In April, 1864, Sellers, as president of the Franklin Institute, read a paper on “A System of Screw Threads and Nuts,” in which he proposed the system of screw threads since variously known as the Sellers, Franklin Institute, or U. S. Standard.[208] They embodied the sixty degree angle and a flat of one-eighth of the pitch at the top and bottom of the thread. In this paper Sellers stated clearly the need for _some_ generally accepted standard, reviewed the various threads then used, particularly the Whitworth thread, with its fifty-five degree angle and rounded corners, which he disapproved of on three grounds; first, that it was difficult to secure a fit at the top and bottom; second, that the angle was difficult to verify; and third, the high cost of making cutting tools which would conform accurately to the standard. He proposed the sixty degree angle as easier to make and already in general use in this country, and the flat top as easy to generate and to verify. He went a step further, and proposed at the same time a standard for bolt-heads and nuts, in which the dimensions were derived from a simple formula and the distance across flats was the same for square and hexagon nuts, so that the same wrench would do for either style of nut. [208] _Journal of the Franklin Institute_, Vol. LXXVII, p. 344. [Illustration: FIGURE 51. WILLIAM SELLERS] This paper had as great influence in America as Whitworth’s paper of 1841 had in England. A committee was appointed to investigate the question and recommend a standard. On this committee, among others, were William B. Bement, C. T. Parry of the Baldwin Locomotive Works, S. V. Merrick, J. H. Towne, and Coleman Sellers. Early in the next year the committee reported in favor of the Sellers standard, the Franklin Institute communicated their findings to other societies, and recommended the general adoption of the system throughout the country. The Sellers’ thread was adopted by the United States Government for all government work in 1868, by the Pennsylvania Railroad in 1869, the Master Car Builders’ Association in 1872, and soon became practically universal. After exhaustive investigation the Sellers’ form of thread was adopted in 1898 by the International Congress for the standardization of screw threads, at Zurich, and is now in general use on the continent of Europe.[209] [209] For the discussion of the Sellers’ screw thread and the circumstances surrounding its adoption, see: _Journal of the Franklin Institute_, Vol. LXXVII, p. 344; Vol. LXXIX, pp. 53, 111; Vol. CXXIII, p. 261; Vol. CXXV, p. 185.