Time Measurement

definition of the second - frequency calibration - atomic clocks - synchronization

What it shows:
There are several services to help the scientist keep time. Some of these can be brought into the lecture hall. Students can listen to the time signals from WWVB (60 kHz signal from Colorado) on a radio receiver or the U.S. Naval Observatory's time service over a telephone line. A publication giving detailed descriptions of the technical services provided by the National Bureau of Standards radio stations is available in the Prep Room. These services are: standard radio frequencies, standard audio frequencies, standard musical pitch, standard time intervals, time signals, astronomical time corrections, radio propagation forecasts, and geophysical alerts. The frequency signals may be displayed on a oscilloscope. On the East Coast of the U.S. (that's us), you can listen to Loran-C, a navigational signal at 100 kHz, to determine frequency and time.

One can also call 1-900-410-TIME. This is the number of the U.S. Naval Observatory's time service. A voice announcement and background ticking allows scientific users to synchronize equipment. The Observatory maintains the Master Clock of the United States. The standard is the resonant frequency of the cesium atom (9,192,631,770 Hz). 1   One clock is not enough. Twenty to twenty-four cesium atomic clocks, kept in separate environmentally controlled vaults, are averaged to determine the time (to 1 billionth of a second per day). About once a year the clocks are adjusted to compensate for the decrease in the rotation rate of the earth. One "leap second" is added by time keeping services around the world.

Setting it up:
The Advanced Physics Laboratory (P191/247) has one or two receivers that can be borrowed. Radio reception is extremely poor throughout the building and the lecture halls are no exception (all the steel-reinforced concrete). Therefore a long (10 or more meters) wire to serve as a crude antenna should be strung up outside, taking care to keep it insulated from ground and as far away from the building as is practical. 2   A long coaxial cable connects the antenna wire to the receiver. The shield should be grounded to minimize noise.

A much easier setup is the Naval Observatory's time service. A modular phone plug is located in the front of the lecture hall. Simply plug in a phone and dial the number (there is a toll charge). The lecturer can hold the radio microphone to the telephone receiver for the class to hear.

Comments:
Although it's mostly a show-n-tell demonstration, it's kind of fun to do it once. Rating *

1 By international agreement, cesium defines the second to be the duration of exactly 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
2 If reception of WWVB is a problem, you can try for WWVH at 2.5, 5, 10, and 15 MHz or WWV at these same frequencies plus 20 and 25 MHz. Obviously it depends where you are. Also receivable from Ottawa, Canada is the Dominion Observatory time signal--CHU at 3.330, 7.355, and 14.670 MHz.