In 1872 the drum armature was devised by Hefner-Alteneck.

For a very large power-generation armature several feet in diameter, using a hollow ring armature requires far less metal and is lighter than a solid core drum armature.

Rudolf Eickemeyer, (1831-1895), born in Altenbamberg, noted inventor with several innovations to hat manufacturing, patent for first symmetrical drum armature and iron-clad dynamo, and first employer of Charles Proteus Steinmetz.