THE EFFECT OF THE EARTH'S ROTATION ON THE VELOCITY OF LIGHT

Transcription

1 THE EFFECT OF THE EARTH'S ROTATON ON THE VELOCTY OF LGHT PART By A. A. MCHELSON AND HENRY G. GALE ASSSTED BY FRED PEARSON ABSTRACT Experimental Test f The-r'Y.-Air was exhausted frm a twelve-inch pipe line laid n the surlace f the grund in the frm f a rectangle 20OX3 feet. Light frm a carbn arc was divided at ne crner by a thinly cated mirrr int direct and reflected beams, which were re:tlected arund the rectangle by mirrrs at the cmers. The tw beams returning t the riginal mirrr prduced interference fringes. The beam traversing the rectangle in a cunter-clckwise directin was retarded. The bserved displacement f the fringes was fund t be 0.23 =.005, agreeing with the cmputed value 0.236=.002 within the limits f experimental errr. A rectangular tract f land at Clearing, llinis, 2010 feet frm east t west and 1113 feet frm nrth t suth, was carefully surveyed and staked by Dr. Kannenstine, and twelve-inch water pipes were laid straight and level arund the entire circuit with a duble line acrss ne end. The general plan f the arrangement is shwn in Figure. Castirn bxes at the crners cntained the mirrrs. Figure 2 shws the details f ne f the crner bxes. Delicate screw-and-ever systems, perated frm utside the bxes thrugh carefully fitted beveled jints, rendered it pssible t adjust the mirrrs readily- abut hrizntal r vertical axes. The bxes were set in heavy cncrete piers, and cnnected t the pipes by flexible jints f canvas and rubber. Similar jints were inserted in the pipe lines, abut fur hundred feet apart, and served as expansin jints. The plane-parallel plates at A and B were lightly cated with gld, and that at C with silver, t reflect and transmit the desired prprtins f light. The mirrrs at D, E, and F were heavily silvered. A telephne system, cnsisting f prtable sets kindly laned by the Chicag Telephne Cmpany, made it pssible fr an bserver at A t direct assistants, ne at each crner, when an adjustment f the mirrrs was necessary. A Wrthingtn air-pump, driven by a 140

2 EARTH'S ROTATON AND VEWCTY OF LGHT 141 Jf' ~ ~ ----.J ~, ~ _---- CO, ~ ~ ~! S d~~~~l ~ ~ ~-- ---~ ; t J f t

3 142 A. A. MCHELSON AND HENRY G. GALE 50 hrse-pwer mtr, reduced the pressure in the pipe t abut a half-inch in three hurs. Mst f the measurements were made when the residual air in the pipes had been reduced t a pressure f abut half an inch t ne inch f mercury. At these pressures the fringes were perfectly steady, and as sharply defined as culd be desired. FG. 2.-Details f cmer bx and mirrr munting The fringes t be measured were thse frmed by the beams ging in ppsite directins abut the circuit ADEF. As a fiducial mark frm which t measure the displacement, a secnd set f fringes was frmed by the mirrr system ABeD. The area inclsed by this circuit was much t small t give a measurable displacement f the fringes, and the shifts actually recrded were thse between the central fringes f the tw sets.

4 EARTH'S ROTATON AND VELOCTY OF LGHT 143 n general the tw sets f fringes will nt cincide in psitin, entirely aside frm any questin f ether drift r the earth's rtatin, unless the tw direct images and the tw reflected images f the surce are exactly superpsed. The central fringes f the set frmed by the mirrrs f the shrt circuit will be halfway between the direct and reflected images f the surce, and the central fringe f the lng line wuld be halfway between the direct and reflected images if there were n difference due t the earth's rtatin. T crrect fr any lack f superpsitin f the tw sets f rays, the bserving telescpe (a six-inch achrmatic bjective, and twinch micrmeter eyepiece) was fcused n the images f the surce (arc r slit) and the apparent displacement f the central fringe f the lng circuit, cmpared with the central fringe f the shrt circuit, was crrected by an amunt equal t the difference in the mean psitins f the tw images fr the tw light circuits. The fringes are mst cnveniently bserved in the verlapping cnes f light an inch r s inside r utside f the fcal plane. Abut half f the detenninatins were made with the arc placed directly in frnt f the windw at A, and abut half with a cndensing lens, slit, and cllimating lens. The secnd arrangement,gave much mre light than the first, but there was n apparent difference in the measured displacements. The calculated value f the displacement n the assumptin f a statinary ether as well as in accrdance with relativity is il=4a w sin q, AV where L1 is the displacement in fringes, A the area in square kilmeters, cp the latitude (41046'), V the velcity f light, UJ the angular velcity f the earth, and A the effective wave-length f the light used. Measurements were made in the labratry, cmparing the fringes prduced by the same set f mirrrs and the same 20-ampere alternating-current arc, with fringes prduced by sdium light frm a bit f glass in an xyhydrgen flame. The light frm the arc was reduced t apprximately the same intensity as in the experiment at Clearing, by transmitting it thrugh a rather narrw slit in a

5 144 A. A. MCHELSON AND HENRY G. GALE rtating disk. The mean f ten determinatins gave x= 5700 ± 50 angstrms. The displacement f the fringes due t the earth's rtatin was measured n many different days, with cmplete readjustments f the mirrrs, with the reflected image smetimes n the right and smetimes n the left f the transmitted image, and by different bservers. The deflectins were averaged usually in sets f twenty, in the rder in which they were taken. The resulting means are given in Table. The entire set f tw hundred and sixty-nine determina- TABLE, Observatin* Displacement Number f Deviatin in Fringes Observatins frm Mean... _ _ , _ _....2$ _ Mean Ttal 269 Av. dev. frm Mean *Observatins and 2 crrected ft direct image nly; z-6 inclusive t withut cllimatr; 7-13 inclusive. with cllimatr.. Obs. Displacement :i:: 005 Calc =.002 tins and their distributin abut the mean value is shwn graphically in Figure 3. The final displacement, expressed as a fractin f a fringe, is 0.23 =.005 bs. O. 236:1:.002 calc. n view f the difficulty f the bservatins, this must be taken t mean that the bseryed and calculated shifts agree within the limits f bservatinal errr. The successful cmpletin f this experiment is due in n small measure t the friendly c-peratin f the fficials f the City f

6 EARTH'S ROTATON AND VEWCTY OF LGHT 145 Chicag, Clnel A. A. Sprague, cmmissiner f public wrks; Mr. H. L. Lucas, superintendent f the Water Pipe Extensin Divisin, and t Mr. B. W. Cullen, superintendent f the Suth District f the same Divisin; t the curtesy f the Clearing.... (» 8 ci.. l e... e : ~.. e ~ ~ rJ 11') Hd M ~ ~ cj Fringe Displacement FG. 3.-Distributin f bservatins ")... ndustrial District, thrugh Mr. Henry H. Prter; t the kindness f the Chicag Telephne Cmpany, and t the hearty and enthusiastic supprt f the University. We are especially indebted t President Burtn, Mr. J. C. Dinsmre, Mr. L. R. Flk, and t Messrs. Thmas O'Dnnell, Jseph Purdy, and Charles Stein. RYERSON PHYSCAL LABORATORY UNVERSTY OF CHCAGO March 1925