Recorded as Ohm, Oehm, Oehme, Ohms, and possibly others, this is a famous German surname. It derives from a pre 7th century word "om" meaning uncle, a word of endearment which did not necessarily mean a close blood relative, but could have the more general translation of a "close older friend of the family". A close younger friend in the same circumstances might have been referred to as "wetter" or cousin. As to how these descriptive expressions progressed into being popular surnames from about the 13th century a.d., is unclear. Arguably they are nicknames and therefore names given to people who habitually used the expression or in this case, were for some reason publically called uncle! An alternative suggestion is that they were theatrical names, used by actors in the travelling theatres of the period who played the part of an uncle, the actor eventually being named after the part he played. What is certain is that as a surname it is one of the earliest recorded in the surviving registers of Germany. The first recording may be that of Johannes Ohm of Hamburg in the year 1298, with Phillipus Om being recorded in the charters of Sponheim in 1306. The international measurement of electrical resistance is the ohm, named after Professor Georg Ohm of Munich, (1787 - 1854).