  A vast class of disordered conducting-insulating compounds close to the
percolation threshold is characterized by nonuniversal values of transport
critical exponents. The lack of universality implies that critical indexes may
depend on material properties such as the particular microstructure or the
nature of the constituents, and that in principle they can be influenced by
suitable applied perturbations leading to important informations about the
origin of nonuniversality. Here we show that in RuO$_2$-glass composites the
nonuniversal exponent can be modulated by an applied mechanical strain,
signaled by a logarithmic divergence of the piezoresistive response at the
percolation threshold. We interpret this phenomenon as being due to a
tunneling-distance dependence of the transport exponent, supporting therefore a
theory of transport nonuniversality proposed some years ago.
