Linear and Non Linear Effects on the Newtonian Gravitational Constant as   deduced from the Torsion Balance
  The Newtonian gravitational constant has still 150 parts per million of uncertainty. This paper examines the linear and nonlinear equations governing the rotational dynamics of the torsion gravitational balance. A nonlinear effect modifying the oscillation period of the torsion gravitational balance is carefully explored. 
Michele Rossi, Lorenzo Zaninetti