the direct current equipment often includes a coil (called a "reactor") that adds inductance in series with the dc line to help smooth the direct current. the inductance typically amounts to between 0.1 h and 1 h. the smoothing reactor can have either an air-core or an iron-core. iron-core coils look like oil-filled high voltage transformers. air-core smoothing coils resemble, but are considerably larger than, carrier frequency choke coils in high voltage transmission lines and are supported by insulators. air coils have the advantage of generating less acoustical noise than iron-core coils, they eliminate the potential environmental hazard of spilled oil, and they do not saturate under transient high current fault conditions. this part of the plant will also contain instruments for measurement of direct current and voltage.