The first book to address the windings for electric machines was published in March 1891 by Professor E. Arnold, the director of the Electrical Engineering Institute at the Grand Ducal Technical University of Karlsruhe, under the title "The Armature Windings of Direct Current Dynamo Machines." In it, he was able to establish, for the first time with the help of his "circuit rule," the common and specific properties of the various windings and disc armatures known at that time, demonstrate the relationship between ring and drum windings, and illustrate the transition from one winding to another. Thus, Prof. Arnold was the first to bring theoretical clarity to the previously opaque field of windings. Thirty years later (1920), Prof. Rudolf Richter, Arnold's successor at the Technical University in Karlsruhe, published his own work.