2. The transformers must convert the current of high electromotive force into a current of such electromotive force as may be desired. The transformers must have a closed magnetic circuit (that is, they must be poleless), in order that all the primary and secondary turns shall possess, relatively to the magnetic field, a like position, also in order that the resistances of the primary and secondary coils shall be so small that they cause practically no loss of electromotive force. Through the fulfilment of both these conditions, it is rendered possible to maintain the secondary tension constant by maintaining the primary tension constant, indifferently whether it is regulated automatically or by hand. To suit this, the transformers must also be arranged into distributive stations of the second order, and derived in parallel from the main leads. [Sidenote: Zipernowsky, Déri, Bláthy, 1885.] In May, 1885, a system of current distribution meeting all the just-mentioned requirements was publicly brought out, giving an illustration of a truly self-regulating system of current distribution. This was the system of Zipernowsky, Déri, and Bláthy. The first two patents concerning this system date from 18th February, 1885, and are entitled, “Improvements in the means for the regulation of alternating electric currents,” No. 34,649, by Carl Zipernowsky and Max Déri, of Budapest; “Improvements in the distribution of alternating currents,” No. 33,951, by Max Déri, of Vienna. The third patent is dated 6th March, 1885, and is entitled, “Improvements in induction apparatus for the purpose of transforming electric currents,” No. 40,414, by Carl Zipernowsky, Max Déri, and Otto Titus Bláthy, of Budapest. The system described in these three patents was immediately afterwards brought forward in the three exhibitions of Budapest, Antwerp, and London (Inventions Exhibition), arousing in technical circles a general and well-earned attention. In the patent documents as well as in the earliest[11] articles in the journals concerning the system, two special forms of transformers are described, viz. that consisting of an iron core with the wire outside, and, secondly, that consisting of copper coils surrounded by iron wire. The transformers shown in Figs. 24 to 28 belong to the last of these classes, that in Fig. 23 to the first. The fundamental principle upon which all these transformers are constructed is that the subdivisions of the iron core run perpendicularly to the copper wires. Transformers such as are shown in Fig. 25 having a ring-shaped iron core wound with copper wire at first employed, later the inventors used in preference the form represented in Fig. 23. [Illustration: FIG. 23.]