Abstract

Improvements in the apparatus formerly used include the use of an eleven-stage electron multiplier tube, the control of the transmitter frequency by the laboratory standard, the substitution of a 1000-watt water-cooled mercury arc for the light source, a new type of Kerr cell, changes in the mirror supports and base line measurements, and the use of an automatic recorder. The apparatus has been completely rebuilt and converted to a.c. operation. The optical system has been changed to permit simpler measurements of the path difference, and electrical circuits have been devised to smooth out the fluctuations due to voltage variations and other causes.

Group velocity is discussed as a correction factor in this and previous measurements. The correction is shown to amount to as much as 7 km/sec. in some cases. Electron transit time is shown to be a limiting factor for this method of measuring the velocity of light. The final result of 2895 observations is given as 299,776±14 km/sec. This includes a group velocity correction and should not be compared with previous results without taking this into consideration. The conclusion is reached that the velocity of light is a constant as nearly as we can measure it at present.

© 1941 Optical Society of America

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References

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  1. M. E. J. G. de Bray, Nature 120, 602 (1927).
    [CrossRef]
  2. O. Mittelstaedt, Physik. Zeits. 30, 165 (1929).
  3. M. E. J. G. de Bray, Nature 133, 464 (1934); F. K. Edmondson, Nature 133, 759 (1934).
    [CrossRef]
  4. R. T. Birge, Nature 134, 771 (1934); Rev. Mod. Phys. 1, 1 (1929); D. C. Miller, Rev. Mod. Phys. 5, 3 (1933).
    [CrossRef]
  5. Michelson, Pease, and Pearson, Astrophys. J. 82, 26 (1935).
    [CrossRef]
  6. Cf. W. C. Anderson, Rev. Sci. Inst. 8, 239 (1937).
    [CrossRef]
  7. Cf. G. W. Pierce, Proc. Am. Acad. Arts Sci. 10, 271 (1925); see also Proc. I. R. E. 16, 1072 (1928).
    [CrossRef]
  8. Private communication to the author.
  9. Rayleigh, Collected Papers (1911), Vol.  I, p. 322; Collected Papers Vol. VI, p. 41; Phil. Mag. 22, 130 (1914).
  10. P. Ehrenfest, Ann. d. Physik 33, 1571 (1910).
    [CrossRef]
  11. W. F. Meggers and Peters, Bull. Bur. Stand. 14, 697 (1918).
    [CrossRef]

1937 (1)

Cf. W. C. Anderson, Rev. Sci. Inst. 8, 239 (1937).
[CrossRef]

1935 (1)

Michelson, Pease, and Pearson, Astrophys. J. 82, 26 (1935).
[CrossRef]

1934 (2)

M. E. J. G. de Bray, Nature 133, 464 (1934); F. K. Edmondson, Nature 133, 759 (1934).
[CrossRef]

R. T. Birge, Nature 134, 771 (1934); Rev. Mod. Phys. 1, 1 (1929); D. C. Miller, Rev. Mod. Phys. 5, 3 (1933).
[CrossRef]

1929 (1)

O. Mittelstaedt, Physik. Zeits. 30, 165 (1929).

1927 (1)

M. E. J. G. de Bray, Nature 120, 602 (1927).
[CrossRef]

1925 (1)

Cf. G. W. Pierce, Proc. Am. Acad. Arts Sci. 10, 271 (1925); see also Proc. I. R. E. 16, 1072 (1928).
[CrossRef]

1918 (1)

W. F. Meggers and Peters, Bull. Bur. Stand. 14, 697 (1918).
[CrossRef]

1911 (1)

Rayleigh, Collected Papers (1911), Vol.  I, p. 322; Collected Papers Vol. VI, p. 41; Phil. Mag. 22, 130 (1914).

1910 (1)

P. Ehrenfest, Ann. d. Physik 33, 1571 (1910).
[CrossRef]

Anderson, W. C.

Cf. W. C. Anderson, Rev. Sci. Inst. 8, 239 (1937).
[CrossRef]

Birge, R. T.

R. T. Birge, Nature 134, 771 (1934); Rev. Mod. Phys. 1, 1 (1929); D. C. Miller, Rev. Mod. Phys. 5, 3 (1933).
[CrossRef]

de Bray, M. E. J. G.

M. E. J. G. de Bray, Nature 133, 464 (1934); F. K. Edmondson, Nature 133, 759 (1934).
[CrossRef]

M. E. J. G. de Bray, Nature 120, 602 (1927).
[CrossRef]

Ehrenfest, P.

P. Ehrenfest, Ann. d. Physik 33, 1571 (1910).
[CrossRef]

Meggers, W. F.

W. F. Meggers and Peters, Bull. Bur. Stand. 14, 697 (1918).
[CrossRef]

Michelson,

Michelson, Pease, and Pearson, Astrophys. J. 82, 26 (1935).
[CrossRef]

Mittelstaedt, O.

O. Mittelstaedt, Physik. Zeits. 30, 165 (1929).

Pearson,

Michelson, Pease, and Pearson, Astrophys. J. 82, 26 (1935).
[CrossRef]

Pease,

Michelson, Pease, and Pearson, Astrophys. J. 82, 26 (1935).
[CrossRef]

Peters,

W. F. Meggers and Peters, Bull. Bur. Stand. 14, 697 (1918).
[CrossRef]

Pierce, G. W.

Cf. G. W. Pierce, Proc. Am. Acad. Arts Sci. 10, 271 (1925); see also Proc. I. R. E. 16, 1072 (1928).
[CrossRef]

Rayleigh,

Rayleigh, Collected Papers (1911), Vol.  I, p. 322; Collected Papers Vol. VI, p. 41; Phil. Mag. 22, 130 (1914).

Ann. d. Physik (1)

P. Ehrenfest, Ann. d. Physik 33, 1571 (1910).
[CrossRef]

Astrophys. J. (1)

Michelson, Pease, and Pearson, Astrophys. J. 82, 26 (1935).
[CrossRef]

Bull. Bur. Stand. (1)

W. F. Meggers and Peters, Bull. Bur. Stand. 14, 697 (1918).
[CrossRef]

Collected Papers (1)

Rayleigh, Collected Papers (1911), Vol.  I, p. 322; Collected Papers Vol. VI, p. 41; Phil. Mag. 22, 130 (1914).

Nature (3)

M. E. J. G. de Bray, Nature 120, 602 (1927).
[CrossRef]

M. E. J. G. de Bray, Nature 133, 464 (1934); F. K. Edmondson, Nature 133, 759 (1934).
[CrossRef]

R. T. Birge, Nature 134, 771 (1934); Rev. Mod. Phys. 1, 1 (1929); D. C. Miller, Rev. Mod. Phys. 5, 3 (1933).
[CrossRef]

Physik. Zeits. (1)

O. Mittelstaedt, Physik. Zeits. 30, 165 (1929).

Proc. Am. Acad. Arts Sci. (1)

Cf. G. W. Pierce, Proc. Am. Acad. Arts Sci. 10, 271 (1925); see also Proc. I. R. E. 16, 1072 (1928).
[CrossRef]

Rev. Sci. Inst. (1)

Cf. W. C. Anderson, Rev. Sci. Inst. 8, 239 (1937).
[CrossRef]

Other (1)

Private communication to the author.

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Figures (13)

Fig. 1
Fig. 1

Plot showing the principal measurements of the velocity of light by direct and indirect methods since 1870.

Fig. 2
Fig. 2

Schematic diagram of the present method for measuring the velocity of light.

Fig. 3
Fig. 3

Glass-sealed Kerr cell with stopcock for draining and refilling without removal from the apparatus. The water-cooling jacket and glass screw pump are similar to those previously used for this purpose.

Fig. 4
Fig. 4

Electron multiplier tube circuit and acorn pentode used for preamplifier stage. The terminals AB were connected to the input of a high gain short wave receiver.

Fig. 5
Fig. 5

Schematic diagram of feed-back circuit used for diminishing extraneous fluctuations in the output voltage.

Fig. 6
Fig. 6

Actual circuit used for fluctuation elimination.

Fig. 7
Fig. 7

Automatic recorder used in final phase of the work.

Fig. 8
Fig. 8

Schematic diagram of the optical arrangement used for simplifying distance measurements.

Fig. 9
Fig. 9

Dual mounting for the concave mirrors (right) and single mounting for plane mirror (left). The micrometer microscope and special inside micrometer calipers for obtaining the mirror positions with respect to the tape are also shown.

Fig. 10
Fig. 10

Automatic motor control circuit for operating the recorder and lathe mirror.

Fig. 11
Fig. 11

Timing circuit for incorporating the date and time on a record by means of a brief exposure. The lamp L is flashed for a time interval independent of the length of time that the slowly acting switch S is closed.

Fig. 12
Fig. 12

Prints made from a series of records taken on the night of June 16–17, 1940. The upper ones are for the short path at about 10:30 p.m., while the lower ones are for the long path at about 2:00 a.m., June 17th. The original records are much clearer than these prints might indicate.

Fig. 13
Fig. 13

Diagram showing in exaggerated form the effect of electron transit time on the final results.

Equations (12)

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c = 2 f s / n ,
2 S + 2 x - 2 y = ( 2 n + 1 ) λ / 2.
2 x + 2 Δ S - 2 y - 2 Δ y = λ / 2.
2 S - 2 Δ S + 2 Δ y = n λ ,
U = V - λ d V / d λ
μ g = μ - λ d μ / d λ .
μ = A + B / λ 2 + C / λ 4 ,
μ g = A + 3 B / λ 2 + 5 C / λ 4 .
( μ - 1 ) × 10 7 = 2726.43 + 12.288 / λ 2 · 10 - 8 + 0.3555 / λ 4 · 10 - 16
( μ g - 1 ) × 10 7 = 2726.43 + 36.864 / λ 2 · 10 - 8 + 1.7775 / λ 4 · 10 - 16 .
n λ = 171.8147 m + 2 Δ y .
c a = 299,894.8 + 34.91 Δ y km / sec . ,