Conference on the Michelson-Morley experiment held at the Mount Wilson observatory Pasadena, California February 4 and 5, 1927

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thus give less certain values for the velocity of relative motion, while at the same time the position of maximum displacement is not disturbed. Thus it is to be expected that the observations for the velocity of motion will not be as precise as the observations for the direction of motion. The two things, magnitude and azimuth of observed relative motion, are quite independent of each other.

It is desirable to have observations equally distributed over the twenty-four hours of the day; since one set requires about fifteen minutes of time, ninety-six sets, properly distributed, will suffice. The making of such a series usually occupies a period of ten days. The observations are finally reduced to one group, and the mean date is considered the date of the epoch. The observations made at Mount Wilson in 1925 correspond to the three epochs, April 1, August i, and September 15, and are more than twice as numerous as all the other ether-drift observations made since 1881. The total number of observations made at Cleveland represents about one thousand turns of the interferometer, while all the observations made at Mount Wilson previous to 1925 correspond to 1200 turns. The 1925 observations consist of 4400 turns of the interferometer, in which over 100,000 readings were made. A group of eight readings gives a value for the magnitude and direction of the ether-drift function, so that 12,500 single measures of the drift were obtained. This required that the observer should walk, in the dark, in a small circle, for a total distance of one hundred miles, while making the readings. Throughout these observations the conditions were exceptionally good. At times there was a fog which rendered the temperature very uniform. Four precision thermometers were hung on the outside walls of the house; often the extreme variation of temperature was not more than one-tenth of a degree, and usually it was less than four-tenths of a degree. Such variations did not at all affect the periodic displacement of the fringes. It may be added that while the readings are being taken, neither the observer nor the recorder can form the slightest opinion as to whether any periodicity is present, much less as to the amount or direction of any periodic effect.

The hundred thousand readings are added in groups of twenty, are averaged, and then plotted in curves. These curves are subjected


to mechanical harmonic analysis for the purpose of determining the azimuth and magnitude of the drift. In this work all the original observations have been used, without any omissions and without the assignment of weights; furthermore, there are no corrections of any kind to be applied to the observed values. The results of the analyses are finally charted in such a way as to show the variation in the azimuth of the drift throughout the day of twenty-four hours for each epoch, and the variation in magnitude is similarly charted.

[The observations of 1925 were described and the details of the results were shown by means of lantern-slide diagrams. A similar report constituted the address of the President of the American Physical Society read at Kansas City on December 29, 1925. This address is printed in full in Science, 63, 433-443, April 30, 1926.]

A calculation based only on the observations of 1925 was made to determine the absolute motion of the earth. The result of this, as reported at the Kansas City meeting, indicated that the solar system is moving toward an apex in the constellation Draco with a velocity which is in excess of 200 km/sec. In order to confirm the Kansas City report, a set of observations consisting of 2020 turns of the interferometer was made at Mount Wilson, corresponding to the epoch February 10, 1926. A complete calculation has now been made, including the observations of both 1925 and 1926, which leads to the following conclusion: The ether-drift experiments at Mount Wilson show, first, that there is a systematic displacement of the interference fringes of the interferometer corresponding to a constant relative motion of the earth and the ether at this observatory of 10 km/sec., with a probable error of 0.5 km/sec.; and, second, that the variations in the direction and magnitude of the indicated motion are just such as would be produced by a constant motion of the solar system in space, with a velocity of 200 km/sec., or more, toward an apex in the constellation Draco, near the pole of the ecliptic, which has a right ascension of 2550 (17 hours) and a declination of +68°; and, third, that the axis across which the observed azimuth of drift fluctuates, because of the rotation of the earth on its axis, points in a northwesterly direction, whereas the simple theory indicates that this axis should coincide with the north and south meridian.