The problem of electric lighting from central-stations is comparatively easy if an area can be obtained immediately surrounding, and within a short distance of, the station, with a right of way for laying down the electric mains direct. This happy state of affairs has not yet been attained, consequently the generating station has more often to be in an out-of-the-way corner of the district to be lighted, and it would be financially impossible to use low-tension currents with correspondingly large mains. The difficulty has been overcome in several ways by the use of high-tension currents in the mains, and has led to the adoption of secondary generators or transformers of electricity, which by induction supply a current of low potential in the house-service. The first to make this plan a practical success was Mr. Goulard, to whom the honour of the introduction is due, although his claims as the first inventor have been recently upset by the decision of the Courts. The relative economy of the supply of electricity by the use of a transformer is clearly shown by the following diagram, Fig. 7. A, B, and C give proportionately the area of cross section of the total mass of copper necessary to supply 5000 16 candle-power glow lamps situated at a mean distance of 4000 feet from the dynamo. A refers to the Edison “three-wire” system, working at a potential or electrical pressure of 200 volts with a fall of potential or loss of energy in the distributing feeders of 10 per cent., average distance from dynamo 4000 feet—the usual conditions on which this system is worked. B shows the size of conductor required for the same work in an installation based on the transformer system, potential 1000 volts, allowing 2·5 per cent. loss in the supply mains—only one-fourth as much as in the direct Edison system at the same average distance from dynamo. If this loss were increased to 10 per cent., and made equal to that in the direct system, viz. 10 per cent., the size of the conductor would be that shown at C. [Illustration: FIG. 7.] [Illustration: FIG. 8.] The graphic diagram, Fig. 8, demonstrates what the relative cost would be with each of the three conditions just named. Mr. Goulard’s first practical application of the secondary generator in this country was the lighting of the Underground Railway Stations, in 1883, from Edgware Road to High Street, the generating dynamo being fixed at the former place. These experiments, which were made by Mr. Kenneth Mackenzie, attracted considerable attention at the time, but it was not until the report of the jurors to the Turin International Exhibition in 1885 was published that companies were formed to instal the Goulard system for lighting an extensive district. [Illustration: FIG. 9.] The principle underlying all transformers is that of the induction coil invented by Ruhmkorff in 1842, but described by Faraday in his “Experimental Researches,” published in 1831-2.