Radio Clock Is Kept on Time by Signals Varying a Beam

From The American Horologist magazine, September 1938

Radio Clock Is Kept on Time
by Signals Varying a Beam

It's time set and corrected by signals sent out from a master clock at the broadcasting station, a radio clock that can be installed as part of an ordinary radio receiver has just been patented. It tells time to the nearest five minutes, but can be modified to show correct time to the nearest second. Without interfering with the regular broadcast program, the clock is set by low-frequency modulations below the audible range. The signals cause reeds to vibrate, and reeds interrupt or let pass a light beam directed toward a screen. The position of the beam indicates the time.