The M100-TAP measures the value of resistance on tap postition changers, typically used on high voltage transformers. Each postition on the selector has as equal value of resitance so that as the tap position is increased or decreased the value of resistance increases or decrease respectively. THe M100-TAP measure the value of the number of taps selected. The M100-TAP can also ne ised to measure the variable resistance 2 or 3 wire.