Resistor

A resistor is the simplest electrical component: a two-terminal device whose sole purpose is to obey Ohm's law with a specific, known, stable value of R. Resistors set bias currents, divide voltages, pull logic lines to well-defined states, absorb unwanted energy in circuit loads, and convert current to measurable voltage drops for signal acquisition.

The three common constructions tell the story. Carbon-film resistors, a thin layer of carbon-composite vapour-deposited onto a ceramic rod, cover 1 Ω to 10 MΩ at 5% tolerance for pennies apiece — the cheap general-purpose type. Metal-film resistors, with the carbon replaced by vapour-deposited nichrome or similar, push tolerance to 0.1% or better and noise to very low levels — the choice for precision analogue circuits and instrumentation. Wire-wound resistors, where a precise length of resistance wire is wound around a ceramic former, handle high power dissipation (tens to hundreds of watts) for dummy-load, motor-starting, and braking applications.

Every resistor is marked with its value (either color bands or printed digits), its tolerance (the worst-case deviation from the nominal), and its power rating (the maximum I²R it can sustain without burning up). Push a resistor past its rating and it becomes a one-shot fuse. Specialised variants include potentiometers (adjustable resistors with a sliding contact, for volume knobs and calibration trims), thermistors (resistance varies strongly with temperature, for temperature sensing), and varistors (resistance drops sharply above a threshold voltage, for surge suppression). All are two-terminal descendants of the same Ohm's-law principle.