What is "decade box"

In prototyping of electronic circuits, a decade box is a type of test equipment that can be used to substitute the interchanging of different values of certain passive components with a single variable output. Decade boxes are made for resistance, capacitance, and inductance, the values of which can be increased incrementally by way of switching the points of contact for the input and output throughout a series of the respective components. The interface for these devices will usually consist of dials or adjustable tape counters, and are operated in-circuit and without any external power source.