TIME INTERVAL-LOCAL

SYNCHRONIZATION

REGIONAL

DATE-UNIVERSAL

TIGER

stantial loss by fire, since the policy was issued on standard time and the fire in question, had it occurred during a period of standard instead of daylight-saving time, would have been within the time still covered by the policy. The birth or death date affected by an individual's crossing the date line can have important bearing on anything from the child's qualifying for age requirements to enter kindergarten to the death benefits to which the family of the deceased are entitled. The subject continues to be a lively issue, and probably will remain so.

TIME AS A STANDARD

The disarray in railroad travel caused by the lack of a standard time system in the late 19th century illustrates one of the primary benefits of standardization-standards promote better understanding and communication. If we agree on a particular standard of time or mass, then we all know what a "minute" or a "kilogram" means.

In working with time and frequency, we have standardization at various levels. With the development of better clocks, people began to see the need for defining more carefully the basic units of time since the minutes or seconds yielded by one clock were measurably different from those yielded by another. As early as 1820, the French defined the second as "1/86,400 of the mean solar day," establishing a standard time interval even though town clocks ticking at the same rate would show different local timethat is, a different date for each town.

In our first chapter we discussed briefly the concepts of time interval, synchronization, and date. In a sense these three concepts represent different levels of standardization. Time interval has a kind of "local" flavor. When one is boiling a three-minute egg, the time in Tokyo is of little concern to him. What he needs to know is how long three minutes is at his location.

King Features Syndicate, Inc..

OKING FEATURES SYNDICATE, INC. 1977-REPRINTED BY PERMISSION OF KING FEATURES SYNDICATE

Synchronization has a somewhat more cosmopolitan flavor. Typically, if we are interested in synchronization we care only that

particular events start or stop at the same time, or that they stay in step. For example, if people on a bus tour are told to meet at the bus at 6:00 P.M., they need only synchronize their watches with the bus driver's watch, to avoid missing the bus. It is of little consequence whether the bus driver's watch is "correct" or not.

The concept of date has the most nearly universal flavor. It is determined according to well-defined rules discussed on page 67, and it cannot be arbitrarily altered by people on bus tours; they do it only at their own peril, for they may well be late for dinner.

There has been a trend in recent years to develop standards in such a way that, if certain procedures are followed, the basic units can be determined. For example, the definition of the second, today, is based upon counting a precise number of oscillations of the cesium atom, as we discussed on page 66. This means that anybody who has the means and materials necessary, and who is clever enough to build a device to count vibrations of the cesium atoms, can determine the second. He doesn't have to travel to Paris. Similarly, the unit of length is defined by a certain wave length of light emitted by the krypton atom.

Concepts such as date, on the other hand-built from the basic units have an arbitrary starting point-such as the birth of Christ which cannot be determined by any physical device.

IS A SECOND REALLY A SECOND?

In our development of the history of timekeeping, we saw that the spinning earth makes a very good timepiece; even today, except for the most precise needs, it is more than adequate. Nevertheless, with the development of atomic clocks we have turned away from the earth definition of the second to the atomic definition. But how do we know that the atomic second is uniform?

One thing we might do to find out is to build several atomic clocks, and check to see if the seconds they generate "side by side" are of equal length. If they are, then we will be pretty certain that we can build clocks that produce uniform intervals of time at the "same" time.

But then how can we be certain that the atomic second itself isn't getting longer or shorter with time? Actually there is no way to tell, if we are simply comparing one atomic clock with another. We must compare the atomic second with some other kind of second. But then if we measure a difference, which second is changing length and which one is not? There would seem to be no way out of this maze. We must take another approach.

TIME:

Instead of trying to prove that a particular kind of clock produces uniform time, the best we can do is agree to take some device be it the spinning earth, a pendulum, or an atomic clock -and simply say that the output of that device helps us define time. In this sense we see that time is really the result of some set of operations that we agree to perform in the same way. This set of operations produces the standard of time; other sets of operations will produce different time scales.

But, one may ask, what if our time standard really does speed up at certain times and slow down at others? The answer is that it really doesn't make any difference, because all clocks built on the same set of operations will speed up and slow down together, so "we will all meet for lunch at the same time"-it's a matter of definition.

Every day hundreds of thousands of people drop nickels, dimes, and quarters into parking meters, coin-operated washers, dryers and dry-cleaning machines, and "fun" machines that give their children a ride in a miniature airplane or on a mechanical horse. Housewives trust their cakes and roasts, their clothing and their fine china to timers on ovens, laundry equipment, and dishwashers. Businesses pay thousands of dollars for the use of a computer's time or for minutes-sometimes fractions of minutes of a communication system's time. We all pay telephone bills based on the number of minutes and parts of minutes we spend talking to Aunt Martha halfway across the nation.

The pumps at the gas station and the scales at the supermarket bear a seal that certifies recent inspection by a standards authority, and assurance that the device is within the accuracy requirements set by law. But who cares about the devices that measure time? What's to prevent a company from manufacturing equipment that runs for 9 minutes and 10 seconds, for instance, instead of the 10 minutes stated on a label? Are there any regulations at all for such things?

Yes indeed. In the United States, the National Bureau of Standards (NBS) has the responsibility for developing and operating standards of time interval (frequency). It is also given the responsibility of providing the "means and methods for making measurements consistent with those standards." As a consequence of these directives, the NBS maintains, develops, and operates a primary frequency standard based on the cesium atom. It also broadcasts standard frequencies based on this primary standard. (See page 73.)

The state and local offices of weights and measures deal with matters of time interval and date, generally by reference to an NBS handbook that deals with such devices as parking meters, parking garage clocks, "time in-time out" clocks, and similar timing devices. The greatest accuracies involved in these devices are about 2 minutes on the date, and about 0.1 percent on time interval. Typically the penalty for violating this code is a fine, a jail sentence, or both, for the first offense.

State standards laboratories seek help from NBS for such duties as calibrating radar "speed guns" used by traffic officers and other devices requiring precise timing. In addition to NBS, there are more than 250 commercial, governmental, and educational institutions in the United States that maintain standards laboratories; some 65 percent of these do frequency and/or time calibrations. So the facilities for monitoring the timing devices that affect the lives of all of us are readily available throughout the land.

In the United States, the United States Naval Observatory (USNO) collects astronomical data essential for safe navigation at sea, in the air, and in space. The USNO maintains an atomic time scale based on a large number of commercial cesium-beam frequency standards. And like NBS, it disseminates its standard, or time scale, by providing time information to several U.S. Navy broadcast stations. The Department of Defense (DOD) has given the USNO the responsibility of tending to the time and frequency needs of the DOD. As a practical matter, however, both the USNO and NBS have a long history of working cooperatively together to meet the needs of a myriad of users.

The responsibility for enforcing the daylight-saving time changes and keeping track of the standard time zones in this country is held by the U.S. Department of Transportation (DOT). And yet another organization—the Federal Communications Commission (FCC) is involved in time and frequency control through its regulation of radio and television broadcasts. Its Code of Federal Regulations-Radio Broadcast Services describes the frequency allocations and the frequency tolerances to which various broadcasters must conform. These include AM stations, commercial and non-commercial FM stations, TV stations, and international broadcasts. The NBS broadcast stations are references which the broadcaster may use to maintain assigned frequency, but the FCC is the enforcing agency.

The development, establishment, maintenance, and dissemination of information generated by time and frequency standards are vitally important services that most of us take for granted and rarely question or think about at all; and they require constant monitoring, testing, comparisons, and adjustment. Those responsible for maintaining these delicate and sensitive standards are constantly seeking better ways to make them more widely available, at less cost to more users. Each year, the demand for better, more reliable, and easier-to-use standards grows; and each year the scientists come up with at least some new concepts and answers to their problems.