and are not suited for high precision applications. For greater precision, a thin film of resistive material, such as carbon or tin oxide, is deposited onto a ceramic base. For very high precision, an extremely thin film of nickel and chromium is used. Because of its thermal properties, the thin film resistor can dissipate more power than a carbon resistor of the same size. These more expensive resistors can also be adjusted to a very small tolerance by cutting a grove in their surface or adding a thin resistance wire, giving stability even at high temperatures.
Tunable resistors, such as rheostats or potentiometers (or pots) make use of a sliding contact. Turning a screw causes the contact to move along the resistive film or wire, changing its value.
In modern electronics individual resistors have been completely replaced by the integrated circuit (IC). Integrated circuits are inexpensive to make, easy to mass produce, and can incorporate many components on a single board. Tiny resistive elements on an IC are created using very small amounts of metals or semiconductors, carefully controlled.
This is the complete article, containing 521 words
(approx. 2 pages at 300 words per page).
