  A cosmic scalar field evolving very slowly in time can account for the
observed dark energy of the Universe. Unlike a cosmological constant, an
evolving scalar field also has local spatial gradients due to gravity. If the
scalar field has a minimal derivative coupling to electromagnetism, it will
cause modifications of Maxwell's equations. In particular, in the presence of a
scalar field gradient generated by Earth's gravity, regions with a magnetic
field appear to be electrically charged and regions with a static electric
field appear to contain electric currents. We propose experiments to detect
such effects with sensitivity exceeding current limits on scalar field
interactions from measurements of cosmological birefringence. If the scalar
field has derivative couplings to fermions, it would also generate observable
effects in precision spin precession experiments.
