Contains a report on a research project.Lincoln Laboratory (Purchase Order DDL-B222)United States Department of the ArmyUnited States Department of the NavyUnited States Department of the Air Force (Contract AF19(122)-458

Stroke, G. W.

Zacharias, J. R.

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IX. ATOMIC BEAMS"Prof. J. R. Zacharias Dr. R. F. C. Vessot E. P. HilarProf. J. G. King H. H. Brown, Jr. G. W. StrokeProf. C. L. Searle R. WeissA. MEASUREMENT OF THE VELOCITY OF LIGHT1. IntroductionThree significant steps in the building of the apparatus for measuring the velocity oflight can now be reported: the completion of the main mechanical elements of the appa-ratus, the successful testing of the guiding of the cavity-tuning piston by dry-boundaryglass-on-brass bearings, and the successful use of a hypodermic syringe to control therate of advance of the guided piston that rests upon this syringe as it advances parallelto itself to tune the cavity in its successive resonance positions.We have completed the 70-mec oscillator that excites the Hg 1 9 8 light source, one ofthe three interferometric photoelectric recording systems, and the microwave resonancedetecting system described by E.P.Hilar in the Quarterly Progress Report of January 15,1958, page 61 [further details will be found in E. P. Hilar's Master's thesis (1)]. Weare in the final building stages of the apparatus for measuring the velocity of light, theproblems of which were outlined in the Quarterly Progress Report of October 15, 1956,page 46. A schematic view of the apparatus is shown in Fig. IX-l, and a photograph ofthe partially assembled system is shown in Fig. IX-2.2. Description of ApparatusA description of the completed parts of the apparatus follows. The schematic viewof Fig. IX-l shows the principle of the design that is carried out in the assembly shownin Fig. IX-2. The completed fused-quartz cavity (2) is being fed from the atomic-clockoscillator, and the resonances are detected through conical irises in the upper plate.The microwave radiation reaches and leaves the cavity through rectangular TE01 wave-guides, the transverse "straight" H fields of which are made to couple into the "radial"H field of the TE01 n mode in the cavity. This arrangement, with an iris diameter of3 mm inside the cavity, leads to very good energy transfer, and, at the same time, togood cavity isolation. The two irises are centered at 0.4804 of the radius from thecenter and placed 90' apart.The apparatus, which is arranged to operate in vacuo, is designed in such a waythat all critical elements (cavity, moving piston, waveguides, and interferometers) arefastened as directly as possible to a single brass plate, 1 inch thick. This plate isThis research was supported in part by Purchase Order DDL-B222 with LincolnLaboratory, which is supported by the Department of the Army, the Department of theNavy, and the Department of the Air Force under Contract AF19(122)-458 with M.I.T.VACUUM TANKCs 1339192.632 MC/SATOMIC CLOCKOSCILLATORVACUUM TANKHg 198 (LIGHT SOURCE)cIII I g'Il 2/ I MICROWAVERESONANCE- f SIGNALPHOTOMULTIPLIE RTUBELIGHT INTERFERENCEFRINGE SIGNAL (5460.7532 A)SILICONEOILVALVENEEDLE VALVERETURNFig. IX-1. Schematic view of velocity-of-light measurement apparatus.Velocity-of-light measurement apparatus partially assembledwith finished fused-quartz cavity and long sliding piston rod.Fig. IX-2.(IX. ATOMIC BEAMS)supported on a sturdy brass ring attached to three brass columns, as shown in Fig. IX-2.In this manner, reliable interferometric distance measurements in the fractional micro-inch domain should be least affected by mechanical distortion, temperature, and possiblevibrations. The parts of the feeding and receiving waveguides inside the vacuum systemare fastened to the inside of a brass ring that rests on the large brass plate, with themicrowave radiation traveling through mica windows from and to the outside connec-tions. The upper stainless-steel vacuum tank rests on this ring. All these mechanicaland microwave parts have been completed with the exception of a choke arrangementinside the vacuum system, between the waveguide sections from the ring to the irises.This choke will ensure complete mechanical isolation of the upper cavity plate fromthe ring and thereby eliminate distortion or temperature effects caused by the wave-guides.The successful principle of the glass-on-brass guiding of the piston without "stick-slip" effects, which was described in the Quarterly Progress Report of October 15, 1956,page 46, has been adopted. It now takes the form of a cylindrical glass piston (2-inchdiameter) sliding between two groups of three tangential brass rods (0. 25-inch diameter).Two rods form a 120* Vee, and the third is spring-loaded to push the cylindrical glasspiston gently against the Vee. The full weight of the piston (except for the frictionalcomponents of the springs which have been proved to be negligible) rests on a hypodermicsyringe filled with silicone oil and fastened to the large brass plate. The good stick-slip-free motion, as detected by a photoelectric interferometer that uses the green line(5460.74 A) of Hgl98, is shown in Fig. IX-3. This should be compared with similarFig. IX-3. Photoelectric-fringe signal as a function of piston advance, obtainedin green Hg 1 9 8 light with dry-boundary glass-on-brass bearings.The piston rod rests on silicone oil-filled hypodermic syringe(1 cycle - 10-5 inch, approximately 4 cps).records from the experimental apparatus that was used to lay the groundwork for thepresent apparatus (3). This result was achieved by optically polishing the outside of the2-inch glass cylinder to a very good scratch-free finish after it had been ground withina few hundred thousandths of an inch on a cylindrical grinder. Measurements of thestraightness of the cylindrical piston still need to be carried out to determine whether--_ __ __(IX. ATOMIC BEAMS)or not, and if so what, improvements are needed. It should be noted that the bearingdesign places greater emphasis on straightness than on circularity in this rod, and onegood way of measuring the straightness is to observe interferometrically the degree towhich parallelism is maintained during the motion of the piston which traverses manyinches. We recall that the motion of the tuning plate is observed by means of threeinterferometers placed 1200 apart, and that they will permit servocorrections of rotationeffects about horizontal axes which are partly caused by the piston shape, if such com-pensations are necessary.G. W. Stroke, J. R. ZachariasReferences1. E. P. Hilar, Resonance detector for a resonant-cavity measurement of the velocityof light, S.M. Thesis, Department of Electrical Engineering, M.I.T., May 26, 1958.2. J. R. Zacharias and G. W. Stroke, Precision cylindrical cavity, Quarterly ProgressReport, Research Laboratory of Electronics, M.I.T., Jan. 15, 1958, p. 57.3. Quarterly Progress Report, Research Laboratory of Electronics, M.I.T., Oct. 15,1956, Figs. VI-3 and VI-4, p. 50.

Atomic Beams

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