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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.

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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.

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Time is so basic a part of our daily lives that we tend to take it for granted and overlook the vital part it plays in industry, scientific research, and many other activities of our present-day world. Almost any activity today that requires precision control and organization rests on time and frequency technology. Its role in these activities is essentially the same as in our own mundane affairs-providing a convenient way to bring order and organization into what would otherwise be a chaotic world.

The difference is mainly one of degree; in our everyday lives we rarely need time information finer or more accurate than a minute or two, but modern electronics systems and machines often require accuracies of one microsecond and better. In this chapter we shall see how the application of precise time and frequency technology helps to solve problems of control and distribution in three key areas of modern industrial society-energy, communication, and transportation-plus a few other usual and unusual uses of time and frequency information.

ELECTRIC POWER

Whether it is generated by nuclear reactors, fossil fuel-burning plants, or a hydroelectric system, electric power is delivered in the United States and Canada at 60 Hz, and at 50 Hz in a good part of the rest of the world. For most of us it is in this aspect of electric power that time and frequency plays its most familiar role. The kitchen wall clock is not only powered by electricity, but its "ticking" rate is tied to the "line" frequency maintained by the power company.

ENERGY

COMMUNICATION
TRANSPORTATION

61.1- EXTRA LOAD LOAD REMOVED

160.00

59.9 +

TIME

The power companies carefully regulate line frequency, so electric clocks keep very good time. The motors that drive tape and record players operate at rates controlled by the line frequency, so that listeners hear the true sound; and electric toothbrushes and shavers, vacuum cleaners, refrigerators, washing and drying machines operate efficiently.

Nevertheless, there are slight variations in frequency that cannot be overcome. If the line load unexpectedly increases in a particular location-such as when many people turn on their television sets at the same time to see a local news flash-power generators in the area will slow down until input energy to the system is increased or until the load is removed. For example, the line frequency may drop to 59.9 Hz for a time and then return to 60.0 Hz when the extra load is removed.

During the period when the line frequency is low, electric clocks will accumulate a time error that remains even though 60 Hz is restored at some later point. To remove this time error, it is the practice of the power companies to increase line frequency above 60 Hz until the time error is removed at which time they drop back to 60 Hz. Generally the time error never exceeds two seconds; and in the United States this error is determined with respect to special time and frequency broadcasts of the National Bureau of Standards. (See page 74.)

But frequency plays a greater role in electric power systems than merely providing a convenient time base for electric clocks. Frequency is a basic quantity that can be measured easily at every point of the system, and thus provides a way to "take the temperature" of the system.

We have seen that frequency excursions are indicative of load variations in power consumption. These variations are used to generate signals that control the supply of energy to the generators, usually in the form of steam-or water at hydroelectric plants. To provide more reliable service, many power companies have formed regional "pools," so that if power demands in a particular region exceed local capability, neighboring companies can fill in with their excess capacity.

Frequency plays an important part in these interconnected systems from several standpoints. First, all electric power in a connected region must be at the same frequency. If an "idle" generator is started up to provide additional power, it must be running in synchronism with the rest of the system before being connected into it. If it is running too slowly, current will flow into its windings from the rest of the system in an attempt to bring it up to speed; and if it is running too fast, excessive current will flow out of its windings in an attempt to slow it down. In either case, these currents may damage the machine.

Besides running at the same electrical frequency as the rest of the system the new generator must also be in step, or in phase with the rest of the system. Otherwise, damaging currents may again flow to try to bring the machine into phase.

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