Weighing delta(E)=c^2 delta(m) on a relativistic high-sensitivity   balance. Its mechanical and thermal equilibrium
  We consider the sticky collision between two sphere of equal rest mass, moving with equal but opposite speeds in a horizontal plane. The collision takes place on the pan of a high sensitivity balance. The conditions of mechanical and thermal equilibrium are studied showing that heat developed in the rest frame of the system has a corresponding inertial mass. 
Bernhard Rothenstein, Stefan Popescu, and George J. Spix