▁system ▁but ▁with ▁different ▁further ▁sub div isions . ▁ ▁Natural ▁units ▁Natural ▁units ▁are ▁physical ▁units ▁of ▁measurement ▁defined ▁in ▁terms ▁of ▁universal ▁physical ▁constants ▁in ▁such ▁a ▁manner ▁that ▁selected ▁physical ▁constants ▁take ▁on ▁the ▁numerical ▁value ▁of ▁one ▁when ▁expressed ▁in ▁terms ▁of ▁those ▁units . ▁ ▁Natural ▁units ▁are ▁so ▁named ▁because ▁their ▁definition ▁re lies ▁on ▁only ▁properties ▁of ▁nature ▁and ▁not ▁on ▁any ▁human ▁construct . ▁Vari ous ▁systems ▁of ▁natural ▁units ▁are ▁possible . ▁ ▁Some ▁other ▁examples ▁are ▁as ▁follows : ▁ ▁Ge ometric ▁unit ▁systems ▁are ▁useful ▁in ▁relativ istic ▁physics . ▁In ▁these ▁systems ▁the ▁base ▁physical ▁units ▁are ▁chosen ▁so ▁that ▁the ▁speed ▁of ▁light ▁and ▁the ▁gravit ational ▁constant ▁are ▁set ▁equal ▁to ▁unity . ▁ ▁Plan ck ▁units ▁are ▁a ▁form ▁of ▁geometric ▁units ▁obtained ▁by ▁also ▁setting ▁the ▁C oul omb ▁constant ▁and ▁the ▁reduced ▁Plan ck ▁constant ▁to ▁unity . ▁They ▁are ▁based ▁only ▁on ▁properties ▁of ▁free ▁space ▁rather ▁than ▁any ▁object ▁or ▁particle . ▁ ▁St oney ▁units ▁are ▁a ▁form ▁of ▁geometric ▁units ▁obtained ▁by ▁also ▁setting ▁the ▁C oul omb ▁constant ▁and ▁the ▁elementary ▁charge ▁to ▁unity . ▁ ▁" Sch r öd inger " ▁units ▁are ▁similar ▁to ▁Plan ck ▁units ▁and ▁also ▁set ▁the ▁elementary ▁charge ▁to ▁unity , ▁but ▁allow ▁the ▁speed ▁of ▁light ▁to ▁float . ▁ ▁Hart ree ▁atomic ▁units ▁are ▁a ▁system ▁of ▁units ▁used ▁in ▁atomic ▁physics , ▁particularly ▁for ▁describing ▁the ▁properties ▁of ▁electrons . ▁The