This is taken from General August V. Kautz's "Customs of Service for Non Commissioned Officers and Soldiers." Written in 1864.
"Section 1577. The rank of non-commissioned officers will be marked by chevrons upon both sleeves of the uniform coat and overcoat, above the elbow, of silk or worsted binding one-half an inch wide, same color as the edging on the coat, points down, as follows:"
"Section 1584. For a corporal—two bars, in worsted."
I hope that this is helpful in some way, I know the Corporals and Sergeant's in my reenacting unit have asked this question before as well.