  We report a systematic study of charge distribution dependency of CMS
Resistive Plate Chamber (RPC) on gap thickness. Prototypes of double-gap RPCs
with six different gap thickness ranging from from 1.0 to 2.0 mm in 0.2-mm
steps have been built with 2-mm-thick phenolic high-pressure-laminated plates.
The efficiencies of the six gaps are measured as a function of the effective
high voltages. We report that the strength of the electric fields of the gap is
decreased as the gap thickness is increased. The distributions of charges in
six gaps are measured. The space charge effect is seen in the charge
distribution at the higher voltages. The logistic function is used to fit the
charge distribution data. Smaller charges can be produced within smaller gas
gap. But the digitization threshold should be also lowered to utilize these
smaller charges.
