FranklinyoudaoicibaDictYouDict[Franklin 词源字典]
Middle English Frankeleyn, attested as a surname from late 12c., from Anglo-French fraunclein "freeholder, land-owner of free but not noble birth," from Old French franc "free" (see frank (adj.)); probably with the Germanic suffix also found in chamberlain.

The Franklin stove (1787) so called because it was invented by U.S. scientist/politician Benjamin Franklin (1706-1790). In early 19c., lightning rods often were called Franklins from his famous experiments with lightning in the 1750s.[Franklin etymology, Franklin origin, 英语词源]
lightning (n.)youdaoicibaDictYouDict
late 13c., present participle of lightnen "make bright," extended form of Old English lihting, from leht (see light (n.)). Meaning "cheap, raw whiskey" is attested from 1781, also sometimes "gin." Lightning bug is attested from 1778. Lightning rod from 1790.