Abstract: A position determining apparatus comprising a taut electrically-conductive member; a pulse generator to feed a pulse of electric current along the member; a magnet located at a variable first point along the member, for example on a float on the surface of a liquid the level of which is to be determined, to apply a magnetic field to the member at that point; and a transducer positioned at a second point along the member to sense the arrival at the second point of a mechanical impulse induced in the member by interaction between the current pulse and the magnetic field as the current pulse passes the magnet. A time measuring circuit responsive to the time interval between the feeding of the current pulse and the arrival of the mechanical impulse at the second point is used to determine the position of the first point.