The Collaborative International Dictionary of English v.0.48:

Steelyard \Steel"yard\ (st[=e]l"y[aum]rd; colloq.
   st[i^]l"y[~e]rd; 277), n. [So named from a place in London
   called the Steelyard, which was a yard in which steel was
   sold.]
   A form of balance in which the body to be weighed is
   suspended from the shorter arm of a lever, which turns on a
   fulcrum, and a counterpoise is caused to slide upon the
   longer arm to produce equilibrium, its place upon this arm
   (which is notched or graduated) indicating the weight; a
   Roman balance; -- very commonly used also in the plural form,
   steelyards.
   [1913 Webster]

WordNet (r) 3.0 (2006):

steelyard
    n 1: a portable balance consisting of a pivoted bar with arms of
         unequal length [syn: steelyard, lever scale, beam
         scale]