Miller D.C. The Ether-Drift Experiment and the Determination of the Absolute Motion of the Earth // Reviews of modern physics, Vol.5, July 1933

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The Morley-Miller Experiments, Cleveland, 1902-1906

The interferometer of wood, 1902

At the International Congress of Physics held in Paris in connection with the International Exposition of 1900, Lord Kelvin gave an address in which he expounded certain theories of the ether, and he explained the significance of the results of the Michelson-Morley experiments as related to these theories.8 Professor Morley and the writer were present and in a later conversation with Lord Kelvin he strongly urged the repetition of the ether-drift experiment with a more powerful apparatus. Morley and Miller then constructed an interferometer designed especially to test the Lorentz-FitzGerald hypothesis. The base of this instrument was in the form of a cross, made of planks of white pine wood about 430 centimeters long, providing a light-path more than three times as long as that used by Michelson and Morley in 1887. The general dimensions, optical parts and methods of observing with this apparatus were the same as for the steel interferometer described in detail in following sections of this paper. The instrument was mounted in the northwest corner room of the basement of the Main Building of Case School of Applied Science and three series of observations were made in August, 1902, and in June, 1903, consisting of 505 turns of the interferometer. A small positive effect was observed, indicated by the square in Fig. 4, which, while slightly larger than that of the previous experiment, was still so small as to indicate that if the reduction of the observed velocity is to be attributed to the hypothetical contraction, the pine is affected by about the same amount as is the sandstone. The changes in the wooden support due to variations in humidity and temperature made it difficult to obtain accurate observations and it was decided to abandon the pine apparatus and to construct one having a base of metal for supporting the heavy parts, while the length of the optical path could be determined by various substances, wood or metal, as desired.

While planning a new apparatus, experiments were made to show that differences of magnetic

8 Lord Kelvin, Rapports présentés an Congrès International de Physique 2, 1 (1900).

attraction on the iron parts of the instrument could not influence the observations. Massive bars of iron were suspended at the opposite ends of one of the long arms of the cross, so that one bar should be parallel to the earths magnetic field while the other was transverse to this field, these relations being reversed on reversing the azimuth of the apparatus. Observations with this load gave the same results as before. In a further experiment, an analytical balance was placed on one arm with which to weigh a bar of iron having a mass of about 1200 grams. It was so oriented that at one azimuth of the apparatus the bar was parallel to the lines of the earths magnetic field, while at another it was transverse to the field. A difference of half a milligram could have been detected but no such difference existed. By observing the effect produced by a known weight on one arm of the interferometer, it was shown that the earths magnetism could not be a disturbing factor.

Description of the new steel interferometer

An appropriation from the Rumford Fund of the American Academy of Arts and Sciences made possible the construction, in 1904, of an entirely new apparatus of steel. The design for the base of the interferometer, made by Professor F. H. Neff of the Department of Civil Engineering of Case School of Applied Science, provided that all optical parts and accessories should be carried by two girders of structural steel, Figs. 5, 10 and 14, each about 430 centimeters long, which intersect in the form of a cross. The purpose of this design was to secure structural symmetry and the utmost rigidity.

The steel cross rests on a circular float of wood, Fig. 5, 150 centimeters in diameter; on the under

Fig. 5. Cross section of the mercury float for the interferometer.

side of the circle is an annulus of wood having an outside diameter of 150 centimeters, an inside diameter of 80 centimeters, and a thickness of 20 centimeters. This float of wood rests on mercury contained in an annular trough of cast iron, of such dimensions as to leave a clearance of about one centimeter around the wood, which space is filled with mercury. It requires about 275 kilograms of mercury to float the entire apparatus which weighs about 1200 kilograms. The float is kept central by a loose-fitting centering pin which sustains no pressure. The annular iron tank is supported by piers of brick or concrete at such a height as to bring the eyepiece of the observing telescope level with the eye of the observer when he takes the posture for easy walking around with the interferometer as it rotates slowly on the mercury. The cast iron trough for the mercury together with the circular wooden float are the same parts as were used in the original Michelson-Morley interferometer of 1887 and these two pieces have been continued in use by the writer to the present time. The other parts of the apparatus of 1887 have been dispersed, excepting only three of the cast iron supports for the mirrors.

The optical flat surfaces were all made in 1902 by that artist-optician, O. L. Petitdidier of Chicago, and proved to be exceptionally perfect; these consist of two plane-parallel plates, each

10.5 × 17.5 centimeters in size, and sixteen plane mirrors of circular shape, 10.25 centimeters in diameter. The general plan of the interferometer is shown in the diagram, Fig. 6, which, however, is not drawn to the exact scale. On a central plate, at the intersection of the arms of the cross, are mounted the half-silvered diagonal mirror, D, and its compensating plate, C, both having been cut from a single plane-parallel disk. At the outer end of each cross-arm, four of the circular mirrors are mounted in a metal plate which is supported in a vertical position. Each of the eighteen mirrors is held by springs against the points of three adjusting screws to permit the necessary adjustments for securing interference. In order to have everything about the two arms as symmetrical as possible, there is no micrometer screw for moving the end mirror parallel to itself, all the adjustment being obtained by means of the three simple screws, as for the other

mirrors. Light from the source, 5, is rendered parallel by the triple-lens condensing system, L, of 15 centimeters diameter, and reaches the half-silvered mirror, D. Part of this light is transmitted to the mirror, I-1; it is successively reflected to mirrors, 2, 3, 4, 5, 6, 7, and 8, having travelled a distance equal to about seven and a half times the length of the arm of the cross. From mirror 8 the light returns by the same path back to D, where it is partially reflected to the observing telescope, T. A second portion of the light incident on D is reflected along the other arm of the cross to II-1, is reflected to and fro and is returned to D and is in part transmitted to the observing telescope. In the actual apparatus, Fig. 10, the mirrors 5 and 7 are above mirrors 3 and 1 instead of at the side of them, and mirrors 6 and 8 are above mirrors 4 and 2. By this system of mirrors the effective length of the arm of the interferometer is greatly increased and in the actual apparatus it is 3203 centimeters, giving a total light-path, going and returning, of 6406 centimeters, equal to about 112,000,000 wave-lengths of the acetylene light used in the experiment. The telescope had an aperture of 3.3 centimeters, a focal length of 35 centimeters, and a magnifying power of thirty-five diameters. The telescope is focussed on the surface of mirror 8

Fig. 6. Plan of the optical paths in the interferometer.

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