Abstract: An improved device for capacitively detecting signals generated in a writing table by a grid of conductive wires that produce a varying electrostatic field near its surface to be picked up through a stack of paper with the table functioning as one plate of a capacitor, and a conductive stylus as the other plate. The improvement consists of grounding the stylus, which has an extended body portion surrounded by a tubular member, extending the tubular member to a point very near the stylus tip where it functions as the second plate of a capacitor, and connecting the tubular member to a preamplifier mounted in the device.

Abstract: An improved apparatus capable of converting the position of a pen-type sensor relative to the surface of a writing table into an electrical signal representative of such position and which may be utilized for control, data input or transmission purposes. The writing table includes two sets of independent, spaced-apart conducting wires which form a grid throughout the writing area. The conducting wires are individually and discretely pulsed sequentially to generate a time varying electrostatic field adjacent to the writing table that can be sensed by the sensor. The signal sensed by the sensor and the signals used to drive the time-varying electrostatic field are electronically processed and converted into electrical signals representing the position of the sensor relative to the writing table.