The original Goulard and Gibbs secondary generator was of the core transformer type, it had an open magnetic circuit and cores which could more or less be inserted into the coils so as to regulate the electro-motive force of the secondary circuit. The transformers were constructed with a number of copper disks or washers; these were placed alternately primary and secondary in a vertical frame, through the centre of which an iron core was fixed, consisting of a bundle of straight iron wires. The core was movable in the coil in the manner of the well-known induction coils, and thereby the electro-motive force of the secondary current could be adjusted. In their latest design the coils are circular in plan and rectangular in section and are surrounded by groups of U-shaped soft iron stampings slipped over from both sides and held together by two circular cast-iron plates with a central bolt. The magnetic lines of force pass through the core, in at one end and out at the other, and are then more or less disseminated through space; it will thus be seen that the path of the lines lies partly in iron and partly in air, and, since air has about seven hundred times more magnetic resistance than iron, it is evident that the number of lines created with a given current must be considerably smaller than would be the case if the path of the lines contained iron only. This constitutes the improvement in the Zippernowsky-Deri-Blathy system of transformer, which has coils similar to the Goulard, but with the iron of the core applied in the form of a ring-shaped shell, surrounding both coils completely. This arrangement can best be described by comparing it to a Gramme armature, in which the copper and the iron have changed places. Imagine what is usually the core in an armature replaced by the primary and secondary coils, and, instead of the winding of insulated copper wire, wind iron wire around the coils, and one of these transformers is the result. In consequence of the lower magnetic resistance of the Class II. transformer, as compared to that of Class I., the electrical output obtainable with equal weights of copper and iron appears to be considerably greater in the former apparatus. Professor Feraris, of Turin, has published some of the results of comparative experiments made with Classes I. and II. and finds that the coefficient of induction is 3·6 times as great with the latter as with the former. There are many varieties of transformers in the market which closely resemble each other; one of the most practical is that designed by Kapp and Snell, of which Fig. 10 is an illustration. U-shaped stampings form the shell and the cores are laid in the double trough. The cover of these troughs is formed from the metal removed from the interior of the stampings, and the whole is held together in a cast-iron frame so arranged as to allow air to circulate through the core and round the coils. The price of these transformers