SPEED OF PROPAGATION OF RADIO WAVES IN AIR1

To determine the speed of radio waves, Vitter and Brieger undertook a measurement of the speed by an interference method, using ultrashort waves.

Their apparatus consisted of a radio transmitter with a frequency of 113.749 MHz, placed on a straight line between a receiver and a reflector—a flat metallic screen, the plane of which was perpendicular to the receiver–transmitter line and which could be moved, counting the displacement with sufficient accuracy.

With this arrangement, two rays enter the receiver antenna: the direct ray from the transmitter and the ray reflected from the screen. The magnitude of the signal in the receiver antenna is determined by the phase difference of the two rays, which in turn depends on the position of the screen. The amount of displacement of the screen corresponding to the occurrence of two adjacent minima or maxima (the need to use maxima or minima arises because of the weakening of the reflected ray as the distance to the screen increases) obviously gives the length of a half-wave. Since the generator frequency was known with considerable accuracy (to 0.03%; the generator was stabilized with a quartz stabilizer), the speed in these experiments was determined with accuracy to the second significant digit. The authors give for it the value \((2.98 \pm 0.07)\cdot 10^{10}\ \mathrm{cm/sec}\).

I. Khleibnikov, Moscow

LITERATURE

1. A. L. Vitter and L. C. Brieger, Phys. Rev., 55, 416, 1939.
2. K. C. Collwell and A. W. Friend, Phys. Rev., 51, 11, 990, 1937.