This method has been modified and commercialized as the Apit, which calculates the difference between two potentials of the root canal with composite sine wave current sources with two different frequencies. This device can make an accurate measurement of the root canal length, even if a strong electrolyte is in the canal. 13 However, the Apit must be calibrated (zero adjustment) at several millimeters from the apical foramen in each individual root canal to eliminate the discrepancies of the root canals.