IT will interest the readers of the SCIENTIfIC ARICAN to know that Nikola Tesla, whose reputation must, naturally, stand upon the. contributions he made to electrical engineering when the art was yet in its comparative infancy, is by training and choice a mechanical engineer, with a strong leaning to that branch of it which is covered by the term “steam engineering.” For several years past he has devoted milch of his attEtion to improvements in thermo-dynamic conversion, and the result of his theories and practical {xperiments is to be found in an entirely new form of prime movers shown in operation at the Waterside station of the. New York Edison Company, who kindly placed the facilities of their great plant at his disposal for carrying on experimental work. By the courtesy of the inventor, we are enabled to publish the accompanying views, representing the testing plant at the Waterside station, which are the frst photographs of this interesting motor that have yet been made public. The basic principle which determined Tesla's investigations was the well-known fact th a t when a fluid (steam, gas or water) is used as a vehicle of energy, the highest possible economy can be obtained only when the changes in velocity and direction of the movement of the fluid are made as gradual and easy as possible. In the present forms of turbines in which the energy is transmitted by pressure, reaction or impact, as in the De Laval, Parsons, and Curtiss types, more or less sudden changes both of speed and direction arc involved, with consequent shocks, vibration and destructive eddies. Furthermore, the introdnction of pistons, blades, buckets, and intercepting devices of this general class, into the path of the fluid involves much delicate and difficult mechanical construction which adds greatly to the cost both of production and maintenance. The desiderata in an ideal turbine group themselves nnder the heads of the theoretical and the mechanical. The theoretically perfect turbine would be one In \ybich the fluid was so controlled from the· inlet to the (xhaust that its energy was delivered to the driving shaft with the least possible losses due to the mechanical means employed. The mechanically perfect turbine would be one which combined simplicity an( cheapness of construction, durability, ease and rapidity of repairs, and a smaIl ratio of weight and space occupied to the power delivered on the shaft. Mr. Tesla maintains that in the turbine which forms the subjprt of this article, he has carried the steam and gas motor a long step forward toward the maximum attainable efficiency, both theoretical and mechanical. That thesp claims are well founded is shown by the fact that in the plant at the Edison station, he is securing an output of 200 horse-power from a single-stage steam turbine with atmospheric exhaust, weighing less than 2 pounds per horse-power, which is contained within a space measuring 2 feet by 3 feet, by 2 feet in height, and which aCfomvlishes these results with a thermal fall of only 130 B.T.U., that is, about one-third of the total drop available. Furthermore, considered from the mechanical standpoint, the turbine. is astonishingly simple and economical in construction, and by the very nature of its construction, should prove to possess such a durability and freedom from wear and breakdown as to place it, in these respects, far in advance of any type of steam or gas motor of the present day. Briefly stated, Tesla's steam motor consists of a set of flat steel disks mounte- on a shaft and rotating within a casing, the steam entering with high velocity at the periphery of the disks, flowing between them in free spiral paths, and finally escaping through exhaust ports at their center. Instead of developing the energy of the steam by pressure, reaction, or impact, on a series of blades or vanes, Tesla depends upon the fluid properties of adhesion and viscosity-the attraction of the steam to the faces of the disks and the resistance of its particles to molecular separation combining in transmitting the velocity energy of the motive fluid to the plates and the shaft. By reference to the accompanying photographs and line drawings, it will be seen that the turbine has a rotor A which in the present case consists of 25 flat steel disks, one thirty-second of an inch in thickness, of hardened and carefully tempered steel. The rotor as assembled is 3% inches wide on the face, by 18 inches in diameter, and when the turbine is running at its maximum working velocity, the material is never under a tensile stress exceeding 50,000 pounds per square inch. The rotor is mounted in a casing D, which is provided with two inlet nozzles, B for use in running direct and B' for reversing. Openings 0 are cut out at the central portion of the disks and these communicate directly with exhaust ports formed, in the side of the casing. In operation, the steam, or gas, as the case may be, is directed on the periphery of the disks through the nozzle B (which may be diverging, straight or converging), where more or less of its expansive energy is converted into velocity energy. When the machine is at rest, the radial and tangential forces due to the pressure and velocity of the steam cause it to travel in a rather short curved path toward the central exhaust opening, as indicated by the full black line in the aecompanying diagram; but as the disks COmm.Dce to 1Qtate and their speed increases, the steam travels in spiral paths the length of which increases until. as September 30, 1 91 J SCIENTIFIC AMERICAN 297 in the case of the present turbine, the particles of the fluid complete a number of turns around the shaft before reaching the exhaust, covering in the meantime a lineal path some 12 to 16 feet in length. During its progress from inlet to exhaust, the velocity and pressure of the steam are reduced until it leaves the exhaust at 1 or 2 pounds gage pressure. The resistance to the passage of the steam or gas between adjoining plates is approximately proportionate to the square of the relative speed, which is at a maximum toward the center of the disks and is equal to the tangential velocity of the steam. Hence the resistance to radial escape is very great, being furthermore enhanced by the centrifugal force acting outwardly. One of the most desirable elements in a perfected turbine is that of reversibility, and we are all familiar with 'the many and frequently cumbersome means which have been employed to secure this end. It will be seen that this turbine is admirably adapted for reversing, since this effect can be secured by merely closing the right-hand valve and opening that on the left. It is evident that the principles of this turbine are equally applicable, by slight modifications of design, for its use as a pump, and we present a photograph of a demonstration model which is in operation in Mr. Tesla's office. This little pump, driven by an electric motor of 1/12 horsepower, delivers 40 gallons per minute against a head of 9 feet. The discharge pipe leads up to a horizontal tube provided with a wire mesh for screening the water and checking the eddies. The water falls through a slot in the bottom of this tube and a'ter passing below a baffle plate flows in a steady stream about % inch thick by 18 inches in width, to a trough from which it returns to the pump. Pumps of this character show an effici.ncy favorably comparing with that of centrifugal pumps and they have the advantage that great heads are obtainable economically in a single stage. The runner is mounted in a two-part volute casing and except for the fact that the place of the buckets, vanes, etc., of the ordinary centrifugal pump is taken by a set of disks, the construction is generally similar to that of pumps of the standard kind. In conclusion, it should be noted that although the experimental plant at the Waterside station develops 200 horse-power with 125 pounds at the supply pipe and free exhaust, it could show an output I of 300 horse-power with the full pressure of the Edison supply circuit. Furthermore, Mr. Tesla states that if it were compounded and the exhaust were led to a low pressure unit, carrying about three times the number of disks contained in the high pressure element, with connection to a condenser affording 28Y to 29 inches of vacuum, the results obtained in the present high-pressure machine indicate that the compound unit would give an output of 600 horse-power, without great increase of dimensions. This estimate is conservative. The testing plant consists of two identical turbines connected by a carefully calibrated torsion spring, the machine to the left being the driving element, the other the brake. In the brake element, the steam- is delivered to the blades in a direction opposite to that of the rotation of the disks. Fastened to the shaft of the brake turbine is a hollow pulley provided with two diametrically opposite narrow slots, and an incandescent lamp placed inside close to the rim. As th. pulley rotates, two flashes of light pass out of the same, and ----- by means of reflecting mirrors and lenses, they Thi are carried around the plant and fall upon two rotating glass mirrors placed back to back on the shaft of the driving turbine so that the center line of the silver coatings coincides with the axis of the shaft. The mirrors are so set that when there is no torsion on the spring, the light beams produce a luminous spot stationary at the zera of the scale. But as soon as load is put on, the beam is deflected through an angle which indicates •directly the torsion. The scaie and spring are so proportioned and adjusted that the horse-power can be read directly from the deflections noted_ The indications of this device are very accurate and have shown that when the turbine is running at 9,000 revolutions Hnder an inlet pressure of 125 pounds to the square inch, and with free exhaust, 200 brake horse-power are developed. The consumption under these conditions of maximum outPlt is 38 pounds of saturated steam per horse-power per hour-a very high efficiency when we consider that the heat-drop, measured by thermometers, is only 130 B.T.U., and that the energy transformation is effected in one stage. Since about three times this number of heat units are available in a modern plant with superheat {nd high vacuum, the above means a consumption of less than 12 pounds per horse-power hour in such turbines adapted to take up the full drop. Under certain conditions, however, very high thermal efficiencies have been obtained which demonstrate that in large machines baSed on this prinCiple, in which a very small slip can be secured, the steam consumption will be much lower and should, Mr_ Tesla states, approximate the theoretical minimum, thus resulting in nearly frictionless tur- bine transmitting almost the entire expansive energy of the steam to the shaft. Some Striking Coal Facts T AST year th e U nit ed States mined 501,5 96,378 Lfshort tons of coal or nearly two-fifths of the year's total production for the world. This coal would load a train stretching back and forth across the United States from the Atlantic to the Pacific 33 times-a train approximately 100,000 miles long. Eleven years ago the United States for the first time surpassed Great Britain with a prOcuction of 253,741,192 tons, only a little more than haIr of last year's output. The mere increase of thB coal output of the United States for 1910 over that of 1909-40,781,762 tons-was greater than the total production of any foreign country except Great Britain, Germany, Austria, Hun-gary, or France. This increase alone was one and one-fifth times as. great as the entire production of the United States in 1870_ Excepting only Great Britain and Germany, -either of the States of Pennsylvania or West Virginia produced in 1910 more coal than any foreign country. For the past seven or eight 10-year periods the coal production for each decade has been about equal to the entire amount of coal previously mined in the United States. Thus in the 10 years between 1885 and - 1895 the production was 1;586,098,641 tons, while the entire amount of coal mined prior to 1895 was only 1,552,080,478 tons. In the 10 years betweeen J 895 and 1905 the production . was 2,832,402,7 46 tons. while all the coai which had been mined prior to 1895 was 3,138,174,119 tons_ Incredible as it may seem, at the present rate of increase the ten-year period between 1905 and 1915 will show a production greater than all the coal mined in the United States prior to 1905. In 1850 the per capita production of coal was a little over one-fourth of a ton. In 1870 the per capita production had increased to nearly one ton; in 1890 it was 2Y tons; in 1900 it was 3Y tons, and in 1910 with the population of 91,972,266 the production was nearly 5112 tons for each person. Last year 725,030 men mined coal in the United States. The great coal production record of 1910 was made in spite of a series of labor strikes participated in by 215,640 men. The loss in wages alone amounted to nearly $30,000,000. The quantity of coal used for making coke in the United States for metallurgical purposes was 52.-187,450 tons. This is additional to by-product coke produced in gas mimufacture. The total production of coal in the United States at the close of 1910 was 8,243,351,259 short tons. This plus the estimated loss incident to mining makes a total exhaustion of 13,395,000,000 tons. The United States ' Geological Surve. estimate?. the original supply of coal in the ground in the United States, exclusive of Alaska, at 3,076,204,000,000 tons. This original supply less the exhaustion at the close of 1910 leaves an apparent supply still available of 3,062,808;972,000 tons, or 99.6 per cent of the original supply. In other words, in all the time since coal mining began in the United States the draft upon the original supply including loss in mining, has amounted to less than one-half of one per cent. At the present rate of production of approximately half a billion tons a year the coal reserve of the United States would therefore last 6,000 years. At the present rate of increase in production. however, these three thousand billion tons of coal in the ground would last only a few generatons. Foreign Students in America ADDRESSING the House of Representatives on the many new activities of the United States diplomatic service, Representative Foster, of Vermont, late chairman of the House Foreign Affairs Committee, recently called attention to the effort made by our diplomatic and consular representatives to advertise the United States as an educational center, an undertaking that has been fruitful of results. One of the outcomes of this program was the formation in Buenos Aires two years ago of a United States University Club, which has been the means of sending at least 20 young Argentinians to this country to be educated. Under the auspices of this club lectures are given on university life in the United States, illustrated with a large number of fppropriate stereopticon views. Negotiations are now under way for an interchange of schoolboys between the Boston High School of Commerce and the preparatory department of the University of La Plata. There are now at least 400 Latin Americans studying in the United States, and the number is steadily increasing_ Through the efforts of our ambassador at Constantinople, supported by the State Department, Columbia University has voted to receive, free of all tuition charges, three students annually from the Ottoman Empire for the next ten years, to pursue courses of study in ·a;y of the departments of the university. These students are to be selected by the Ottoman government. with the advice and approval of the ambassador at Constantinople. The education of Chinese students in America, a matter in which the United States government has always taken a kindly interest, is assuming ever larger proportions. These students now number between 800 and 900. Half of these are “government students,” supported by the different Chinese provinces, and by the remitted portion of the Boxer indemnity fund. To insure that the indemnity students coming to the United States should not start with a serious handicap, but be fully prepared to enter the American colleges, an academy has been establifhed in Peking by the Chinese government, where these students receive preliminary instuctions under American teachers.