Edison had installed his historic first great central-station system in New York on the multiple arc system covered by his feeder and main invention, which resulted in a notable saving in the cost of conductors as against a straight two-wire system throughout of the "tree" kind. He soon foresaw that still greater economy would be necessary for commercial success not alone for the larger territory opening, but for the compact districts of large cities. Being firmly convinced that there was a way out, he pushed aside a mass of other work, and settled down to this problem, with the result that on November 20, 1882, only two months after current had been sent out from Pearl Street, he executed an application for a patent covering what is now known as the "three-wire system." It has been universally recognized as one of the most valuable inventions in the history of the lighting art. [13] Its use resulted in a saving of over 60 per cent. of copper in conductors, figured on the most favorable basis previously known, inclusive of those calculated under his own feeder and main system. Such economy of outlay being effected in one of the heaviest items of expense in central-station construction, it was now made possible to establish plants in towns where the large investment would otherwise have been quite prohibitive. The invention is in universal use today, alike for direct and for alternating current, and as well in the equipment of large buildings as in the distribution system of the most extensive central-station networks. One cannot imagine the art without it. [Footnote 13: For technical description and illustration of this invention, see Appendix.]