Positioning a cursor on the display screen of a computer

A computer system comprises a keyboard ( 11 ), a display screen ( 13 ), a first pair of position-sensing electrodes ( 18.1, 18.2 ), a second pair of position-sensing electrodes ( 20.1, 20.2 ), a signal injection electrode ( 22 ), and an oscillator ( 27 ). The oscillator injects a signal via the signal injection electrode and the operator's left hand (L) into the operator's body, and this creates a field around the operator's right hand (R). The position-sensing electrodes are arranged underneath the keyboard and sense the strength of the field. This enables the position of the operator's right hand (R) in an X-Y plane above the keyboard to be determined. To this end the position-sensitive electrodes are connected via synchronous detectors ( 34, 44 ) and an analogue-to-digital converter ( 36 ) to a microprocessor ( 46 ), which operates to control the position of a cursor on the display screen.