The position of the finger F as a function of time relative to electrode pairs (El, E3) and (E2, E4) can be determined according to the ratio of the potentials at respective electrodes of the pair, i.e. ( / EI/ E3) and I/ ) . This is illustrated in Figure 1(e). During the process of the finger sliding or touching along the device surface, the electrical fields on the opposite electrodes change accordingly. If the electric potential of ground and infinity is assumed to be 0 V, then the electric potential of a point charge can be written as: where Q is the amount of charge, r is the distance to the point charge and k is Coulomb’s constant. The distance between two opposite electrodes (El and E3) is assumed to be l . After contact with the silicone rubber surface, the finger with charge of + Q moves away from the silicone rubber surface to a distance of h . The touch point on the silicone rubber surface is with charge of - Q correspondingly. If the distance between the touch point and El is x , then the distance between the touch point and E3 is l-x. Thus, the electric potentials of El and E3 ( / EI and / $_{E3}$) can be expressed as: Their ratio can then be derived as: