In 1994, Nina Allen provided the root hairs and Joe Kunkel provided the software, 3DVIS, which allowed the two investigators, working at the MBL National Vibrating Probe Facility, to apply a Calcium selective electrode, with electronics designed by Applicable Electronics, to measure the small current of calcium ions entering the root hair tip. The tip of the root hair is about 6Ám in diameter and the disc of inward conducting root hair cell membrane is about 4Ám in diameter. This dimension was estimated by using a 2Ám tip Ca selective electrode which was scanned at 4Ám steps across the tip of a root hair. At that time, it was the smallest structure so far resolved by the NVPi.
Subsequently, in 1997, Louis Cardenas, visiting Peter Hepler's lab at UMass has persued the role of calcium in the Vicia faba root hair response to the NOD factor and has used the vibrating Calcium selective electrode at the UMass Vibrating Probe Facility to demonstrate that both the area and the intensity of the Calcium current at the tip of the root hair are modulated by NOD factor (Cardenas, 1998 Dissertation; Cardenas et al., 1999). This study applied the principle of following the predictable rate of decline in apparent current eminating from a disc shaped source measured as one recedes along an axis from its center. The rate of decline is proportional to the radius of the disc source. The intensity of the source is an independent parameter also estimable. This methodology is one of the major quantitative strengths of this developing technology.