Englehard Industries Lab quality Silver-silver chloride reference cell with 100' test lead. This was the cadillac of all corrosion testing equipment and provides consistent and verifiable readings. It is far superior to the Pro Mariner unit I am also including. I would use the Pro Mariner on simple jobs but if I was involved with a serious corrosion claim I used the Englehard unit. In 1976 this probe cost over 0 because of the quality of the silver-silver chloride half cell. The Pro Mariner was about 0! If you are serious about measuring corrosion potentials, the Englehard probe is the best there is.