Abstract: A balance mechanism for automatically lifting known weights in a single-arm balance comprised of two plates, a first plate supporting the known weights and a pan for an unknown mass to be weighed and a second counterbalancing plate coupled to the first through a box lever having diagonal corners connected to a support member through flexible elements that will permit the box lever to pivot about a diagonal axis in response to a vertical force at one free corner connected to a corner of one balance plate or in response to a vertical force at the other free corner connected to a corner of the other balance plate. One balance plate is secured in space by two taut rectangular loop lines, one line being connected taut to diagonally opposite corners of the plate and to elements affixed to the support member in a position over the free corners of the plate, and the other line being connected taut to the free corners of the plate and to elements affixed to the support member in a position over the other two corners.