Both Essen and Bergstrand subsequently improved their
apparatus and figure 10 shows Essen's final form of cylindrical
microwave cavity resonator which, because it could be placed in a
vacuum, did not have to be corrected for the effect of the air.
You will see that the device is the microwave equivalent of a closed
organ pipe. The microwaves entered at E and the wavelength
inside the pipe at R was measured by displacing the piston P
(which essentially formed the lower end-plate of the cavity) from
one resonant position to the next by means of end-gauges measured
by optical interferometry. When this measured wavelength is
multiplied by frequency one obtains a velocity which still has to be
corrected for the effects of the cylindrical walls. This correction
is difficult to eliminate entirely and depending how it was evaluated
Essen (7) obtained values of C0 of 299792 or 299793 km/s. He decided
the true value must lie between these limits and published a value
Co 299792.5 T: i km/s, allowing for other expected systematic
errors.
Figure 11 shows a diagram of Bergstrand's Geodimeter. In
tended primarily for the measurement of geodetic base-lines it has
now been used for many determinations of the velocity of light.
Basically the apparatus consists of an accurate crystal controlled
oscillator of frequence about 10 Mc/s (MHz) used to drive a "Kerr-
cell" light modulator which then directs a beam of light modulated
at 20 Mc/s onto a distant plane mirror (or "corner-cube" reflector).
Simultaneously the oscillator is used to alter (with electrically
variable phase lag) the sensitivity of a photo-multiplier onto which
the reflected light is directed. By this means the phase of the
io8
Kerr cell
Polaroids
Jo plane mirror
Variable
electrical
delay unit.
Crystal
controlled
osc.
Variable light
delay unit
L Phototube
From plane mirror
Null indicator
Fig. 11. Schematic diagram of the Geodimeter.
