Standard design underground reference electrodes, also known as bagged electrodes, are the original reference electrode design for permanent underground applications. They remain in widespread use today because they have proven long term durability. The reference electrode is contained in a cotton bag filled with bentonite-gypsum backfill. Since the entire surface of the bag is in contact with the surrounding soil, there is very low earth contact resistance. This is the primary reason that bagged electrodes are the preferred design for use in high resistance semi-dry soils or locations with seasonal dry periods.