decade

A decade is a set of precision resistors wired in a specific configuration that allows you to dial in resistance values in steps of 10 times a base unit. If your base resistor is 1 ohm, a decade box gives you 1, 10, 100, 1000 ohms and so on. Each step is selectable by a rotary switch or stepped switch, making it possible to set any resistance from 1 to 999,999 ohms (or whatever range the box covers) without building custom networks for each value.

The internal circuit uses series and parallel combinations of fixed resistors. A typical unit has multiple decades stacked together, each covering one order of magnitude. A five-decade box has five independent switches, one for each power of ten. When you turn the 10s dial to 3 and the 1s dial to 7, the box presents 37 ohms to your circuit. The resistors themselves are precision wirewound or metal film types, usually 0.1% or better tolerance, because the whole point is repeatability.

Standard uses and variants

Decade boxes are workhorses in laboratories, manufacturing test stations, and field service. Technicians use them to calibrate instruments, bridge unbalanced sensor outputs, or substitute known values in troubleshooting. Capacitor decades and inductance decades follow the same principle, giving you 0.1 to 9.9 microfarads or similar ranges. Some modern units are electronic, using relays or solid-state switches instead of mechanical dials, but the underlying function stays the same.

The name comes directly from the decimal structure. Ten units per stage, ten possible values per switch. The term has stuck since the mechanical era because it describes the architecture more clearly than "resistor selector" or "variable resistor" ever could. A variable rheostat lets you slide smoothly across a range; a decade lets you stop at exact, repeatable points.

Mechanical decades can wear. The switch contacts oxidize, developing contact resistance that corrupts the precision you paid for. Sealed units with gold-plated contacts hold up better, but nothing lasts forever. Electronic decades bypass mechanical wear but need stable voltage supplies and temperature compensation to avoid drift. Both types are aging out of everyday use as digital sources and arbitrary waveform generators handle more calibration work, yet they remain essential backup tools because they need no power, introduce no harmonics, and are impossible to fool.