What instrument is installed in an electrical circuit to vary the resistance?
Electrical ballast Electrical ballast An electrical ballast is a device placed in line with the load to limit the amount of current in an electrical circuit. It may be a fixed or variable resistor. A familiar and widely used example is the inductive ballast used in fluorescent lamps to limit the current through the tube, which would otherwise rise to a destructive level due to the negative differential resistance of the tube's voltage-current characteristic. Ballasts vary greatly in complexity. They may be as simple as a resistor, inductor or capacitor (or a combination of these) wired in series with the lamp; or
Electrical resistance survey used on hard or very dry surfaces that preclude electrical contact necessary for probe resistance systems. While these show promise for archaeological applications, currently available systems operating on this principle lack sufficient spatial resolution and sensitivity. Survey usually involves walking with the instrument along closely spaced parallel traverses, taking readings at regular intervals. In most cases, the area to be surveyed is staked into a series of square or rectangular survey "grids" (terminology can vary). With the corners of the grids as known reference points, the instrument operator uses tapes or marked ropes as a guide when collecting data. In