  Stability arguments suggest that the Kaluza-Klein (KK) internal scalar field,
$\Phi$, should be coupled to some external fields. An external bulk real scalar
field, $\psi$, minimally coupled to gravity is proved to be satisfactory. At
low temperature, the coupling of $\psi$ to the electromagnetic (EM) field
allows $\Phi$ to be much stronger coupled to the EM field than in the genuine
five dimensional KK theory. It is shown that the coupling of $\Phi$ to the
geomagnetic field may explain the observed dispersion in laboratory
measurements of the (effective) gravitational constant. The analysis takes into
account the spatial variations of the geomagnetic field. Except the high PTB
value, the predictions are found in good agreement with all of the experimental
data.
