the surface 20a of substrate 20 from the top of raised portion 22v as shown in Figure l(d)-I. Because of the work function difference between skin and the material of the substrate (e.g. silicone rubber), physical contact between the two dielectric surfaces with differing electron affinities creates opposite charges on the respective surfaces. There are positive charges on the surface of the finger and negative charges on the surface 22a of the substrate 20. As the finger F passes over the raised portion 22v and approaches surface 20a, the positive charges on finger F gradually balance the negative charges on surface 20a. Therefore, different amounts of charge are forced to flow from each electrode terminal El and E3 to ground via respective resistors R1 and R3, thus generating currents while the finger F approaches the first contact point 20al on surface 20a. When the finger F contacts the surface 20a of substrate 20 (Figure l(d)-II), the electrical field is confined to the space between the finger surface and substrate 20 so that the electrode (El, E3) voltage will not be affected by the charges at the touch point 20al. When the finger F separates from surface 20a and moves far enough away from the first contact point 20a 1 on substrate 20 (Figure l(d)-III), the negative charges on the surface 20a of substrate 20 will induce opposite charges on the electrode terminals (El, E3) again. Therefore, currents with different magnitude from ground to each electrode terminal will be formed while the finger F moves away. When the finger F reaches the top of the next raised portion 22v of the grid structure 22, the maximum charge on the electrodes El and E3 is reached (Figure l(d)-IV). When the finger F crosses over the grid 22 and approaches the second contact point 20a2 on surface 20a (Figure l(d)-V), the charges on the two electrode terminals El, E3 will be repelled and forced to flow to ground. Therefore, a negative current will be formed, and the above process and phenomenon are repeated as the finger traverses the grid 22. Based on the different current and voltage output of the four electrodes E1-E4, the location of contact can be determined. Because of the way that the grid 22 is configured, the continuous sliding trajectory and movement of the finger F on the sensor surface 20a are recorded by continuous pixel positions, and thus motion parameters (trajectory, speed and acceleration) of the finger F can be inferred. 

