▁atomic ▁units ▁have ▁been ▁chosen ▁such ▁that ▁several ▁constants ▁rel ating ▁to ▁the ▁electron ▁are ▁all ▁equal ▁to ▁one : ▁the ▁electron ▁mass , ▁the ▁elementary ▁charge , ▁the ▁C oul omb ▁constant ▁and ▁the ▁reduced ▁Plan ck ▁constant . ▁The ▁unit ▁of ▁energy ▁in ▁this ▁system ▁is ▁the ▁total ▁energy ▁of ▁the ▁electron ▁in ▁the ▁Bo hr ▁atom ▁and ▁called ▁the ▁Hart ree ▁energy . ▁The ▁unit ▁of ▁length ▁is ▁the ▁Bo hr ▁radius . ▁ ▁Electron ic ▁units ▁are ▁similar ▁to ▁St oney ▁units ▁but ▁set ▁the ▁electron ▁mass ▁to ▁unity ▁and ▁allow ▁the ▁gravit ational ▁constant ▁to ▁float . ▁They ▁are ▁also ▁similar ▁to ▁atomic ▁units ▁but ▁set ▁the ▁speed ▁of ▁light ▁to ▁unity ▁and ▁allow ▁the ▁Plan ck ▁constant ▁to ▁float . ▁ ▁Quant um ▁elect rod ynam ical ▁units ▁are ▁similar ▁to ▁the ▁electronic ▁system ▁of ▁units ▁except ▁that ▁the ▁pro ton ▁mass ▁is ▁normal ised ▁rather ▁than ▁the ▁electron ▁mass . ▁ ▁Non - standard ▁units ▁Non - standard ▁measurement ▁units ▁also ▁found ▁in ▁books , ▁newsp apers ▁etc ., ▁include : ▁ ▁Area ▁ ▁The ▁American ▁football ▁field , ▁which ▁has ▁a ▁playing ▁area ▁ ▁long ▁by ▁ ▁wide . ▁This ▁is ▁often ▁used ▁by ▁the ▁American ▁public ▁media ▁for ▁the ▁sizes ▁of ▁large ▁buildings ▁or ▁par ks . ▁It ▁is ▁used ▁both ▁as ▁a ▁unit ▁of ▁length ▁( , ▁the ▁length ▁of ▁the ▁playing ▁field ▁ex cluding ▁goal ▁areas ) ▁and ▁as ▁a ▁unit ▁of ▁area ▁( ), ▁about ▁. ▁ ▁British ▁media ▁also ▁frequently