The question is now before us, “In what manner must the primary electromotive force vary to effect this?” Consider an iron core, having on two different parts round it, two rings of wire. This iron core may now be magnetised by bringing near to it in the line of its axis a permanent magnet. On drawing the latter quickly away, an electromotive force will be momentarily produced in both the wire rings, and the electromotive force will be proportional to the number of the disappearing lines of force. This number, in consequence of the dispersion of the lines of force, will be very different at different parts of the magnetised core. The induced electromotive forces in the windings of the wire will also be different. The equality of these electromotive forces, which is so important, can only be attained if all the windings are in relatively the same position with regard to the magnetic field. The circuits of both coils being closed, the one having a current flowing through it, the other through a suitable resistance, besides the condition mentioned in the last sentence, another must be fulfilled; this is, the internal resistance must be practically zero, i.e. the difference of potential between the terminals shall equal to all intents and purposes the total electromotive force. We have now to examine how far the already observed constructions of transformers fulfilled these demands. A transformer in which the windings lie relatively in the same position to the magnetic field can quite well be bi-polar. All that is necessary for this is that the coils be wound on to the core next to one another; this is most simply managed in a transformer having a ratio of 1:1. This law was first determined by Maxwell. The apparatus of Strumbo shows such a method of winding already carried out. Thus it may be seen that of bi-polar transformers, those which, with regard to the constancy of the secondary tension, are most suitable, are quite useless on account of their ratio being 1:1, although they are destined for the series method of connection. The connection of proper transformers in parallel can only be made with such apparatus as, notwithstanding their ratio of transformation, possess windings having the same relative position to the magnetic field—this is only the case with non-polar transformers. Besides this quality of non-polar transformers, their magnetic resistance is so low that the condition of very low internal resistance is easily fulfilled. The following conditions of a self-regulating and economical system of current distribution with transformers result, therefore, from the foregoing explanations:— 1. The generator of current must give a great difference of potential as constant as possible at the terminals of the transformers, and also independent of the number fed.