▁the ▁amp ere ▁is ▁defined ▁in ▁terms ▁of ▁other ▁units .) ▁Other ▁quantities , ▁such ▁as ▁power ▁and ▁speed , ▁are ▁derived ▁from ▁the ▁base ▁set : ▁for ▁example , ▁speed ▁is ▁distance ▁per ▁unit ▁time . ▁Histor ically ▁a ▁wide ▁range ▁of ▁units ▁was ▁used ▁for ▁the ▁same ▁type ▁of ▁quantity : ▁in ▁different ▁context s , ▁length ▁was ▁measured ▁in ▁inches , ▁feet , ▁yards , ▁f ath oms , ▁ro ds , ▁ch ains , ▁f url ongs , ▁miles , ▁na ut ical ▁miles , ▁stad ia , ▁le agues , ▁with ▁conversion ▁factors ▁which ▁were ▁not ▁powers ▁of ▁ten . ▁Such ▁arrang ements ▁were ▁satisf actory ▁in ▁their ▁own ▁context s . ▁ ▁The ▁preference ▁for ▁a ▁more ▁universal ▁and ▁consistent ▁system ▁( based ▁on ▁more ▁rational ▁base ▁units ) ▁only ▁gradually ▁spread ▁with ▁the ▁growth ▁of ▁science . ▁Ch anging ▁a ▁measurement ▁system ▁has ▁substantial ▁financial ▁and ▁cultural ▁costs ▁which ▁must ▁be ▁offset ▁against ▁the ▁advantages ▁to ▁be ▁obtained ▁from ▁using ▁a ▁more ▁rational ▁system . ▁However ▁pressure ▁built ▁up , ▁including ▁from ▁scient ists ▁and ▁engine ers ▁for ▁conversion ▁to ▁a ▁more ▁rational , ▁and ▁also ▁intern ation ally ▁consistent , ▁basis ▁of ▁measurement . ▁ ▁In ▁anti qu ity , ▁systems ▁of ▁measurement ▁were ▁defined ▁locally : ▁the ▁different ▁units ▁might ▁be ▁defined ▁independently ▁according ▁to ▁the ▁length ▁of ▁a ▁king ' s ▁thumb ▁or ▁the ▁size ▁of ▁his ▁foot , ▁the ▁length ▁of ▁st ride , ▁the ▁length ▁of ▁arm , ▁or ▁maybe ▁the