  We present a detailed study of the static enhancement effects of electric
charges in micron-length single-walled carbon nanotubes, using theoretically an
atomic charge-dipole model and experimentally electrostatic force microscopy.
We demonstrate that nanotubes exhibit at their ends surprisingly weak charge
enhancements which decrease with the nanotube length and increase with the
nanotube radius. A quantitative agreement is obtained between theory and
experiments.
