The inverse of the smallest Lyapunov exponent is the localisation
length 
. The renormalised length 
is
found to obey a scaling theory(MacKinnon & Kramer 1981, MacKinnon & Kramer 1983) such that
which has solutions of the form
where 
is a characteristic length scale which can be identified
with the localisation length of the insulator and which scales as the
reciprocal of the resistivity of the metallic phase(MacKinnon & Kramer 1983).
In 3D (6) always has a fixed point 
which corresponds
to the metal-insulator transition. The behaviour close to the
transition can be found by linearising (6) and solving to
obtain
where 
is the disorder 
or 
, 
and 
represent the critical 
and disorder respectively, and 
and
are constants. By comparing (7) and (8) an
expression for can be obtained in the form
so that the localisation length exponent 
is given by 
. Since it is well known(Wegner 1976, Abrahams et al. 1979) that the
conductivity exponent 
is related to by 
then by
fitting (8) to the data and calculating 
both exponents
can be obtained.
