a writing instrument comprising a housing including a flexible diaphragm and a sidewall forming a cavity therewithin , the diaphragm serving to distort in response to a movement of the sidewall , a plurality of strain gages attached to selected locations on the diaphragm , each strain gage having an electrical resistance and being responsive to distortion of the diaphragm and operative to produce a change in its electrical resistance corresponding to the distortion at its respective location , a reservoir capable of holding writing fluid and formed to be carried within the cavity , the reservoir including external ribs for frictionally engaging the sidewall and having a shaft extending therefrom , the shaft having a distal end for writing on a surface and a passageway for supplying writing fluid to the distal end , the distal end being movable in response to contact with the surface , whereby when the shaft is moved , such movement is transmitted through the reservoir and the sidewall to the diaphragm causing it to distort , and whereby such distortion causes the electrical resistance of the strain gages to change , and a bridge circuit responsive to the change in electrical resistance of the plurality of strain gages and operative to produce a signal representative of the movement of the distal end .