Electric Field Sensor systems (see below) enable intuitive man-machine-interaction based on natural gestures. The MARS Bag is another electric field sensor device. Carried like handbags, MARS Bags recognise the movements of the carriers' body and the surrounding people and reacts with sound. Once it is touched it will be personalized by a biosign, e.g. fingerprints. It is a wireless wearable device, powered by a small solar panel. MARS Bags function as a prototype for wearable mobile interaction and communication. They initiate a non-standardized communication by signifying encounters acoustically. MARS Bags do not substitute communicational practice. Rather, they are meant to initiate and comment communication between passers-by.
The principles of electric field sensing (EFS) were initially used in a musical instrument called Theremin, invented in the 1920s. A performer playing this instrument is given the opportunity to affect the pitch and the volume of the sound by moving his/her hand in the vicinity of two antennas. The principles of electric field sensing are based on detecting the change of characteristics of alternate electrical current induced by the movement of the user's hand(s) in the vicinity of several electrodes/antennas which constitute the complex electrical circuit.