THE EFFECT OF THE EARTH’S ROTATION ON
THE VELOCITY OF LIGHT
PART II
By A. A. MICHELSON AND HENRY G. GALE Assisted by FRED PEARSON
ABSTRACT
Experimental Test of Theory.—Air was exhausted from a twelve-inch pipe line laid on the surface of the ground in the form of a rectangle 2010×1113 feet. Light from a carbon arc was divided at one comer by a thinly coated mirror into direct and reflected beams, which were reflected around the rectangle by mirrors at the comers. The two beams returning to the original mirror produced interference fringes. The beam traversing the rectangle in a counter-clockwise direction was retarded. The observed displacement of the fringes was found to be 0.230±.005, agreeing with the computed value 0.236±.002 within the limits of experimental error.
A rectangular tract of land at Clearing, Illinois, 2010 feet from east to west and 1113 feet from north to south, was carefully surveyed and staked by Dr. Kannenstine, and twelve-inch water pipes were laid straight and level around the entire circuit with a double line across one end.
The general plan of the arrangement is shown in Figure 1. Cast-iron boxes at the corners contained the mirrors. Figure 2 shows the details of one of the corner boxes. Delicate screw-and-lever systems, operated from outside the boxes through carefully fitted beveled joints, rendered it possible to adjust the mirrors readily about horizontal or vertical axes. The boxes were set in heavy concrete piers, and connected to the pipes by flexible joints of canvas and rubber. Similar joints were inserted in the pipe lines, about four hundred feet apart, and served as expansion joints.
The plane-parallel plates at A and B were lightly coated with gold, and that at C with silver, to reflect and transmit the desired proportions of light. The mirrors at D, E, and F were heavily silvered. A telephone system, consisting of portable sets kindly loaned by the Chicago Telephone Company, made it possible for an observer at A to direct assistants, one at each corner, when an adjustment of the mirrors was necessary. A Worthington air-pump, driven by a
Fig. 1.—Ground plan, showing arrangement of mirrors