Why do we call them coppers?

According to the Merriam Webster Dictionary:

Around the year 1700, the slang verb cop entered English usage, meaning “to get ahold of, catch, capture.” By 1844, cop showed up in print, and soon thereafter the -er suffix was added, and a policeman became a copper, one who cops or catches and arrests criminals. Copper first appeared in print in 1846, the use of cop as a short form copper occured in 1859.